[Federal Register: February 15, 2005 (Volume 70, Number 30)]
[Rules and Regulations]
[Page 7799-7827]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr15fe05-13]
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Part II
Department of Transportation
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Federal Aviation Administration
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14 CFR Part 25
Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/
100B SUD/400/400D/400F Airplanes; Flammability Reduction Means (Fuel
Tank Inerting); Final Special Conditions; Rule
[[Page 7800]]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. NM270; Special Conditions No. 25-285-SC]
Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/
100B/300/ 100B SUD/400/400D/400F Airplanes; Flammability Reduction
Means (Fuel Tank Inerting)
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
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SUMMARY: These special conditions are issued for the Boeing Model 747-
100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F series
airplanes. These airplanes, as modified by Boeing Commercial Airplanes,
include a new flammability reduction means that uses a nitrogen
generation system to reduce the oxygen content in the center wing fuel
tank so that exposure to a combustible mixture of fuel and air is
substantially minimized. This system is intended to reduce the average
flammability exposure of the fleet of airplanes with the system
installed to a level equivalent to 3 percent of the airplane operating
time. The applicable airworthiness regulations do not contain adequate
or appropriate safety standards for the design and installation of this
system. These special conditions contain the additional safety
standards the Administrator considers necessary to ensure an acceptable
level of safety for the installation of the system and to define
performance objectives the system must achieve to be considered an
acceptable means for minimizing development of flammable vapors in the
fuel tank installation.
DATES: The effective date of these special conditions is March 17,
2005.
FOR FURTHER INFORMATION CONTACT: Mike Dostert, Propulsion and
Mechanical Systems Branch, FAA, ANM-112, Transport Airplane
Directorate, Aircraft Certification Service, 1601 Lind Avenue SW.,
Renton, Washington 98055-4056; telephone (425) 227-2132, facsimile
(425) 227-1320, e-mail mike.dostert@faa.gov.
SUPPLEMENTARY INFORMATION:
Background
Boeing Commercial Airplanes intends to modify Model 747 series
airplanes to incorporate a new flammability reduction means (FRM) that
will inert the center fuel tanks with nitrogen-enriched air (NEA).
Though the provisions of Sec. 25.981, as amended by amendment 25-102,
will apply to this design change, these special conditions address
novel design features.
Regulations used as the standard for certification of transport
category airplanes prior to amendment 25-102, effective June 6, 2001,
were intended to prevent fuel tank explosions by eliminating possible
ignition sources from inside the fuel tanks. Service experience of
airplanes certificated to the earlier standards shows that ignition
source prevention alone has not been totally effective at preventing
accidents. Commercial transport airplane fuel tank safety requirements
have remained relatively unchanged throughout the evolution of piston-
powered airplanes and later into the jet age. The fundamental premise
for precluding fuel tank explosions has involved establishing that the
design does not result in a condition that would cause an ignition
source within the fuel tank ullage (the space in the tank occupied by
fuel vapor and air). A basic assumption in this approach has been that
the fuel tank could contain flammable vapors under a wide range of
airplane operating conditions, even though there were periods of time
in which the vapor space would not support combustion.
Fuel Properties
Jet fuel vapors are flammable in certain temperature and pressure
ranges. The flammability temperature range of jet engine fuel vapors
varies with the type and properties of the fuel, the ambient pressure
in the tank, and the amount of dissolved oxygen released from the fuel
into the tank. The amount of dissolved oxygen in a tank will also vary
depending on the amount of vibration and sloshing of the fuel that
occurs within the tank.
Jet A fuel is the most commonly used commercial jet fuel in the
United States. Jet A-1 fuel is commonly used in other parts of the
world. At sea level and with no sloshing or vibration present, these
fuels have flammability characteristics such that insufficient
hydrocarbon molecules will be present in the fuel vapor-air mixture, to
ignite when the temperature in the fuel tank is below approximately 100
[deg]F. Too many hydrocarbon molecules will be present in the vapor to
allow it to ignite when the fuel temperature is above approximately 175
[deg]F. The temperature range where a flammable fuel vapor will form
can vary with different batches of fuel, even for a specific fuel type.
In between these temperatures the fuel vapor is flammable. This
flammability temperature range decreases as the airplane gains altitude
because of the corresponding decrease of internal tank air pressure.
For example, at an altitude of 30,000 feet, the flammability
temperature range is about 60 [deg]F to 120 [deg]F.
Most transport category airplanes used in air carrier service are
approved for operation at altitudes from sea level to 45,000 feet.
Those airplanes operated in the United States and in most overseas
locations use Jet A or Jet A-1 fuel, which typically limits exposure to
operation in the flammability range to warmer days.
We have always assumed that airplanes would sometimes be operated
with flammable fuel vapors in their fuel tank ullage (the space in the
tank occupied by fuel vapor and air).
Fire Triangle
Three conditions must be present in a fuel tank to support
combustion. These include the presence of a suitable amount of fuel
vapor, the presence of sufficient oxygen, and the presence of an
ignition source. This has been named the ``fire triangle.'' Each point
of the triangle represents one of these conditions. Because of
technological limitations in the past, the FAA philosophy regarding the
prevention of fuel tank explosions to ensure airplane safety was to
only preclude ignition sources within fuel tanks. This philosophy
included application of fail-safe design requirements to fuel tank
components (lightning design requirements, fuel tank wiring, fuel tank
temperature limits, etc.) that are intended to preclude ignition
sources from being present in fuel tanks even when component failures
occur.
Need To Address Flammability
Three accidents have occurred in the last 13 years as the result of
unknown ignition sources within the fuel tank in spite of past efforts,
highlighting the difficulty in continuously preventing ignition from
occurring within fuel tanks. Between 1996 and 2000 the National
Transportation Safety Board (NTSB) issued recommendations to improve
fuel tank safety that included prevention of ignition sources and
addressing fuel tank flammability (i.e., the other two points of the
fire triangle).
The FAA initiated safety reviews of all larger transport airplane
type certificates to review the fail-safe features of previously
approved designs and also initiated research into the feasibility of
amending the regulations to address fuel tank flammability. Results
from the safety reviews indicated a significant number of single
[[Page 7801]]
and combinations of failures that can result in ignition sources within
the fuel tanks. The FAA has adopted rulemaking to require design and/or
maintenance actions to address these issues; however, past experience
indicates unforeseen design and maintenance errors can result in
development of ignition sources. These findings show minimizing or
preventing the formation of flammable vapors by addressing the
flammability points of the fire triangle will enhance fuel tank safety.
On April 3, 1997, the FAA published a notice in the Federal
Register (62 FR 16014), Fuel Tank Ignition Prevention Measures, that
requested comments concerning the 1996 NTSB recommendations regarding
reduced flammability. That notice provided significant discussion of
the service history, background, and issues related to reducing
flammability in transport airplane fuel tanks. Comments submitted to
that notice indicated additional information was needed before the FAA
could initiate rulemaking action to address all of the recommendations.
Past safety initiatives by the FAA and industry to reduce the
likelihood of fuel tank explosions resulting from post crash ground
fires have evaluated means to address other factors of the fire
triangle. Previous attempts were made to develop commercially viable
systems or features that would reduce or eliminate other aspects of the
fire triangle (fuel or oxygen) such as fuel tank inerting or ullage
space vapor ``scrubbing'' (ventilating the tank ullage with air to
remove fuel vapor to prevent the accumulation of flammable
concentrations of fuel vapor). Those initial attempts proved to be
impractical for commercial transport airplanes due to the weight,
complexity, and poor reliability of the systems, or undesirable
secondary effects such as unacceptable atmospheric pollution.
Fuel Tank Harmonization Working Group
On January 23, 1998, the FAA published a notice in the Federal
Register that established an Aviation Rulemaking Advisory Committee
(ARAC) working group, the Fuel Tank Harmonization Working Group
(FTHWG). The FAA tasked the FTHWG with providing a report to the FAA
recommending regulatory text to address limiting fuel tank flammability
in both new type certificates and the fleet of in service airplanes.
The ARAC consists of interested parties, including the public, and
provides a public process to advise the FAA concerning development of
new regulations. [Note: The FAA formally established ARAC in 1991 (56
FR 2190, January 22, 1991), to provide advice and recommendations
concerning the full range of the FAA's safety-related rulemaking
activity.]
The FTHWG evaluated numerous possible means of reducing or
eliminating hazards associated with explosive vapors in fuel tanks. On
July 23, 1998, the ARAC submitted its report to the FAA. The full
report is in the docket created for this ARAC working group (Docket No.
FAA-1998-4183). This docket can be reviewed on the U.S. Department of
Transportation electronic Document Management System on the Internet at
http://dms.dot.gov.
The report provided a recommendation for the FAA to initiate
rulemaking action to amend Sec. 25.981, applicable to new type design
airplanes, to include a requirement to limit the time transport
airplane fuel tanks could operate with flammable vapors in the vapor
space of the tank. The recommended regulatory text proposed, ``Limiting
the development of flammable conditions in the fuel tanks, based on the
intended fuel types, to less than 7 percent of the expected fleet
operational time (defined in this rule as flammability exposure
evaluation time (FEET)), or providing means to mitigate the effects of
an ignition of fuel vapors within the fuel tanks such that any damage
caused by an ignition will not prevent continued safe flight and
landing.'' The report included a discussion of various options for
showing compliance with this proposal, including managing heat input to
the fuel tanks, installation of inerting systems or polyurethane fire
suppressing foam, and suppressing an explosion if one occurred.
The level of flammability defined in the proposal was established
based on a comparison of the safety record of center wing fuel tanks
that, in certain airplanes, are heated by equipment located under the
tank, and unheated fuel tanks located in the wing. The ARAC concluded
that the safety record of fuel tanks located in the wings with a
flammability exposure of 2 to 4 percent of the FEET was adequate and
that if the same level could be achieved in center wing fuel tanks, the
overall safety objective would be achieved. The thermal analyses
documented in the report revealed that center wing fuel tanks that are
heated by air conditioning equipment located beneath them contain
flammable vapors, on a fleet average basis, in the range of 15 to 30
percent of the fleet operating time.
During the ARAC review, it was also determined that certain
airplane types do not locate heat sources adjacent to the fuel tanks
and have significant surface areas that allow cooling of the fuel tank
by outside air. These airplanes provide significantly reduced
flammability exposure, near the 2 to 4 percent value of the wing tanks.
The group therefore determined that it would be feasible to design new
airplanes such that airplane operation with fuel tanks that were
flammable in the flammable range would be limited to nearly that of the
wing fuel tanks. Findings from the ARAC report indicated that the
primary method of compliance available at that time with the
requirement proposed by the ARAC would likely be to control heat
transfer into and out of fuel tanks. Design features such as locating
the air conditioning equipment away from the fuel tanks, providing
ventilation of the air conditioning bay to limit heating and to cool
fuel tanks, and/or insulating the tanks from heat sources, would be
practical means of complying with the regulation proposed by the ARAC.
In addition to its recommendation to revise Sec. 25.981, the ARAC
also recommended that the FAA continue to evaluate means for minimizing
the development of flammable vapors within the fuel tanks to determine
whether other alternatives, such as ground-based inerting of fuel
tanks, could be shown to be cost effective.
To address the ARAC recommendations, the FAA continued with
research and development activity to determine the feasibility of
requiring inerting for both new and existing designs.
FAA Rulemaking Activity
Based in part on the ARAC recommendations to limit fuel tank
flammability exposure on new type designs, the FAA developed and
published amendment 25-102 in the Federal Register on May 7, 2001 (66
FR 23085). The amendment included changes to Sec. 25.981 that require
minimization of fuel tank flammability to address both reduction in the
time fuel tanks contain flammable vapors, (Sec. 25.981(c)), and
additional changes regarding prevention of ignition sources in fuel
tanks. Section 25.981(c) was based on the FTHWG recommendation to
achieve a safety level equivalent to that achieved by the fleet of
transports with unheated aluminum wing tanks, between 2 to 4 percent
flammability. The FAA stated in the preamble to Amendment 25-102 that
the intent of the rule was to--
* * * require that practical means, such as transferring heat
from the fuel tank (e.g., use of ventilation or cooling air), be
incorporated into the airplane design if heat sources were
[[Page 7802]]
placed in or near the fuel tanks that significantly increased the
formation of flammable fuel vapors in the tank, or if the tank is
located in an area of the airplane where little or no cooling
occurs. The intent of the rule is to require that fuel tanks are not
heated, and cool at a rate equivalent to that of a wing tank in the
transport airplane being evaluated. This may require incorporating
design features to reduce flammability, for example cooling and
ventilation means or inerting for fuel tanks located in the center
wing box, horizontal stabilizer, or auxiliary fuel tanks located in
the cargo compartment.
Advisory circulars associated with Amendment 25-102 include AC
25.981-1B, ``Fuel Tank Ignition Source Prevention Guidelines,'' and AC
25.981-2, ``Fuel Tank Flammability Minimization.'' Like all advisory
material, these advisory circulars describe an acceptable means, but
not the only means, for demonstrating compliance with the regulations.
FAA Research
In addition to the notice published in the Federal Register on
April 3, 1997, the FAA initiated research to provide a better
understanding of the ignition process of commercial aviation fuel
vapors and to explore new concepts for reducing or eliminating the
presence of flammable fuel air mixtures within fuel tanks.
Fuel Tank Inerting
In the public comments received in response to the 1997 notice,
reference was made to hollow fiber membrane technology that had been
developed and was in use in other applications, such as the medical
community, to separate oxygen from nitrogen in air. Air is made up of
about 78 percent nitrogen and 21 percent oxygen, and the hollow fiber
membrane material uses the absorption difference between the nitrogen
and oxygen molecules to separate the NEA from the oxygen. In airplane
applications NEA is produced when pressurized air from an airplane
source such as the engines is forced through the hollow fibers. The NEA
is then directed, at appropriate nitrogen concentrations, into the
ullage space of fuel tanks and displaces the normal fuel vapor/air
mixture in the tank.
Use of the hollow fiber technology allowed nitrogen to be separated
from air, which eliminated the need to carry and store the nitrogen in
the airplane. Researchers were aware of the earlier system's
shortcomings in the areas of weight, reliability, cost, and
performance. Recent advances in the technology have resolved those
concerns and eliminated the need for storing nitrogen on board the
airplane.
Criteria for Inerting
Earlier fuel tank inerting designs produced for military
applications were based on defining ``inert'' as a maximum oxygen
concentration of 9 percent. This value was established by the military
for protection of fuel tanks from battle damage. One major finding from
the FAA's research and development efforts was the determination that
the 9 percent maximum oxygen concentration level benchmark, established
to protect military airplanes from high-energy ignition sources
encountered in battle, was significantly lower than that needed to
inert civilian transport airplane fuel tanks from ignition sources
resulting from airplane system failures and malfunctions that have much
lower energy. This FAA research established a maximum value of 12
percent as being adequate at sea level. The test results are currently
available on FAA Web site: http://www.fire.tc.faa.gov/pdf/tn02-79.pdf
as FAA Technical Note ``Limiting Oxygen Concentrations Required to
Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures,'' report
number DOT/FAA/AR-TN02/79. As a result of this research, the quantity
of NEA that is needed to inert commercial airplane fuel tanks was
lessened so that an effective FRM can now be smaller and less complex
than was originally assumed. The 12 percent value is based on the
limited energy sources associated with an electrical arc that could be
generated by airplane system failures on typical transport airplanes
and does not include events such as explosives or hostile fire.
As previously discussed, existing fuel tank system requirements
(contained in earlier Civil Air Regulation (CAR) 4b and now in 14 Code
of Federal Regulations (CFR) part 25) have focused solely on prevention
of ignition sources. The FRM is intended to add an additional layer of
safety by reducing the exposure to flammable vapors in the heated
center wing tank, not necessarily eliminating them under all operating
conditions. Consequently, ignition prevention measures will still be
the principal layer of defense in fuel system safety, now augmented by
substantially reducing the time that flammable vapors are present in
higher flammability tanks. We expect that by combining these two
approaches, particularly for tanks with high flammability exposure,
such as the heated center wing tank or tanks with limited cooling,
risks for future fuel tank explosions can be substantially reduced.
Boeing Application for Certification of a Fuel Tank Inerting System
On November 15, 2002, Boeing Commercial Airplanes applied for a
change to Type Certificate A20WE to modify Model 747-100/200B/200F/
200C/SR/SP/100B/300/100B SUD/400/400D/400F series airplanes to
incorporate a new FRM that inerts the center fuel tanks with NEA. These
airplanes, approved under Type Certificate No. A20WE, are four-engine
transport airplanes with a passenger capacity up to 624, depending on
the submodel. These airplanes have an approximate maximum gross weight
of 910,000 lbs with an operating range up to 7,700 miles.
Type Certification Basis
Under the provisions of Sec. 21.101, Boeing Commercial Airplanes
must show that the Model 747-100/200B/200F/200C/SR/SP/100B/300/100B
SUD/400/400D/400F series airplanes, as changed, continue to meet the
applicable provisions of the regulations incorporated by reference in
Type Certificate No. A20WE, or the applicable regulations in effect on
the date of application for the change. The regulations incorporated by
reference in the type certificate are commonly referred to as the
``original type certification basis.'' The regulations incorporated by
reference in Type Certificate A20WE include 14 CFR part 25, dated
February 1, 1965, as amended by Amendments 25-1 through 25-70, except
for special conditions and exceptions noted in Type Certificate Data
Sheet A20WE.
In addition, if the regulations incorporated by reference do not
provide adequate standards with respect to the change, the applicant
must comply with certain regulations in effect on the date of
application for the change. The FAA has determined that the FRM
installation on the Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/
100B SUD/400/400D/400F series airplanes must also be shown to comply
with Sec. 25.981 at amendment 25-102.
If the Administrator finds that the applicable airworthiness
regulations (14 CFR part 25) do not contain adequate or appropriate
safety standards for the Boeing Model 747-100/200B/200F/200C/SR/SP/
100B/300/100B SUD/400/400D/400F series airplanes because of a novel or
unusual design feature, special conditions are prescribed under the
provisions of Sec. 21.16.
In addition to the applicable airworthiness regulations and special
conditions, the Model 747-100/200B/
[[Page 7803]]
200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F series airplanes must
comply with the fuel vent and exhaust emission requirements of 14 CFR
part 34 and the acoustical change requirements of Sec. 21.93(b).
Special conditions, as defined in Sec. 11.19, are issued in
accordance with Sec. 11.38 and become part of the type certification
basis in accordance with Sec. 21.101.
Special conditions are initially applicable to the model for which
they are issued. Should the type certificate for that model be amended
later to include any other model that incorporates the same or similar
novel or unusual design feature, or should any other model already
included on the same type certificate be modified to incorporate the
same or similar novel or unusual design feature, the special conditions
would also apply to the other model under the provisions of Sec.
21.101.
Novel or Unusual Design Features
Boeing has applied for approval of an FRM to minimize the
development of flammable vapors in the center fuel tanks of Model 747-
100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F series
airplanes. Boeing also plans to seek approval of this system on Boeing
Model 737, 757, 767, and 777 airplanes.
Boeing has proposed to voluntarily comply with Sec. 25.981(c),
amendment 25-102, which is normally only applicable to new type designs
or type design changes affecting fuel tank flammability. The provisions
of Sec. 21.101 require Boeing to also comply with Sec. Sec. 25.981(a)
and (b), amendment 25-102, for the changed aspects of the airplane by
showing that the FRM does not introduce any additional potential
sources of ignition into the fuel tanks.
The FRM uses a nitrogen generation system (NGS) that comprises a
bleed-air shutoff valve, ozone converter, heat exchanger, air
conditioning pack air cooling flow shutoff valve, filter, air
separation module, temperature regulating valve controller and sensor,
high-flow descent control valve, float valve, and system ducting. The
system is located in the air conditioning pack bay below the center
wing fuel tank. Engine bleed air from the existing engine pneumatic
bleed source flows through a control valve into an ozone converter and
then through a heat exchanger, where it is cooled using outside cooling
air. The cooled air flows through a filter into an air separation
module (ASM) that generates NEA, which is supplied to the center fuel
tank, and also discharges oxygen-enriched air (OEA). The OEA from the
ASM is mixed with cooling air from the heat exchanger to dilute the
oxygen concentration and then exhausted overboard. The FRM also
includes modifications to the fuel vent system to minimize dilution of
the nitrogen-enriched ullage in the center tank due to cross-venting
characteristics of the existing center wing fuel tank vent design.
Boeing originally proposed that the system be operated only during
flight and that the center tank would continue to be inert on landing
and remain inert during normal ground procedures. Boeing has more
recently stated that the FRM design may include the capability to be
operated on the ground.
Boeing has proposed that limited dispatch relief for operation with
an inoperative NGS be allowed. Boeing has initially proposed a 10-day
master minimum equipment list (MMEL) relief for the system. Boeing
originally proposed that there be no cockpit or maintenance indication
onboard for the NGS, and that periodic maintenance, using ground
service equipment, be performed to verify system operation. More
recently Boeing has stated that to meet operator needs and system
reliability and availability objectives, built-in test functions would
be included and system status indication of some kind would be
provided. In addition, indications would be provided in the cockpit on
certain airplane models that have engine indicating and crew alerting
systems. The reliability of the system is expected to be designed to
achieve a mean time between failure (MTBF) of 5000 hours or better.
Discussion
The FAA policy for establishing the type design approval basis of
the FRM design will result in application of Sec. Sec. 25.981(a) and
(b), amendment 25-102, for the changes to the airplane that might
increase the risk of ignition of fuel vapors. Boeing will therefore be
required to substantiate that changes introduced by the FRM will meet
the ignition prevention requirements of Sec. Sec. 25.981(a) and (b),
amendment 25-102 and other applicable regulations.
With respect to compliance with Sec. 25.981(c), AC 25.981-2
provides guidance in addressing minimization of fuel tank flammability
within a heated fuel tank, but there are no specific regulations that
address the design and installation of an FRM that inerts the fuel
tank. Since amendment 25-102 was adopted, significant advancements in
inerting technology have reduced the size and complexity of inerting
systems. Developments in inerting technology have made it practical to
significantly reduce fuel tank flammability below the levels required
within the rule. However, due to factors such as the limited
availability of bleed air and electrical power, it is not considered
practical at this time to develop systems for retrofit into existing
airplane designs that can maintain a non-flammable tank ullage in all
fuel tanks or during all operating conditions. These special conditions
include additional requirements above that of amendment 25-102 to Sec.
25.981(c) to minimize fuel tank flammability, such that the level of
minimization in these special conditions would prevent a fuel tank with
an FRM from being flammable during specific warm day operating
conditions, such as those present when recent accidents occurred.
Definition of ``Inert''
For the purpose of these special conditions, the tank is considered
inert when the bulk average oxygen concentration within each
compartment of the tank is 12 percent or less at sea level up to 10,000
feet, then linearly increasing from 12 percent at 10,000 feet to 14.5
percent at 40,000 feet and extrapolated linearly above that altitude.
The reference to each section of the tank is necessary because fuel
tanks that are compartmentalized may encounter localized oxygen
concentrations in one or more compartments that exceed the 12 percent
value. Currently there is not adequate data available to establish
whether exceeding the 12 percent limit in one compartment of a fuel
tank could create a hazard. For example, ignition of vapors in one
compartment could result in a flame front within the compartment that
travels to adjacent compartments and results in an ignition source that
exceeds the ignition energy (the minimum amount of energy required to
ignite fuel vapors) values used to establish the 12 percent limit.
Therefore, ignition in other compartments of the tank may be possible.
Technical discussions with the applicant indicate the pressure rise in
a fuel tank that was at or near the 12 percent oxygen concentration
level would likely be well below the value that would rupture a typical
transport airplane fuel tank. While this may be possible to show, it is
not within the scope of these special conditions. Therefore, the effect
of the definition of ``inert'' within these special conditions is that
the bulk average of each individual compartment or bay of the tank must
be evaluated and shown to meet the oxygen concentration limits
specified in the definitions section of these special conditions (12
percent or less at sea level) to be considered inert.
[[Page 7804]]
Determining Flammability
The methodology for determining fuel tank flammability defined for
use in these special conditions is based on that used by ARAC to
compare the flammability of unheated aluminum wing fuel tanks to that
of tanks that are heated by adjacent equipment. The ARAC evaluated the
relative flammability of airplane fuel tanks using a statistical
analysis commonly referred to as a ``Monte Carlo'' analysis that
considered a number of factors affecting formation of flammable vapors
in the fuel tanks. The Monte Carlo analysis calculates values for the
parameter of interest by randomly selecting values for each of the
uncertain variables from distribution tables. This calculation is
conducted over and over to simulate a process where the variables are
randomly selected from defined distributions for each of the variables.
The results of changing these variables for a large number of flights
can then be used to approximate the results of the real world exposure
of a large fleet of airplanes.
Factors that are considered in the Monte Carlo analysis required by
these special conditions include those affecting all airplane models in
the transport airplane fleet such as: A statistical distribution of
ground, overnight, and cruise air temperatures likely to be experienced
worldwide, a statistical distribution of likely fuel types, and
properties of those fuels, and a definition of the conditions when the
tank in question will be considered flammable. The analysis also
includes factors affecting specific airplane models such as climb and
descent profiles, fuel management, heat transfer characteristics of the
fuel tanks, statistical distribution of flight lengths (mission
durations) expected for the airplane model worldwide, etc. To quantify
the fleet exposure, the Monte Carlo analysis approach is applied to a
statistically significant number (1,000,000) of flights where each of
the factors described above is randomly selected. The flights are then
selected to be representative of the fleet using the defined
distributions of the factors described previously. For example, flight
one may be a short mission on a cold day with an average flash point
fuel, and flight two may be a long mission on an average day with a low
flash point fuel, and on and on until 1,000,000 flights have been
defined in this manner. For every one of the 1,000,000 flights, the
time that the fuel temperature is above the flash point of the fuel,
and the tank is not inert, is calculated and used to establish if the
fuel tank is flammable. Averaging the results for all 1,000,000 flights
provides an average percentage of the flight time that any particular
flight is considered to be flammable. While these special conditions do
not require that the analysis be conducted for 1,000,000 flights, the
accuracy of the Monte Carlo analysis improves as the number of flights
increases. Therefore, to account for this improved accuracy appendix 2
of these special conditions defines lower flammability limits if the
applicant chooses to use fewer than 1,000,000 flights.
The determination of whether the fuel tank is flammable is based on
the temperature of the fuel in the tank determined from the tank
thermal model, the atmospheric pressure in the fuel tank, and
properties of the fuel quantity loaded for a given flight, which is
randomly selected from a database consisting of worldwide data. The
criteria in the model are based on the assumption that as these
variables change, the concentration of vapors in the tank
instantaneously stabilizes and that the fuel tank is at a uniform
temperature. This model does not include consideration of the time lag
for the vapor concentration to reach equilibrium, the condensation of
fuel vapors from differences in temperature that occur in the fuel
tanks, or the effect of mass loading (times when the fuel tank is at
the unusable fuel level and there is insufficient fuel at a given
temperature to form flammable vapors). However, fresh air drawn into an
otherwise inert tank during descent does not immediately saturate with
fuel vapors so localized concentrations above the inert level during
descent do not represent a hazardous condition. These special
conditions allow the time during descent, where a localized amount of
fresh air may enter a fuel tank, to be excluded from the determination
of fuel tank flammability exposure.
Definition of Transport Effects
The effects of low fuel conditions (mass loading) and the effects
of fuel vaporization and condensation with time and temperature
changes, referred to as ``transport effects'' in these special
conditions, are excluded from consideration in the Monte Carlo model
used for demonstrating compliance with these special conditions. These
effects have been excluded because they were not considered in the
original ARAC analysis, which was based on a relative measure of
flammability. For example, the 3 percent flammability value established
by the ARAC as the benchmark for fuel tank safety for wing fuel tanks
did not include the effects of cooling of the wing tank surfaces and
the associated condensation of vapors from the tank ullage. If this
effect had been included in the wing tank flammability calculation, it
would have resulted in a significantly lower wing tank flammability
benchmark value. The ARAC analysis also did not consider the effects of
mass loading which would significantly lower the calculated
flammability value for fuel tanks that are routinely emptied (e.g.,
center wing tanks). The FAA and JAA have determined that using the ARAC
methodology provides a suitable basis for determining the adequacy of
an FRM system.
The effect of condensation and vaporization in reducing the
flammability exposure of wing tanks is comparable to the effect of the
low fuel condition in reducing the flammability exposure of center
tanks. We therefore consider these effects to be offsetting, so that by
eliminating their consideration, the analysis will produce results for
both types of tanks that are comparable. Using this approach, it is
possible to follow the ARAC recommendation of using the unheated
aluminum wing tank as the standard for evaluating the flammability
exposure of all other tanks. For this reason, both factors have been
excluded when establishing the flammability exposure limits. During
development of these harmonized special conditions, the FAA and the
European Joint Aviation Authorities (JAA) agreed that using the ARAC
methodology provides a suitable basis for determining the flammability
of a fuel tank and consideration of transport effects should not be
permitted.
Flammability Limit
The FAA, in conjunction with the Joint Airworthiness Authorities
(JAA) and Transport Canada, has developed criteria within these special
conditions that require overall fuel tank flammability to be limited to
3 percent of the fleet average operating time. This overall average
flammability limit consists of times when the system performance cannot
maintain an inert tank ullage, primarily during descent when the change
in ambient pressures draws air into the fuel tanks and those times when
the FRM is inoperative due to failures of the system and the airplane
is dispatched with the system inoperative.
Specific Risk Flammability Limit
These special conditions also include a requirement to limit fuel
tank flammability to 3 percent during ground operations, takeoff, and
climb phases of
[[Page 7805]]
flight to address the specific risk associated with operation during
warmer day conditions when accidents have occurred. The specific risk
requirement is intended to establish minimum system performance levels
and therefore the 3 percent flammability limit excludes reliability
related contributions, which are addressed in the average flammability
assessment. The specific risk requirement may be met by conducting a
separate Monte Carlo analysis for each of the specific phases of flight
during warmer day conditions defined in the special conditions, without
including the times when the FRM is not available because of failures
of the system or dispatch with the FRM inoperative.
Inerting System Indications
Fleet average flammability exposure involves several elements,
including--
The time the FRM is working properly and inerts the tank
or when the tank is not flammable;
The time when the FRM is working properly but fails to
inert the tank or part of the tank, because of mission variation or
other effects;
The time the FRM is not functioning properly and the
operator is unaware of the failure; and
The time the FRM is not functioning properly and the
operator is aware of the failure and is operating the airplane for a
limited time under MEL relief.
The applicant may propose that MMEL relief is provided for aircraft
operation with the FRM unavailable; however, it is considered a safety
system that should be operational to the maximum extent practical.
Therefore, these special conditions include reliability and reporting
requirements to enhance system reliability so that dispatch of
airplanes with the FRM inoperative would be very infrequent. Cockpit
indication of the system function that is accessible to the flightcrew
is not an explicit requirement, but may be required if the results of
the Monte Carlo analysis show the system cannot otherwise meet the
flammability and reliability requirements defined in these special
conditions. Flight test demonstration and analysis will be required to
demonstrate that the performance of the inerting system is effective in
inerting the tank during those portions of ground and the flight
operations where inerting is needed to meet the flammability
requirements of these special conditions.
Various means may be used to ensure system reliability and
performance. These may include: System integrity monitoring and
indication, redundancy of components, and maintenance actions. A
combination of maintenance indication and/or maintenance check
procedures will be required to limit exposure to latent failures within
the system, or high inherent reliability is needed to assure the system
will meet the fuel tank flammability requirements. The applicant's
inerting system does not incorporate redundant features and includes a
number of components essential for proper system operation. Past
experience has shown inherent reliability of this type of system would
be difficult to achieve. Therefore, if system maintenance indication is
not provided for features of the system essential for proper system
operation, system functional checks at appropriate intervals determined
by the reliability analysis will be required for these features. At a
minimum, proper function of essential features of the system should be
validated once per day by maintenance review of indications or
functional checks, possibly prior to the first flight of the day. The
determination of a proper interval and procedure will follow completion
of the certification testing and demonstration of the system's
reliability and performance prior to certification.
Any features or maintenance actions needed to achieve the minimum
reliability of the FRM will result in fuel system airworthiness
limitations similar to those defined in Sec. 25.981(b). Boeing will be
required to include in the instructions for continued airworthiness
(ICA) the replacement times, inspection intervals, inspection
procedures, and the fuel system limitations required by Sec.
25.981(b). Overall system performance and reliability must achieve a
fleet average flammability that meets the requirements of these special
conditions. If the system reliability falls to a point where the fleet
average flammability exposure exceeds these requirements, Boeing will
be required to define appropriate corrective actions, to be approved by
the FAA, that will bring the exposure back down to the acceptable
level.
Boeing proposed that the FRM be eligible for a 10-day MMEL dispatch
interval. The Flight Operations Evaluation Board (FOEB) will establish
the approved interval based on data the applicant submits to the FAA.
The MMEL dispatch interval is one of the factors affecting system
reliability analyses that must be considered early in the design of the
FRM, prior to FAA approval of the MMEL. Boeing requested that the
authorities agree to use of an MMEL inoperative dispatch interval for
design of the system. Boeing data indicates that certain systems on the
airplane are routinely repaired prior to the maximum allowable
interval. These special conditions require that Boeing use an MMEL
inoperative dispatch interval of 60 hours in the analysis as
representative of the mean time for which an inoperative condition may
occur for the 10-day MMEL maximum interval requested. Boeing must also
include actual dispatch inoperative interval data in the quarterly
reports required by Special Condition III(c)(2). Boeing may request to
use an alternative interval in the reliability analysis. Use of a value
less than 60 hours would be a factor considered by the FOEB in
establishing the maximum MMEL dispatch limit. The reporting requirement
will provide data necessary to validate that the reliability of the FRM
achieved in service meets the levels used in the analysis.
Appropriate maintenance and operational limitations with the FRM
inoperative may also be required and noted in the MMEL. The MMEL
limitations and any operational procedures should be established based
on results of the Monte Carlo assessment, including the results
associated with operations in warmer climates where the fuel tanks are
flammable a significant portion of the FEET when not inert. While the
system reliability analysis may show that it is possible to achieve an
overall average fleet exposure equal to or less than that of a typical
unheated aluminum wing tank, even with an MMEL allowing very long
inoperative intervals, the intent of the rule is to minimize
flammability. Therefore, the shortest practical MMEL relief interval
should be proposed. To ensure limited airplane operation with the
system inoperative and to meet the reliability requirements of these
special conditions, appropriate level messages that are needed to
comply with any dispatch limitations of the MMEL must be provided.
Confined Space Hazard Markings
Introduction of the FRM will result in NEA within the center wing
fuel tank and the possibility of NEA in compartments adjacent to the
fuel tank if leakage from the tank or NEA supply lines were to occur.
Lack of oxygen in these areas could be hazardous to maintenance
personnel, the passengers, or flightcrew. Existing certification
requirements do not address all aspects of these hazards. Paragraph
II(f) of the special conditions requires the applicant to provide
markings to emphasize the potential hazards associated with confined
spaces and areas where a hazardous atmosphere
[[Page 7806]]
could be present due to the addition of an FRM.
For the purposes of these special conditions, a confined space is
an enclosed or partially enclosed area that is big enough for a worker
to enter and perform assigned work and has limited or restricted means
for entry or exit. It is not designed for someone to work in regularly,
but workers may need to enter the confined space for tasks such as
inspection, cleaning, maintenance, and repair. (Reference U.S.
Department of Labor Occupational Safety & Health Administration (OSHA),
29 CFR 1910.146(b).) The requirement in the special conditions does not
significantly change the procedures maintenance personnel use to enter
fuel tanks and are not intended to conflict with existing government
agency requirements (e.g., OSHA). Fuel tanks are classified as confined
spaces and contain high concentrations of fuel vapors that must be
exhausted from the fuel tank before entry. Other precautions such as
measurement of the oxygen concentrations before entering a fuel tank
are already required. Addition of the FRM that utilizes inerting may
result in reduced oxygen concentrations due to leakage of the system in
locations in the airplane where service personnel would not expect it.
A worker is considered to have entered a confined space just by putting
his or her head across the plane of the opening. If the confined space
contains high concentrations of inert gases, workers who are simply
working near the opening may be at risk. Any hazards associated with
working in adjacent spaces near the opening should be identified in the
marking of the opening to the confined space. A large percentage of the
work involved in properly inspecting and modifying airplane fuel tanks
and their associated systems must be done in the interior of the tanks.
Performing the necessary tasks requires inspection and maintenance
personnel to physically enter the tank, where many environmental
hazards exist. These potential hazards that exist in any fuel tank,
regardless of whether nitrogen inerting has been installed, include
fire and explosion, toxic and irritating chemicals, oxygen deficiency,
and the confined nature of the fuel tank itself. In order to prevent
related injuries, operator and repair station maintenance organizations
have developed specific procedures for identifying, controlling, or
eliminating the hazards associated with fuel-tank entry. In addition
government agencies have adopted safety requirements for use when
entering fuel tanks and other confined spaces. These same procedures
would be applied to the reduced oxygen environment likely to be present
in an inerted fuel tank.
The designs currently under consideration locate the FRM in the
fairing below the center wing fuel tank. Access to these areas is
obtained by opening doors or removing panels which could allow some
ventilation of the spaces adjacent to the FRM. But this may not be
enough to avoid creating a hazard. Therefore, we intend that marking be
provided to warn service personnel of possible hazards associated with
the reduced oxygen concentrations in the areas adjacent to the FRM.
Appropriate markings would be required for all inerted fuel tanks,
tanks adjacent to inerted fuel tanks and all fuel tanks communicating
with the inerted tanks via plumbing. The plumbing includes, but is not
limited to, plumbing for the vent system, fuel feed system, refuel
system, transfer system and cross-feed system. NEA could enter adjacent
fuel tanks via structural leaks. It could also enter other fuel tanks
through plumbing if valves are operated or fail in the open position.
The markings should also be stenciled on the external upper and lower
surfaces of the inerted tank adjacent to any openings to ensure
maintenance personnel understand the possible contents of the fuel
tank. Advisory Circular 25.981-2 will provide additional guidance
regarding markings and placards.
Affect of FRM on Auxiliary Fuel Tank System Supplemental Type
Certificates
Boeing plans to offer a service bulletin that will install the FRM
on existing in-service airplanes. Some in-service airplanes have
auxiliary fuel tank systems installed that interface with the center
wing tank. The Boeing FRM design is intended to provide inerting of the
fuel tank volume of the 747 and does not include consideration of the
auxiliary tank installations. Installation of the FRM on existing
airplanes with auxiliary fuel tank systems may therefore require
additional modifications to the auxiliary fuel tank system to prevent
development of a condition that may cause the tank to exceed the 12
percent oxygen limit. The FAA will address these issues during
development and approval of the service bulletin for the FRM.
Disposal of Oxygen-Enriched Air (OEA)
The FRM produces both NEA and OEA. The OEA generated by the FRM
could result in an increased fire hazard if not disposed of properly.
The OEA produced in the proposed design is diluted with air from a heat
exchanger, which is intended to reduce the OEA concentration to non-
hazardous levels. Special requirements are included in these special
conditions to address potential leakage of OEA due to failures and safe
disposal of the OEA during normal operation.
To ensure that an acceptable level of safety is achieved for the
modified airplanes using a system that inerts heated fuel tanks with
NEA, special conditions (per Sec. 21.16) are needed to address the
unusual design features of an FRM. These special conditions contain the
additional safety standards that the Administrator considers necessary
to establish a level of safety equivalent to that established by the
existing airworthiness standards.
Discussion of Comments
Notice of Proposed Special Conditions No. 25-03-08-SC for the
Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/
400F series airplanes was published in the Federal Register on December
9, 2003 (68 FR 68563). Thirteen commenters responded to the notice.
General Comments
Comment: One commenter supports the special conditions but states
that ignition source prevention must still be provided. The commenter
believes that the combination of flammability reduction and ignition
source prevention is the most effective means to prevent fuel tank
explosions.
FAA Reply: The safety assessment required by Special Federal
Aviation Regulation (SFAR) No. 88, Fuel Tank System Fault Tolerance
Evaluation, identifies design and maintenance changes that are needed
to prevent ignition sources in transport category airplanes. The FAA is
developing a number of airworthiness directives (ADs) to address
ignition sources resulting from single failures in all fuel tanks and
combinations of failures in tanks that have been classified as high
flammability. We will not issue ADs to address combinations of failures
in high flammability tanks if the FRM is installed because of the
significant improvement in fuel tank safety offered by the FRM required
by this special condition. We are not considering a change to the
current ignition prevention analysis requirements that include assuming
a flammable ullage. No changes were made as a result of this comment.
Comment: Two commenters believe the special conditions for the FRM
are
[[Page 7807]]
not appropriate because the special conditions are written to fit the
applicant's proposed design of an inerting system to reduce
flammability of fuel tanks and are therefore considered ``prejudiced.''
One of these commenters adds that regulatory guidance should be
unprejudiced and available before development of any design.
FAA Reply: We do not concur. As stated earlier in this document,
these special conditions are specific to certification of an FRM based
on inerting technology. As discussed in AC 25.981-2, inerting, as well
as other technologies such as cooling, is an acceptable means of
compliance with Sec. 25.981(c). No changes were made as a result of
this comment.
Comment: Two commenters believe the limited FRM, as described in
the special conditions, would not comply with the requirements of
Sec. Sec. 25.981(c) and 25.1309 for new airplane designs (post
amendment 25-102) with high flammability fuel tanks.
FAA Reply: As stated earlier, these special conditions apply
specifically to certification of an FRM for applicable Boeing Model 747
series airplanes and do not apply to new airplane designs. However, we
have determined that an FRM that complies with these special conditions
would meet the intent of Sec. 25.981(c). No changes were made as a
result of this comment.
Comment: One commenter would support rulemaking to investigate
amending Sec. 25.981 (and revising AC 25.981-2) to:
Clarify that ``minimization of flammable vapors'' in
accordance with Sec. 25.981(c) is to be accomplished through design
features ensuring the tank will have inherent low flammability (e.g.
venting, cooling, control of heat transfer, etc.); and
Eliminate the possibility of compliance for future
airplane designs through the installation of a limited FRM.
FAA Reply: On February 17, 2004, the FAA Administrator announced
plans to issue a notice of proposed rulemaking that will require
approximately 3,800 Airbus and Boeing planes be fitted with systems
that reduce the presence of flammable vapors in fuel tanks. This
proposal could require airlines to install new systems to reduce fuel
tank flammability on existing and newly produced larger passenger jets.
We are also considering amending Sec. 25.981(c) and revising AC
25.981-2 to further limit fuel tank flammability. No changes were made
as a result of these comments.
Comment: The commenter requests that before proceeding with any
further regulatory activities, the FAA should provide additional
detailed information on whether SFAR 88 changes are sufficient to cover
the requirements of Sec. 25.981. The commenter believes that ``SFAR 88
meets the requirement of Sec. 25.981(c)(2) and does not understand the
need to also address Sec. 25.981(c)(1).'' This commenter also states
that harmonization with the European Aviation Safety Agency (EASA) on
these special conditions is essential for industry.
FAA Reply: We do not concur with the commenter's first statement. A
direct relationship between SFAR 88 and Sec. 25.981(c)(1), or Sec.
25.981(c)(2), does not exist. SFAR 88 addresses ignition source
prevention, while Sec. 25.981(c)(1) acknowledges an ignition source
may be present under some remote circumstances. Section 25.981(c)(2)
assumes that an ignition can occur--in essence that SFAR 88 was not
successful and also flammable vapors are present--and requires that the
resulting ignition of flammable vapor will not prevent continued safe
flight and landing. The FAA has fully coordinated these special
conditions with the JAA/EASA. No changes were made as a result of these
comments.
Comment: One commenter notes that although the special condition
requirements for system reliability and performance are very specific,
they do not address the qualification standards that the system will
have to meet. Additional guidance on this subject would be appropriate.
Another commenter expresses concern about use of the terms ``intended''
and ``expected'' in the special conditions when relating to an FRM. It
is the commenter's opinion that the use of these terms indicates that
the applicant is not confident that their design ``will'' or ``shall''
contribute to the overall safety of the airplanes.
FAA Reply: We do not concur. In the preamble to the special
conditions, we state that the applicant is required to show compliance
with the applicable airworthiness regulations and special conditions.
In part, the applicable regulations, Sec. 25.1301 and Sec. 25.1309,
require the applicant to show that the equipment ``functions properly
when installed'' and ``is designed to ensure that they perform their
intended functions under any foreseeable operating condition.''
Irrespective of any wording in the preamble to the special conditions,
the special conditions include requirements to address foreseeable
specific safety issues that are not addressed by the current
regulations. Any airplane that meets the requirements of the special
conditions will maintain the level of safety intended by the applicable
requirements of the Code of Federal Regulations (CFR). No changes were
made as a result of these comments.
Comment: One commenter states that there are various statements
made throughout the special conditions that refer to reliability and
maintenance of the system. It is the commenter's opinion that these
statements are specific to implementation, and the actual approach
should be derived using standard methodology used for certification of
the airplane.
FAA Reply: To achieve the desired safety level of the FRM, we
believe the special condition requirements for determining reliability
and maintainability of the FRM are necessary. This is to ensure that
the FRM is an acceptable means by which the development of flammable
vapors in the center wing tank is minimized as required by Sec.
25.981. No changes were made as a result of this comment.
Comment: One commenter notes that ``inert'' is not defined
consistently throughout the special conditions. The commenter suggests
the use of only one definition and proposes the definition used in
special condition paragraph I. Definitions. The same commenter also
requests clarification if linear extrapolation of oxygen concentration
can be used for aircraft ceilings above 40,000 feet, and clarification
of the difference between the terms ``bulk'' and ``bulk average.''
FAA Reply: We concur that the definition of inert needs to be
consistent throughout the special conditions and have therefore
modified the definition of inert in the preamble to incorporate the
definition of inert provided in paragraph I. Definitions of the special
conditions. With respect to aircraft altitudes above 40,000 feet, we
have added that linear extrapolation can continue for oxygen
concentration from 14.5 percent at 40,000 feet to the required
operating altitude. Concerning the use of bulk and bulk average in the
special conditions, we have modified the preamble and special
conditions to consistently use the term ``bulk average'' when referring
to the fuel temperature or oxygen concentration within the fuel tank.
Comment: The commenter requests that the FAA clarify if the FRM is
a safety enhancement system or a safety system. The commenter notes
that in the preamble discussion of the ``Inerting System Indication,''
the FAA states that the applicant may propose master minimum equipment
list (MMEL) relief
[[Page 7808]]
be provided for airplane operation with the FRM unavailable. The
system, however, is considered a safety system that should be
operational to the maximum extent practical. If this system is
considered a safety system, then a form of redundancy will have to be
built in. At this time, the applicant's design does not show any
redundancy.
FAA Reply: The FRM is a safety system designed to provide an
additional layer of protection to the ignition prevention means already
in place. The system by itself is not intended to be fully redundant
since it provides a second layer of protection. The FRM is intended to
be a safety enhancement system that provides an additional layer of
protection by reducing the exposure to flammable vapors in the heated
center wing fuel tank. This protection, when added to ignition
prevention measures, will substantially reduce the likelihood of future
fuel tank explosions in the fleet. The applicant has proposed a 10-day
MMEL relief period, but the Flight Operations Evaluation Board (FOEB)
will determine and approve the appropriate MMEL intervals based on data
the applicant submits to the FAA. The applicant must show that the
fleet average flammability exposure of a tank with an FRM installed is
equal to or less than 3 percent, including any time when the system is
inoperative. No changes were made as a result of these comments.
Comment: One commenter says the cost of the FRM is substantial and
justification for it is debatable. The commenter believes the FRM will
put a heavy economic burden on the slowly recovering airline industry
and only supports the adoption of an FRM on new type designs and newly
built airplanes as an improvement in fuel system safety. This commenter
also says that considering the potential affects of this subject on the
European airline industry, joint European position activity is critical
to ensure that decisions are based on safety grounds and not on
political motivations.
FAA Reply: We do not concur with the commenter regarding the impact
of cost associated with the issuance of the special conditions. These
special conditions are unique to the applicant's certification of an
FRM for the applicable Boeing Model 747 series airplanes and do not
mandate that an FRM must be added to an operator's 747 fleet. They have
been fully harmonized with EASA. The FAA announcement of issuance of a
notice of proposed rulemaking that would propose retrofit and
production incorporation of FRM into U.S-registered airplanes is a
separate rulemaking effort that will require a cost benefit analysis
and will be published for public comment. No changes were made as a
result of this comment.
Comment: One commenter notes that the applicant has planned a 3-
month, in-service evaluation (ISE) of the FRM. It is the opinion of two
other commenters that a 4,000-hour (12 month) ISE should be specified
before certification of the FRM because--
It adds complexity,
It has not yet been retrofitted in an in-service airplane,
It has no proven track record for reliability, and
Ground and flight tests are not sufficient to demonstrate
overall reliability of the system.
The commenters say that maintenance and performance features of the
system were designed to support a 10-day relief under the MMEL program.
If the demonstrated performance and reliability of the system meet
design objectives, then the FAA should support the planned relief.
Another commenter recommends a one-year in-service evaluation (ISE)
program following the first installation of an FRM and prior to FRM
installation on a production airplane. This commenter says that past
experience has shown reliability and system degradation by oil
contamination scenarios, with the engine and APU being the source, and
carbon particle buildup on components similar to those required by the
proposed FRM, due to airport and airplane turbine exhausts. This
commenter believes that one year would be an adequate time for the
manufacturer to develop and provide corrective actions for
discrepancies or reliability issues with the FRM that are identified
during the ISE program.
FAA Reply: We do not concur with the commenters. The industry
commonly conducts ISE through cooperative efforts between the type
certificate holder and the airlines prior to fleetwide introduction of
changes. While the FAA agrees an ISE might be appropriate, we
traditionally do not mandate it. An ISE can be part of a manufacturer's
incorporation strategy for optional equipment. FAA certification of a
system is required before an ISE can be conducted on a U.S.-registered
transport category airplane; therefore, an ISE is not related to
certification requirements. The reliability reporting requirements in
the special conditions will provide data to determine if actions are
needed to correct discrepancies and improve system reliability after
certification of the system. No changes were made as a result of these
comments.
Comment: Three commenters request that the FAA consider 9 percent
as the maximum oxygen concentration at sea level. One commenter
disagrees with the premise that the wing fuel tanks offer an acceptable
minimum level of flammability exposure and is concerned about using
this minimum level for development of inerting systems. The commenters
believe that the maximum oxygen concentration of 12 percent at sea
level should be considered as a level of reduced flammability rather
than inert, and that 9 percent should be used as the long-term goal for
defining a tank as inert. Another commenter states that 12 percent
oxygen concentration will not protect the center or wing fuel tanks
from external hazards and that 9 percent should be used to protect the
tanks. The commenter requests clarification of why 12 percent oxygen
concentration at sea level is specified in the special conditions
instead of the maximum 9 percent.
Three commenters want the minimum oxygen concentration percentage
at sea level to be 10 percent. They refer to paragraph 7(a)(1) of AC
25.981-2, which reads: ``An oxygen concentration of 10 percent or less
by volume is acceptable for transport airplane fuel tanks inerted with
nitrogen, without additional substantiation.'' One commenter believes
this acceptable oxygen concentration establishes a minimum acceptable
performance standard in terms of the threat (ignition source energy),
and 10 percent or less should be the average design concentration for
each fuel cell with no area at a concentration greater than 11.5
percent. Another commenter says that 10 percent contradicts the
definition of ``inert,'' as proposed, and would like the FAA to provide
the acceptable oxygen concentration level (percentage by volume) and
the fundamental justification for this level. Minimum performance
inherent in the AC method must be guaranteed. The final commenter would
like to know if AC 25.981-2 will be revised if the FAA believes that 12
percent is adequate.
Two commenters referenced applying an adequate safety factor to the
maximum 12 percent oxygen concentration limit. One commenter referenced
various reports they believe support the use of a 20 percent safety
margin that should be applied to the FRM. The commenter states that the
FAA uses safety factors in design of aircraft structure, components,
and systems and to deviate from good design practice is not in the
interest of public
[[Page 7809]]
safety. This commenter suggests that the FAA follow industry practice.
FAA Reply: We do not concur with the commenters. The special
condition requirement of 12 percent maximum oxygen concentration at sea
level is based on FAA oxygen content testing and review of other test
data, such as Navy gunfire tests. These data show that 12 percent
oxygen concentration will prevent a fuel tank explosion for airplane
system failure and malfunction-generated ignition sources.
Additionally, data from the Navy testing provided in document NWC TP
7129, ``The Effectiveness of Ullage Nitrogen-Inerting Systems Against
30 mm High-Explosive Incendiary Projectiles,'' dated May 1991, shows
that 12 percent oxygen concentrations are also very effective at
mitigating the effects of a high-energy incendiary projectile
puncturing the fuel tank ullage.
We plan to revise AC 25.981-2 to include the definition of inert
that is used in these special conditions.
Summary
Comment: The commenter refers to the statement in the summary
paragraph that the regulations do not contain adequate or appropriate
safety standards. The commenter considers this statement invalid and
fails to comprehend what is missing in the regulations to adequately
address certification of an FRM and why special conditions would be
required. The commenter agrees with the FAA that the FRM installation
must comply with Sec. 25.981 at amendment 25-102, the fuel vent and
exhaust emission requirements of part 34, and the acoustical
requirements of Sec. 21.93(b). The commenter also believes that
Sec. Sec. 25.831(b), 25.1301, 25.1307, 25.1309, 25.1316, 25.1321,
25.1322, 25.1357, 25.1431, 25.1438, and 25.1461 might also apply.
FAA Reply: Many of the regulations quoted by the commenter are
applicable, and compliance with these requirements must be shown for
certification of the FRM for the applicable Boeing Model 747 series
airplanes. However, part 25 regulations do not contain adequate or
appropriate safety standards for the performance of the FRM. The basis
to issue special conditions is addressed in Sec. 21.16. No changes
were made as a result of this comment.
Background
Comment: This commenter believes ignition source prevention has
failed. The commenter points to the 1997 notice, in which the FAA
requested industry comments on the mitigation of hazards posed by
flammable fuel tank vapors. In that notice, the FAA cites 13 fuel tank
explosion/ignition events and three non-operational events, for a total
of 16 during the 1959-1996 timeframe, before the Thailand B737 center
wing tank explosion. The commenter says that since the ignition sources
for the last three accidents are unknown, an FRM must safeguard against
unknown ignition sources of unknown ignition energy. A significant
number of single failures and combinations of failures can result in
ignition sources within fuel tanks; therefore an acceptable system must
safeguard against all (except extremely improbable) ignition sources
within the fuel tank. The commenter also notes that approximately 550
people lost their lives in these explosions.
FAA Reply: The ignition prevention safety reviews conducted
following the 1996 accident revealed many previously unknown single
component failures that could result in ignition sources within the
fuel tanks. We will issue additional ADs, where necessary, to require
design or maintenance actions to address these newly discovered
deficiencies. The safety reviews also identified combinations of
failures that could result in an ignition source. Because service
experience and analysis indicated that these combinations were less
likely to occur, we determined that it was not practical to address
them in existing airplanes. The safety reviews also confirmed that
unforeseen design and maintenance errors exist and result in
development of ignition sources. As discussed earlier in this document,
the NTSB recommendations included not just preventing ignition sources,
but also reducing fuel tank flammability. The NTSB concluded that ``a
fuel tank design and certification philosophy that relies solely on the
elimination of all ignition sources, while accepting the existence of
fuel tank flammability, is fundamentally flawed because experience has
demonstrated that all possible ignition sources cannot be determined
and reliably eliminated.'' Therefore, the purpose of these special
conditions is not to address additional rulemaking for prevention of
ignition sources but to certificate a specific fuel tank FRM for Boeing
Model 747 series airplanes. No changes were made as a result of this
comment.
Comment: The commenter states that service experience of airplanes
certificated to the earlier standards shows that ignition source
prevention alone has not been totally effective at preventing
accidents. The commenter notes that after the TWA 800 accident, fuel
tank system rulemaking activity started in such an excessive way that
the FAA has mandated over 50 ADs and proposed changes to part 25. After
other fuel tank explosion accidents prior to the flight TWA 800
accident, the FAA did not change the design standards of fuel tank
systems. SFAR 88 was the first real rulemaking activity where the FAA
mandated ignition source reduction throughout the fleet. Those changes
are not incorporated at this time. The commenter therefore believes the
FAA cannot say that the past service experience for ignition source
prevention alone has not been totally effective in preventing
accidents. Currently, the results of ignition source prevention
measures are unknown.
This same commenter also believes that the addition of SFAR 88 and
an FRM will not reduce the chance of maintenance induced errors and may
have an opposite effect in that it could introduce the risk of further
human factors errors.
FAA Reply: We do not concur. Past experience shows that detailed
design reviews, similar to those required by SFAR 88, have not been
effective at eliminating ignition sources. Following an accident in
1976, we conducted an exhaustive investigation and design review of the
lightning protection features of the fuel tank system, including full
scale testing of the wing. From this, we mandated design changes to
improve lightning protection of the system. Subsequent review of the
airplane design required by SFAR 88 revealed the need for additional
bonding modifications that will be mandated. Failure of other
components within the fuel tank system and components adjacent to the
fuel tank could also cause ignition sources. These examples show that
it is very difficult to identify all ignition sources during design.
Additionally, past experience also indicates unforeseen design and
maintenance errors can result in development of ignition sources.
We have issued multiple ADs to address ignition source prevention
and believe that implementation of design changes intended to prevent
ignition sources identified by SFAR 88 will prevent about 50 percent of
future fuel tank explosions. The more significant changes to fuel tank
systems resulting from the SFAR 88 activity include:
Features to prevent dry running of fuel pumps within the
fuel tanks;
Ground fault protection of fuel pump power supplies for
pumps or wires exposed to the fuel tank ullage;
Additional electrical bonds on some components;
Electrical energy limiters on wiring entering fuel tanks
that are normally
[[Page 7810]]
emptied and located within the fuselage contour;
Electrical bond integrity checks; and
Improved maintenance programs.
While we believe these modifications and maintenance program
changes will significantly improve safety, the results of the safety
reviews conducted as part of SFAR 88 show there is uncertainty in the
effectiveness of ignition source prevention alone. The addition of an
FRM will significantly improve fuel tank safety by reducing or
preventing flammable vapors in the fuel tank and will incorporate fail-
safe features into the fuel tank system that account for design and
maintenance errors. No changes were made as a result of these comments.
Fuel Properties
Comment: The commenter says that the new generation airplanes
(B737NG, B757, B767, and B777) are not certified to use Jet B or JP-4
wide-cut fuels. The commenter also points out that AD 85-11-52R1
prohibits the use of Jet B and JP-4 on Boeing Model 737-300 series
airplanes.
FAA Reply: We do not concur. While wide-cut fuels are not commonly
used in the world fleets, some of the airplanes mentioned do allow at
least limited use. Other models are certified for unrestricted use.
Significant use of lower flash-point fuels could affect the percentage
of time the fuel tanks are flammable. Therefore, to achieve consistent
flammability exposure, the flash point of the approved fuels must be
considered in the analysis used for demonstrating compliance. No
changes were made as a result of these comments.
Fire Triangle
Comment: The commenter points to the FAA statement, ``Because of
technological limitations in the past, the FAA philosophy regarding the
prevention of fuel tank explosions to ensure airplane safety was to
only preclude ignition sources within fuel tanks.'' It is the
commenter's opinion that there never was a technological limitation.
The commenter refers to a test the FAA conducted in the 1970s of a
nitrogen fuel tank inerting system on a DC-9 airplane, and that system
maintained oxygen concentration less than 8 percent under all normal
and emergency flight conditions. The commenter also listed other
airplanes that use NEA, liquid nitrogen, and explosion suppressant
systems to minimize fuel tank flammability. The commenter further
points out that in March 2002, the Aviation Rulemaking Advisory
Committee (ARAC) concluded that fuel tank inerting may provide safety
benefits and warrants continued industry and government research. Then,
in December 2002, an on-board nitrogen generator intended to pump the
inert gas into an emptying fuel tank was unveiled. The commenter states
that all of this demonstrates the capabilities of industry.
FAA Reply: While we agree with the commenter that the earlier
systems were available, we do not agree that they were practical for
commercial transport airplanes because of the cost, complexity, weight,
and poor reliability of the systems. The FRM that will be certified for
installation on Boeing Model 747 series airplanes reduces fuel tank
flammability by inerting the tanks with nitrogen using hollow fiber
membrane technology that does not require installation of an air
compressor to produce NEA, thereby reducing cost, complexity, and
weight. As previously discussed, more recent research has found that a
simpler inerting system that reduces the oxygen concentration of the
fuel tank to 12 percent or less at sea level is sufficient in achieving
the desired safety level. No changes were made as a result of these
comments.
Fuel Tank Harmonization Working Group
Comment: The commenter points to several references throughout the
preamble discussion to a flammability exposure of 2 to 4 percent and
requests that this be changed to 5 percent. The commenter says that the
ARAC, in their 1998 report, estimated wing fuel tank exposure as 5
percent. The commenter also points to the reference to 3 percent
flammability value for the wing fuel tanks in the preamble discussion
of ``Definition of Transport Effects'' and requests that this also be
changed to 5 percent.
FAA Reply: We concur in part. Although the ARAC report did identify
a flammability exposure of 2 to 6 percent in the Task Group 8 section,
in other locations of the report a generalized value of 5 percent was
used. In the original discussion in the proposed special conditions, we
incorrectly referenced a range of 2 to 4 percent instead of the actual
value of 2 to 6 percent. We consider the estimated range that was based
on a flammability analysis of a number of different airplane models to
be more representative of the wing fuel tank flammability range across
various airplane models. No changes were made as a result of these
comments.
Comment: The commenter says that the data presented in the
discussion of the Fuel Tank Harmonization Working Group should be for
historical reasons, and the criteria used for determining the need for
an FRM should be AC 25.981-2.
FAA Reply: We do not concur. The purpose of AC 25.981-2 is to
provide guidance for demonstrating compliance with Sec. 25.981(c) to:
Minimize fuel tank flammability; and
Mitigate the hazards if ignition of the fuel vapors
occurs.
The AC does not provide criteria to determine if a system is required
to reduce flammability in fuel tanks.
We infer from the commenter's remarks that they believe these
special conditions will mandate the installation of an FRM, which is
not the case. These special conditions do not represent rulemaking to
mandate the reduction of a fuel tank flammability system. Instead, they
are required to support certification of novel features of the FRM not
addressed by existing regulations, and include additional requirements
to address warm day operations during ground, takeoff, and climb
portions of the flight where previous accidents have occurred. No
changes were made as a result of these comments.
Comment: One commenter considers the flammability range of l5 to 30
percent of fleet operating time for fuel tanks containing flammable
vapors, as documented in the ARAC report, a large range. This range
indicates that the actual percent depends on assumptions. This
commenter believes that a Monte Carlo analysis should not be a part of
the certification process as it is an analysis that is based on flawed
assumptions. The commenter considers use of statistical methods more
consistent with the FAA philosophy for fail-safe designs. The commenter
believes that aviation safety would be undesirably low if a Monte Carlo
analysis was used for the design and certification of navigation and
guidance systems, ground proximity warning systems, weather radar, wind
shear avoidance, engine fire protection, etc. Another commenter also
contends that the assumptions used in the Monte Carlo analysis are not
supported by historical data.
FAA Reply: We do not concur with the first comment. The 15-30
percent addresses the range of average flammability exposures across
the airplane models in the fleet. Specific airplane models will have a
fixed average flammability exposure. We do agree that variations in
assumptions for the analysis could result in large differences in the
results of the
[[Page 7811]]
flammability analysis. For this reason, the special conditions
incorporate specific parameters that must be used when determining fuel
tank flammability. The Monte Carlo methodology has been used in a wide
range of industries to address safety concerns. Previous ARAC
activities recommended use of the Monte Carlo method for calculating
average fuel tank flammability exposure. This methodology has recently
been used by industry to evaluate the flammability exposure of fuel
tanks as part of the SFAR 88 activities. We therefore expect the
applicant as well as industry already have a good understanding of how
to use the model. No changes were made as a result of these comments.
FAA Rulemaking Activity
Comment: The commenter notes that the ARAC recommendations
referenced in this discussion did not use the word ``reduction.'' The
commenter believes that the word ``reduction'' in Sec. 25.981(c) needs
further study. The commenter also says that the 2 to 4 percent
flammability of unheated aluminum wing fuel tanks should not be used as
a criterion in the special conditions, and notes that AC 25.981-2 does
not specifically address the center wing fuel tank like the special
conditions but includes all tanks (including wing tanks).
FAA Reply: We do not concur with the comment concerning the use of
unheated aluminum wing fuel tanks as the criterion for an acceptable
level of fuel tank flammability. AC 25.981-2 does provide clarification
under section 5, paragraph (d)(3), that the intent of Sec. 25.981 is
``to require that the exposure to formation or presence of flammable
vapors is equivalent to that of an unheated wing tank in the transport
airplane being evaluated.'' The special conditions incorporate the
intent of Sec. 25.981(c) and also include additional requirements for
warm day conditions where previous accidents have occurred. The special
conditions also include requirements to address novel design features
that are not covered under the applicable airworthiness standards of
part 25. No changes were made as a result of these comments.
Fuel Tank Inerting
Comment: Two commenters say the applicant's proposed design does
not include an essential verification system (NEA sensors and
indication) to ensure that the appropriate nitrogen concentrations will
be directed into the fuel tank to displace the fuel vapors in the
ullage space. One commenter compares this to the statement in the
discussion of ``Criteria for Inerting'' that the combination of
ignition prevention and reduction of flammable vapors in the tank will
substantially reduce the number of future fuel tank explosions.
FAA Reply: We do not concur. To comply with the special conditions,
the applicant must demonstrate that the FRM meets the specific
performance and reliability requirements. An indication system would be
required if it is shown that the FRM cannot meet these requirements
unless one is installed. No changes were made as a result of these
comments.
Comment: The commenter requests that the reference to ``using the
size difference'' in the first paragraph be changed to ``using the
absorption difference,'' as this would more accurately reflect how
hollow fiber membranes function.
FAA Reply: We concur with the commenter and revised the sentence to
read: ``* * * the hollow fiber membrane material uses the absorption
difference between the nitrogen and oxygen molecules to separate the
NEA from the oxygen.''
Comment: The commenter says that it does not have to be pressurized
air from the airplane engines that is used to produce NEA; compressed
air from any source can be used.
FAA Reply: We agree, however these special conditions address a
specific system design for the applicable Boeing Model 747 series
airplanes using bleed air from the airplane engines to generate NEA. We
recognize there may be other means to achieve the same goal. No changes
were made as a result of this comment.
Comment: The commenter contends that technology has not kept up
with the need to eliminate the need for stored nitrogen because hollow
fiber technology does not produce enough NEA to inert the center tank
during all phases of flight, including descent. Hollow fiber
technology, as described in the special conditions, will not inert the
wing tanks.
FAA Reply: We do not concur. The applicant has selected hollow
fiber technology as a means to produce NEA to inert the center wing
tank on Model 747 series airplanes. The applicant must show that the
FRM will inert the center tank. Hollow fiber technology could be used
to inert wing fuel tanks; however, there is no requirement in the
special conditions to do so. No changes were made as a result of this
comment.
Criteria for Inerting
Comment: The commenter requests that this discussion be revised as
shown below. The commenter says the FAA proposed wording implies that
the 9 percent military and 12 percent commercial oxygen concentration
values are intended to be equivalent. The 9 percent is a military limit
for zero exposure. The 12 percent is a benchmark for evaluating
minimization of flammability exposure, equivalent to wing tanks.
Criteria for Inerting
Earlier fuel tank inerting designs produced for military
applications were based on defining ``inert'' as a maximum oxygen
concentration of 9 percent. One major finding from the research and
development efforts conducted by the FAA was the determination that
the 9 percent maximum oxygen concentration limit established to
protect military airplanes was significantly lower than necessary to
prevent significant pressure rise for the majority of ullage
conditions. This FAA research supports a value of 12 percent as a
benchmark at sea level for determining when the likelihood of
significant pressure rise is low. The test results are currently
available on FAA Web site: http://www.fire.tc.faa.gov, and will be
published in FAA Technical Note `Limiting Oxygen Concentrations
Required to Inert Jet Fuel Vapors Existing at Reduced Fuel Tank
Pressures,' report number DOT/FAA/AR-TN02/79.
It should be noted that the 12% benchmark is not intended to
claim that ignition is impossible below 12%. 14 CFR 25.981 (c)
requires minimization of flammability, not elimination. ARAC
evaluations concluded complete elimination of flammability was
impractical and unnecessary. 14 CFR 25.981(c) was based on reducing
flammability exposure to equal or less than wing tanks, which have
an acceptable safety history. The 12% benchmark is used to divide
exposure time when significant pressure rise is unlikely, from
exposure time when significant pressure rise is more likely. Testing
indicates there is also significant ability to inhibit ignition for
many fuel vapor conditions when oxygen content is above 12%, but no
credit is taken for these conditions.
As a result of this research and the 12 percent benchmark, the
quantity of nitrogen-enriched air that is needed to inert commercial
airplane fuel tanks was reduced. This reduction in nitrogen-enriched
air, coupled with advancements in design technology, facilitates the
development of an effective flammability reduction system that
approaches simple and practical.''
FAA Reply: We do not concur. The 12 percent requirement in the
special conditions is based on testing of flammability using electrical
ignition sources caused by airplane system failures. It is not intended
to address combat threats. However, data from the Navy tests concludes
that inerting to 9 percent oxygen has little benefit over 12 percent
for protection of fuel tanks from overpressure caused by ignition from
30 millimeter Hi energy incendiary rounds.
[[Page 7812]]
No changes were made as a result of this comment.
Type Certification Basis
Comment: The commenter points to two statements concerning
compliance with Sec. 25.981, which appear to be confusing regarding
applicability to the FRM. First, the commenter asks for clarification
as to the extent to which Sec. 25.981 is applied to the system. The
commenter assumes it is only those areas exposed to fuel vapor under
normal operation. The commenter also points to paragraph two of the
``Novel or Unusual Design Features,'' which states that compliance is
required for the changed aspects of the airplane by showing that the
FRM does not introduce any additional potential ignition risk into the
fuel tanks.
FAA Reply: There are two aspects of the FRM concept. First, it is
the means chosen to achieve the requirements of Sec. 25.981(c) to
minimize fuel tank flammability for the applicable 747 series
airplanes. In this case, the applicant chose to introduce NEA into the
center wing tank and assure that it is dispersed throughout. Having
made that choice, the applicant is required to ensure that the changes
introduced by the system (i.e., FRM) do not introduce any potential
ignition sources into the tank. No changes were made as a result of
this comment.
Comment: The commenter says that compliance with Sec. 25.981
applies to certification of fuel tanks and not to the installation of
an inerting system, although fuel tank inerting may be one way to show
compliance with Sec. 25.981(c)(1).
FAA Reply: We do not concur. The applicant has proposed to
voluntarily comply with Sec. 25.981(c), amendment 25-102, for
certification of the performance of an FRM to reduce flammability in
the center wing fuel tanks of Model 747 series airplanes. Additionally,
as stated in the preamble to these special conditions, the applicant
must also ensure that installation of an FRM will meet the ignition
source prevention requirements of Sec. 25.981(a) and (b), as well as
all the other applicable part 25 regulations. No changes were made as a
result of this comment.
Comment: The commenter requests that the 747-Classics effectivity
be removed from the special conditions. The commenter says that few
747-Classics remaining in service may fall within the total 3 percent
exposure criteria, and failing that should pose a far lower risk for
the following reasons:
The majority of ignition reduction modifications (IRM),
including the improved maintenance procedures, will be implemented
prior to any reasonable FRM compliance date;
AD 98-20-40 fuel quantity indicating system protection
upgrade has been fully incorporated on all 747-Classics; and
With the two 737 accidents, it appeared that the center
wing tank (CWT) fuel pumps were inadvertently left running with an
empty CWT, and although it could not be confirmed that the pumps were
at fault, the IRM requirement to automatically (or otherwise) shut
pumps off at low pressure will eliminate this possible ignition source.
There may be an argument that the older airplanes are at a greater risk
and therefore should be FRM protected, but the historical events and
sample in-tank inspections tend to rebuff this proposition.
FAA Reply: We disagree with the commenter that the center wing fuel
tank on 747 Classic airplanes falls within the 3 percent fleet average
flammability exposure criteria because initial flammability exposure
analyses of these airplane models has shown the flammability to be well
above 3 percent. We estimate there are currently about 95 747-100, -
200, and -300 airplanes in service today in the United States. Though
ignition source prevention ADs have been incorporated on these
airplanes and additional ADs will be incorporated as a result of SFAR
88 rulemaking, as we said earlier in this document experience
demonstrates that all possible ignition sources cannot be determined
and reliably eliminated. Reducing or preventing flammable vapors from
forming in high flammability fuel tanks will significantly improve fuel
tank safety. These special conditions support certification of the
applicant's FRM design for possible installation on Boeing Model 747
series airplanes. These special conditions do not mandate any changes
to current airplanes. No changes were made as a result of these
comments.
Novel or Unusual Design Features
Comment: The commenter requests that the phrase ``by showing that
fuel tanks'' in the second paragraph of this discussion be deleted
because the beginning of the sentence establishes the requirement to
comply with Sec. 25.981(a), and (b). The method of compliance is the
applicant's responsibility.
FAA Reply: We do not concur with the commenter. This last phrase
provides a condensed explanation to the reader of what is required for
compliance with Sec. 25.981(a) and (b). No changes were made as a
result of this comment.
Comment: This comment concerns the discussion of how the applicant
proposes to operate the FRM. The commenter says the applicant must be
allowed the freedom to design the system and must ensure that all
features of the FRM are addressed properly so that hazardous conditions
do not occur and the system complies with Sec. Sec. 25.1301 and
25.1309 and other applicable requirements.
Another commenter requests that the system description be replaced
by the following to focus on requirements and not prescribe design:
The proposed FRM uses a nitrogen generation system (NGS). Engine
bleed air will flow through an air separation module (ASM) that will
separate the air stream into nitrogen-enriched air (NEA), which will
be supplied to the center fuel tank, and oxygen-enriched air (OEA),
which will be exhausted overboard. The FRM will also include
modifications to the fuel vent system. Certain features of the FRM
may introduce a hazard to the airplane if not properly addressed.
FAA Reply: We do not concur with the commenters. This section of
the special conditions preamble appropriately defines what the novel or
unusual design features of the FRM are that require special conditions
under Sec. 21.16. No changes were made as a result of these comments.
Comment: This commenter says the special conditions do not
adequately address the descent control valve function as it relates to
the high flow versus low flow mode. The Monte Carlo analysis is not
based on test data or historical data to predict the effectiveness of
the NGS on descent.
FAA Reply: We do not concur. The special conditions require that
the applicant validate the inputs to the Monte Carlo analysis by ground
and flight tests and substantiate that distribution of NEA is effective
at inerting the fuel tank for the performance conditions required. No
changes were made as a result of these comments.
Comment: It is the commenter's opinion that the proposed 10-day
MMEL relief for the system is unjustified. The commenter says all
components are Line Replaceable Units (LRU) that can be replaced within
``typical'' turn around time. A long relief time defeats the purpose of
the system. If limited dispatch relief is granted, then it should be
restricted to conditions (cold temperature) in which development of
flammable vapors in the fuel tank is of low probability. The commenter
points to AC 25.981-2,
[[Page 7813]]
paragraph 4(h), which addresses limited operations based on outside air
temperature.
FAA Reply: The special conditions do not approve an MMEL dispatch
interval. As stated previously, even though the applicant has proposed
a 10-day MMEL dispatch interval, the Flight Operations Evaluation Board
(FOEB) will determine and approve the appropriate MMEL relief intervals
based on data submitted by the applicant. The applicant must show that
the fleet average flammability exposure of a tank with an FRM installed
is equal to or less than 3 percent, including operating time with an
FRM. No changes were made as a result of these comments.
Comment: This commenter says the MMEL procedure is a result of
system design (safety system or not, redundancy, etc.) and reliability
of the system. It is up to the applicant to design their system to
satisfy both the regulations and their customers.
FAA Reply: We concur. The special conditions require the applicant
to submit data that show compliance with the special conditions for
their proposed MMEL dispatch interval. The FOEB will assess the data in
determining if the interval is appropriate. No changes were made as a
result of this comment.
Comment: The commenter contends that the existing technology for
hollow fiber technology presently has a mean time between failure
(MTBF) of less than 2,000 hours, which is different than the 5,000
hours identified in this section.
FAA Reply: To comply with the specific reliability requirements,
the applicant will have to consider the MTBF or life limit of the
hollow fiber technology in their FRM design. The design and compliance
with the special conditions will dictate what the MTBF will be. No
changes were made as a result of this comment.
Discussion
Comment: Three commenters contend that the statement ``* * * due to
factors such as the limited availability of bleed air and electrical
power, it is not considered practical at this time to develop systems
for retrofit * * *'' is not appropriate and is incorrect. One commenter
says this issue would be better addressed in documentation and
discussion rather than this section of the special conditions. The
discussion should be limited to the issues considered and the data
presented in the proposed special conditions. The second commenter says
that on all commercial airplanes during normal operation (all engines
operating and all generators operating), excess bleed-air and
electrical power is available. The last commenter requests removal of
the words ``Since amendment 25-102 was adopted,* * * it is not
considered practical at this time to develop systems for retrofit into
existing airplane designs that can maintain a non-flammable tank ullage
in all fuel tanks or during all operating conditions.'' The commenter
says the wording suggests that a more stringent requirement than that
established by amendment 25-102 has been demonstrated to be practical.
The FAA has not proposed, substantiated, or adopted rulemaking to
support this statement. Changes to the requirements of Sec. 25.981(c)
are not the subject of these special conditions.
FAA Reply: We do not concur with the commenters but believe
clarification is needed to fully understand the context of the
statement that is at issue. As stated earlier, the FAA Administrator
has made public statements concerning our intention to propose
rulemaking that would amend Sec. 25.981(c). During the public process
following issuance of any proposal, comments will be welcome. The
purpose of this statement in the special conditions is to provide
justification for the level of performance required within the
proposal. Although the complexity and sizing of inerting technology has
been reduced such that it is a viable method for reduction of
flammability in fuel tanks, there are still restrictions in existing
airplanes today that would limit an inerting system from being 100
percent effective at inerting the fuel tank during all operating
conditions. No changes were made as a result of these comments.
Comment: One commenter expresses concern that an FRM that complies
with Sec. 25.981(c), amendment 25-102, may not preclude fuel tanks
from routinely being flammable under the specific operating conditions
present when recent accidents occurred. The commenter says that if the
FAA believes the above statement is true, then it has not specified the
right regulations. The commenter believes a repeat of the Philippine,
TWA, or Thai incidents would be prevented by compliance with Sec.
25.981(c).
FAA Reply: The FRM is intended to add an additional layer of safety
for high flammability fuel tanks by reducing the existence of flammable
vapors in the center wing tank. It is important to recognize that this
system does not totally eliminate flammable vapors in the tank during
all operating conditions. The special conditions include requirements
that will address specific risk elements for warm day ground and climb
profiles where accidents have occurred which is a more stringent
requirement than Sec. 25.981(c). The FRM will augment the ignition
source prevention measures in substantially reducing the risk for
future fuel tank explosions. No changes were made as a result of these
comments.
Definition of Inert
Comment: One commenter believes that 12 percent oxygen
concentration at sea level cannot be assured unless the oxygen
percentage within the ullage of the fuel tank is monitored and
measured. The commenter says oxygen monitoring by percentage is needed
to verify if the center wing fuel tank is inert per the definition
supplied in the special conditions, and to determine if the inerting
system is inoperative. The commenter says there is a need to know the
oxygen concentration in the center tank for airplanes operated in
warmer climates. If NEA is lost, the risk factor needs to be accounted
for in the analysis. If it is lost because of a leak surrounding the
NGS, there will be a higher than normal oxygen level in that
compartment. The commenter would encourage further investigation,
testing, and analysis of existing data to support the definition of
inert in all locations and all fuel tanks for the Model 747 series
airplanes and eventually on the Model 737, 757, 767, and 777 airplanes,
as referenced in the ``Novel or Unusual Design Features'' discussion.
Two commenters believe that the level of oxygen concentration
should be monitored at the most critical location in the fuel tank to
verify adequate system operation. One of the commenters believes that
an indication should be generated if the oxygen concentration in the
fuel tank rises above the maximum allowable concentration for greater
than a specified time. This would prevent transient conditions from
generating nuisance indications. The other commenter says that the
system indications should monitor adequate system performance
throughout the flight profile, which is something a periodic ground
check cannot ensure. Besides the obvious safety and reliability
benefits, it is not understood how else the reporting requirements of
special condition III(c) could be met. Although AC 25.981-2 does not
require cockpit indications for an inerting system, this commenter
would support rulemaking intended to revise AC 25.981-2.
Two commenters believe that an indication system that displays the
inerting system functionality should be available to the flightcrew.
Relying solely on preflight or ground crew checks leaves out a valuable
resource for
[[Page 7814]]
monitoring the system status. The flightcrew should be aware if the
system is functioning. If it is not, changes in the flight profile
should be made to ensure the airplane is out of the regime where the
center fuel tank is in the most danger.
FAA Reply: We do not concur with the commenters. There are no
requirements in the special conditions for oxygen concentration
monitoring, but there is nothing that precludes a monitoring system and
associated crew indications from being developed. While monitoring of
oxygen concentrations is one means of determining system performance,
other indications such as pressure measurements, flow measurements,
valve positions etc., as well as periodic functional checks may be used
to provide assurance that the system is functional. The concerns listed
by the commenters are included in the analysis and testing the
applicant must perform to show that the FRM meets the special condition
flammability and reliability requirements. No changes were made as a
result of these comments.
Comment: The commenter requests the word ``localized'' in the
second sentence of the first paragraph in this section be deleted. The
commenter also requests that the rest of the paragraph after the second
sentence (i.e., ``Currently there is * * * be considered inert'') be
deleted. The commenter believes the addition of a requirement to
individually address all tank compartments is not in accordance with
the principles used to date to develop a practical and commercially
viable system that will minimize the average fleet flammability
exposure. It is already conservative to estimate flammability based on
average fuel temperature because the average fuel temperature is
typically higher than the majority of the tank surfaces. This approach
represents the theoretical flammability of a tank where all the tank
surfaces are at this uniform temperature. In reality, when the fuel
temperature is high enough to result in evolution of sufficient vapors
to cause a flammable ullage near the fuel surface, the temperatures of
the sides and top of the fuel tank are cooler, resulting in
condensation that significantly reduces the actual flammability of the
tank ullage.
FAA Reply: We concur, in part, with the commenter. We have revised
the definition of ``flammable'' in the special conditions to read,
``With respect to a fluid or gas, flammable means susceptible to
igniting readily or to exploding (14 CFR part 1, Definitions). A non-
flammable ullage is one where the gas mixture is too lean or too rich
to burn and/or is inert per the definition below.''
We do not concur with the comment that the bulk average fuel
temperature should be used to determine flammability. The ARAC used a
bulk average fuel temperature to provide a comparative flammability
level for various fuel tanks on different airplane models. The ARAC
used a simplified methodology that assumed the fuel tank was one large
volume and that the liquid fuel and fuel vapor in the tank would mix,
forming a uniform mixture. In this case, using the bulk average fuel
temperature would provide a realistic representation of the actual fuel
tank flammability.
This simplified approach, however, does not reflect the actual
design of some fuel tanks. In reality, some fuel tanks have
significantly different flammability exposures within different
compartments of the fuel tank due to barriers installed in the tank, to
prevent sloshing of fuel. These barriers do not allow significant
mixing of the fuel and vapors. For example, some center fuel tanks
extend from the center wing box out into the wing. Other tanks located
in the center wing box have barriers that create separate compartments
within the tank. In these cases, the portion of the fuel tank in the
wing or that exposed to a cold air source may be much cooler and little
mixing within the different portions of the fuel tank would occur. If
the fuel temperature in the part of the tank located in the wing or
other colder section were used in the analysis, the results would not
represent the actual flammability of those portions of the tank where
cooling did not occur. We have therefore modified the special
conditions to revise the discussion in appendix 2 to address those
airplanes that have significantly different flammability exposures
within different compartments of the fuel tank due to the design of the
tank, such as a center fuel tank that extends from the center wing box
out into the wing. For these fuel tanks, the appendix requires
evaluation of the compartment with the highest flammability for each
flight phase. We do not expect that determining which compartment to
evaluate will require a detailed analysis of each compartment. In most
cases, a qualitative assessment, considering ambient temperatures and
other relevant factors will be sufficient.
Determining Flammability
Comment: This commenter says the Monte Carlo analysis should also
consider the center tank theoretically in an unheated condition, not
heated by adjacent equipment.
FAA Reply: We do not concur. The Monte Carlo analysis as used in
these special conditions is specific for determining fuel tank
flammability exposure and certifying an FRM that reduces the
flammability of a specific center wing tank. No changes were made as a
result of this comment.
Comment: This commenter points out that in the second paragraph of
the ``Flammability'' discussion the FAA says ``to quantify the fleet
exposure, the Monte Carlo analysis approach is applied to a
statistically significant number (1,000,000) of flights where each of
the factors described above is randomly selected.'' Table 6 in appendix
2 of the special conditions defines lower flammability limits if the
applicant chooses to use fewer than 1,000,000 flights. The commenter
says the number of runs should be defined as ``when the average results
become stable,'' and the criteria for assessing these results should
then be 3 percent.
FAA Reply: We do not concur. Monte Carlo analyses in general
require the applicant to run a large number of cases for the results to
be accurate. The special conditions contain a method for an applicant
to run fewer cases if they are able to show that they meet the required
3 percent fleet average and 3 percent warm day flammability exposure
limits for the fuel tank under evaluation. No changes were made as a
result of this comment.
Comment: The commenter requests that the following sentence be
added to the end of the last paragraph of the ``Flammability''
discussion: ``However, fresh air drawn into an otherwise inert tank
during descent does not immediately saturate with fuel vapors, and
hence localized concentrations above the inert level during descent do
not represent a hazardous condition.'' This is because fresh air drawn
into the fuel tank through the vent during descent is not flammable,
and will not cause the tank to become flammable during descent. Fresh
air near the vent has not had the time necessary to mix with the bulk
tank ullage, and thus will not be inert. However, the same lack of
mixing time also precludes the presence of a flammable vapor level in
this same region. Counting these non-hazardous periods as ``flammable''
would increase system size, weight, and associated costs with no
benefit.
FAA Reply: We concur and have modified the preamble discussion of
``Determining Flammability'' to add the following sentence: ``However,
fresh air drawn into an otherwise inert tank during descent does not
immediately saturate with fuel vapors; hence, localized concentrations
above the inert
[[Page 7815]]
level during descent do not represent a hazardous condition.''
Definition of Transport Effects
Comment: One commenter says the FAA statement that the effects of
mass loading and the effects of fuel vaporization and condensation with
time and temperature changes have been excluded is flawed, because FAA
documents clearly indicate that ``transport effects'' are important.
Another commenter also believes that the analysis model should include
``transport effects'' as well as flammability effects on heated
unusable (empty, 0 quantity indication) fuel in the center wing tank.
This second commenter says the fuel temperature within a specific
compartment of the tank could be within the flammable range for the
fuel type being used if the tank was empty and heat sources were next
to the compartment.
FAA Reply: We do not concur with the commenters. As stated in the
definition of ``transport effects'' in the special conditions and the
earlier discussion, this term includes two physical phenomena that
affect the concentration of fuel vapor in the fuel tank ullage. The
first is referred to as low fuel conditions or ``mass loading.'' At low
fuel quantities there may be insufficient fuel in the fuel tank at a
given pressure and temperature for the concentration of fuel vapor to
reach the equilibrium level that would form if fuel were added to the
tank.
The second is the change in fuel vapor concentration in the fuel
tank ullage caused by fuel condensation and vaporization. This change
in fuel vapor concentration is caused by temperature variations on the
fuel tank surfaces that result in a vapor concentration different from
the concentration calculated using the bulk average fuel temperature.
We excluded both of these effects because they were not considered
in the original methodology ARAC used to establish the proposed
flammability requirements. If this effect had been included in the wing
tank flammability exposure calculation, it would have resulted in a
significantly lower wing tank flammability exposure benchmark value.
The ARAC analysis also did not consider the effects of the low fuel
condition (or ``mass loading''), which would lower the calculated
flammability exposure value for fuel tanks that are routinely emptied,
such as center wing tanks. As explained earlier, when the amount of
fuel is reduced to very low quantities within a fuel tank, there may be
insufficient fuel in the tank to allow vaporization of fuel to the
concentration that would be predicted for any particular temperature
and pressure.
No changes were made as a result of these comments.
Flammability Limit
Comment: The commenter requests that the reference to ``during
descent'' be changed to ``after high rate descent'' to more accurately
reflect conditions.
FAA Reply: We do not concur. The commenter provided no
substantiation to clarify why they believe the tank would be able to
maintain an inert ullage during descent mode that is not classified as
a high rate of descent. Both the performance of the FRM and the rate of
descent may impact the oxygen concentration level in the fuel tank and
both need to be considered. No changes were made as the result of this
comment.
Comment: The commenter says that the 3 percent exposure criteria,
referenced in this discussion, appears to be premised on the good
service history of main and non-heated reserve fuel tanks. However,
heated center wing tanks (CWTs) make up only a small percentage of the
total number of tanks in use. If the exposure times for non-heated
tanks are summed, it is likely to be close to the total overall
exposure period for heated CWTs. If exposure period were the only
criterion, then one would expect to see non-heated tank incidents. It
is probable that the operating requirements (fuel remaining in tanks)
have as much to do with the good service history as the exposure level.
SFAR 88 Ignition Reduction Modifications will significantly reduce the
ignition risk of the heated CWT to a level where perhaps they are not
quite as safe as the main tanks but on a false premise. If the non-
heated tanks had an average 6 percent exposure, it is unlikely that the
service history would differ. Setting the exposure design criteria to 3
percent or lower may not be as relevant as indicated in these special
conditions, and even a small shift upward could significantly influence
the cost of installation and maintenance. A more important criterion
could be the fact that many CWT components remain uncovered for the
majority of time, with the possibility of an intermittent latent
ignition type defect coming into play when inerting is unavailable.
Therefore, the commenter states it may be more appropriate to consider
additional MMEL limitations to help mitigate whatever is the remaining
exposure risk. This may include ensuring that if CWT components fail,
power is removed and not reapplied until the component is replaced and/
or some fuel is left in the CWT under certain defect conditions. It
should also be noted that it is important to ensure that inerting does
not become a substitute over time for the quick and effective clearance
of CWT defects.
FAA Reply: We agree with the commenter concerning the limitations
of ignition source prevention. Minimization of ignition sources, such
as component failure, removal of power, etc., was the goal of SFAR 88
but it is recognized that absolute elimination of ignition sources is
not possible. Flammability reduction provides a significant improvement
in fuel tank safety in conjunction with ignition source prevention but,
as such, it is important to recognize that this system will not
necessarily eliminate all flammable vapors at all operating conditions.
However, the warm day flammability exposure requirements in these
special conditions would prevent fuel tank flammability during
conditions where the past three fuel tank explosions occurred. By
combining the two approaches, the risks for fuel tank explosions can be
substantially reduced. Compliance with the special conditions will also
ensure that neither the performance nor the reliability of the FRM will
be greater than 1.8 percent of the fleet average flammability exposure,
thereby further minimizing the exposure risk. The MMEL for each
airplane model was reviewed as part of SFAR 88 and limitations on
operations. We do not believe that additional MMEL requirements would
be needed unless the FRM is unable to meet the performance,
reliability, or warm day requirements in the special conditions. No
changes were made as a result of these comments.
Specific Risk Flammability Limit
Comment: The commenter says that because the issue of fuel tank
flammability is primarily one of specific risk, they do not understand
why the Monte Carlo analysis does not include MMEL relief and dispatch
with the FRM inoperative in the evaluation of specific risk against the
requirement of special condition paragraph II (b).
FAA Reply: We did not include the effect of MMEL in special
condition paragraph II (b) because the intent is to address the
performance of the FRM under warm day conditions on the ground, in
takeoff, and in climb, which are high risk. The fleet average
flammability exposure includes the affects of reliability and including
this in the warm day (that is, specific risk) is redundant. No changes
were made as a result of this comment.
Comment: The commenter requests that reference to ``conducting a
separate
[[Page 7816]]
Monte Carlo'' be changed to ``analyzing a subset of the fleet average
Monte Carlo'' to more accurately reflect how the analysis has been
developed.
FAA Reply: We do not agree. The applicant can analyze either a
subset of an overall analysis or conduct a separate Monte Carlo for the
warm day ground, takeoff, and climb cases. The applicant is still
required to run the analysis to meet the fuel tank flammability
exposure limit for the number of simulated flights as shown in Table 6
of appendix 2. No changes were made to the special conditions because
the method has not been limited.
Inerting System Indications
Comment: The commenter says the four elements (when the FRM is
operating and inerts the tank, when the FRM is operating but does not
inert the tank, when the FRM is not operating properly and the operator
is unaware of the failure, and when the FRM is not operating and is on
the MMEL) mentioned in the first paragraph of this discussion should be
included for fleet average flammability exposure. Paragraph II (e) of
the special condition states that ``sufficient accessibility for
maintenance personnel, or the flightcrew, must be provided to FRM
status indications that are necessary to meet the reliability
requirements of paragraph II (a) of these special conditions.'' The way
this special condition is written is unclear and leaves it to the
applicant's opinion of what the ``status indication'' should be. The
commenter would therefore like to see this special condition explicitly
address the four elements mentioned above.
FAA Reply: We do not concur. The special conditions require the
overall FRM reliability to meet a minimum standard and allow the
applicant to optimize the design. The type of indications that would be
required to meet the reliability requirements is design dependent;
therefore, the special conditions do not require specific indications.
No changes were made as a result of this comment.
Comment: This commenter believes it would be cost beneficial and
easier for operators if the look and feel of the FRM indication system
is the same across all fleets. Operators already deal with different
indication design philosophy across different fleets, so the argument
of consistency is not appropriate. Where possible and depending on
cost, a strong consideration should be made to align the FRM indication
with existing indication philosophy. In the case of the 747-400, this
should be by way of an Engine Indication and Crew Alert System (EICAS)
status message.
FAA Reply: We do not concur. As stated earlier, the special
conditions do not dictate a specific design but rather state that
indication and/or maintenance checks will be required to ensure that
the performance and reliability of the FRM meets the special condition
requirements. The look and feel of an indication system is beyond the
scope of these special conditions. No changes were made as a result of
these comments.
Comment: The commenter believes that an FRM requires a redundant
system to address any future foreseeable events and/or conditions.
Consideration should be given to apply the FRM on newly certificated
airplanes, and only where it is feasible to existing airplanes.
FAA Reply: We do not concur. As stated earlier, the FRM is intended
to be a system that provides an additional layer of protection by
reducing the exposure to flammable vapors in the heated center wing
fuel tank. This protection, when added to ignition prevention measures,
will substantially reduce the likelihood of future fuel tank explosions
in the fleet. These special conditions are only applicable to
certification of an FRM for the affected 747 series airplanes for which
an application was received. No changes were made as a result of these
comments.
Comment: The special conditions state that, ``at a minimum, proper
function of essential features of the system should be validated once
per day by maintenance review of indications or functional checks,
possibly prior to the first flight of the day.'' This is a specific
implementation and is taken to be for 747 series airplanes only. If the
special condition material is intended to be used for other projects,
the sentence should be ``proper function of essential features of the
system should be monitored.''
FAA Reply: The special conditions require that the FRM for the
applicable 747 airplanes meet specific performance and reliability
requirements. Various design methods to ensure this may include a
combination of system integrity monitoring and indication, redundancy
of components, and maintenance actions. The initial 747 FRM design
features, as presented to the FAA, would require daily monitoring of
system performance to meet the reliability requirements. Daily checks
may not be needed on all FRM and are only one way of monitoring proper
function of essential system features. Continuous system monitoring by
maintenance computers with associated maintenance messages may also be
used. A combination of maintenance indication or maintenance check
procedures could be used to limit exposure to latent failures within
the system, or high inherent reliability may be used to make sure the
system will meet the fuel tank flammability exposure requirements.
The type of FRM indications and the frequency of checking system
performance (maintenance intervals) must be determined as part of the
FRM fuel tank flammability exposure analysis. These special conditions
will be used as the starting point for developing special conditions
for other airplane models, listed in the preamble, for which the
applicant is considering certification of an FRM. No changes were made
to these special conditions as a result of these comments because they
are applicable to the 747.
Comment: Two commenters question the same discussion in the
preamble, specifically the sentence that reads, ``if system maintenance
indication is not provided for features of the system essential for
proper system operation, system functional checks will be required for
these features. They believe that, at a minimum, proper function of
essential features of the system should be validated once per day by
maintenance review of indications or functional checks, possibly prior
to the first flight of the day.'' The comments indicate the commenter
interpreted the statement to mean that daily checks are required. One
commenter says that accomplishing the functional checks prior to the
first flight of the day is not practical, because maintenance personnel
are not available at all destinations. It could be 2 to 3 days before
the affected airplanes would be at an appropriate location where
maintenance is available. The validation check would better align with
the operators' maintenance programs if the interval were based on
flight hours. The applicant and airplane operators have discussed this
topic at length, and believe that an interval of 75 flight hours would
provide a conservative validation of the system's functionality and
allow the check to be accomplished by qualified maintenance personnel.
The commenters also say there is no historical data to support FRM
validation only once per day. They recommend continuous monitoring.
FAA Reply: As discussed earlier, we concur with the commenters that
the need for daily checks will depend on the FRM design. The preamble
discussion was not intended to mandate daily checks by maintenance
personnel. As noted earlier, the need for system
[[Page 7817]]
functional checks and the interval between the checks will be
established based on the level of ``system maintenance indication
provided for features of the system essential for proper system
operation'' and the reliability of the system. If continual system
monitoring is provided or features of the system have high inherent
reliability, daily checks would not be needed to meet the reliability
requirements in these special conditions. As we stated in the preamble,
the determination of a proper interval and procedure will follow
completion of the certification testing and demonstration of the
system's reliability and performance prior to certification. The time
interval between system health checks and maintenance will be
established by the reliability analysis, any airworthiness limitations,
and the FOEB. We agree with the commenter that providing a design with
continuous system monitoring is desirable; however, we do not agree
that this feature should be required by the special conditions because
it would mandate specific design features and not allow design freedom.
No change was made as a result of these comments.
Comment: Concerning accomplishment of a daily check for proper
function of the FRM, the commenter says past experience has shown that
extended ground time and maintenance induced errors can happen. The
commenter also contends this is contradictory to the statement that,
``determination of a proper interval and procedure will follow
completion of the certification testing * * *.'' The commenter
recommends that the maintenance review board (MRB) procedure, outlined
in AC 121-22, be used to develop the Instructions for Continued
Airworthiness.
FAA Reply: Instructions for Continued Airworthiness are established
as part of certification of the FRM to the performance and reliability
requirements in these special conditions. The MRB procedure, as
outlined in AC 121-22, will be used to define how an MRB will be
conducted. No changes were made as a result of these comments.
Comment: Concerning the MMEL dispatch inoperative interval, four
commenters believe the proposed MMEL interval of 10 days should be
shortened and the FRM be operational to the maximum extent practical.
One commenter says 10 days represents approximately 2.74 percent of a
year, and contends that the FRM components (bleed-air control valve,
ozone converter, heat exchanger, filter, and ASM) can be readily
removed and replaced by a line mechanic during a typical turnaround.
The commenter believes that several of the FRM components can cause
system malfunction (produce low quality NEA) without any indication.
These malfunctions cannot be predicted by analysis or by test. A second
commenter notes that the FAA and industry have adopted a 3-day MMEL
relief interval for other inoperative safety systems, such as flight
data recorders, while another commenter states that catastrophic events
brought about the development of an FRM; therefore, the importance of
such a system is easily seen.
FAA Reply: We do not concur with the commenters regarding setting a
specific MMEL interval in the special conditions. The FOEB process, as
previously discussed, will determine the appropriate MMEL dispatch
interval. No changes were made as a result of these comments.
Comment: One commenter believes that if the reliability analysis
shows that a 10-day MMEL will allow the overall fleet flammability
exposure limit to meet the requirements listed in the special
conditions, then the 10-day MMEL should be acceptable. A second
commenter requests clarification that the MMEL relief will be
determined using standard methods, and that the reference to warm
climates in the last paragraph of this section refers to inclusion in
the Monte Carlo analysis and not to a limitation in the MMEL specific
to warm ambient temperatures.
FAA Reply: The standard processes (FOEB review), as discussed
above, will be used to determine the appropriate MMEL dispatch
interval. These same processes may also determine if a limitation is
needed in the MMEL for warm day operation based on the results of the
analysis. No changes were made as a result of these comments.
Comment: The commenter says that if the FRM is inoperative, there
might be some conditions in which the percentage of oxygen
concentration is as high as 30 percent while the airplane is in the
climb flight profile. An operational consideration might be to transfer
fuel into the center tank or to carry extra fuel in that tank until
level cruise is attained. This procedure addresses the internal energy
sources discussed in current advisory circulars. The commenter contends
that whether or not the FRM is in low or high flow mode, it cannot keep
up with the need due to pressure and temperature changes and out-
gassing of the fuel.
FAA Reply: We do not concur. The special conditions require that
the flammability analysis take into account any periods where the FRM
is inoperative or does not have the capacity to maintain a non-
flammable fuel tank ullage. We agree with the commenter that out-
gassing of dissolved air in the fuel may affect the oxygen
concentration in the fuel tank during certain flights. These special
conditions require that this factor be considered when determining the
portion of the flammability exposure evaluation time (FEET) when the
FRM cannot maintain a non-flammable ullage. This portion of the fleet
average flammability exposure is limited to 1.8 percent. The special
condition requirements are intended to provide an additional layer of
protection to the existing certification standards that require designs
to preclude fuel tank ignition sources. This balanced risk management
approach of precluding ignition sources and reducing flammability
exposure in certain fuel tanks provides two independent layers for
preventing fuel tank explosions in those tanks. No changes were made as
a result of these comments.
Comment: The commenter requests that the entire discussion of
``Inerting System Indications'' be reworded. It is the commenter's
position that the special conditions should establish the certification
requirements not already established by existing part 25 requirements.
The commenter says that the reliability requirement for the FRM is
clearly established in paragraph II (a)(2) of the special conditions as
to not contribute more than 1.8% overall fleet flammability exposure.
The commenter believes the required inspections and associated
inspection intervals should be developed by the applicant in support of
complying with the 1.8% limit. The applicant should have the
flexibility to design a system that has high reliability (at higher
equipment cost) with fewer inspections required, or lower reliability
and higher frequency of inspection with less time allowed for MMEL
dispatch. The commenter also believes that this is consistent with
Sec. 25.981(c), amendment 25-102, where it specifically states that
``minimize'' means to incorporate practicable design methods to reduce
the likelihood of flammable vapors.
FAA Reply: We do not concur. The special conditions do provide the
applicant with flexibility to design the FRM either to higher
reliability and longer inspection intervals or lower reliability with
more frequent inspections, as long as the contributions for either
performance of the system or its reliability are not greater than 1.8
percent of the total 3 percent fleet average flammability exposure. The
approved maintenance procedures and
[[Page 7818]]
intervals established by the FOEB will be based on the applicant's
fleet average flammability exposure data submitted to the FAA. No
changes were made as a result of these comments.
Affect of FRM on Auxiliary Fuel Tank System Supplemental Type
Certificates
Comment: The commenter believes the applicant should validate, as
part of the certification effort, that the performance and reliability
requirements for the FRM are met for any approved combination of
auxiliary fuel tank installations. The commenter does not understand
how installation of an FRM on an airplane with auxiliary fuel tanks can
be adequately assessed ``during development and approval of the service
bulletin for the FRM.''
FAA Reply: We concur and have added a requirement in special
condition II (a)(3) for the applicant to ``identify critical features
of the fuel tank system to prevent an auxiliary fuel tank installation
from increasing the flammability exposure of the center wing tank above
that permitted under paragraph II (a)(1) and (2) and to prevent
degradation of the performance and reliability of the FRM.'' We have
also added a requirement under paragraph III (a)(3) to establish
airworthiness limitations to address these features.
Disposal of Oxygen-Enriched Air
Comment: One commenter refers to the statement, ``the OEA produced
in the proposed design is diluted with air from a heat exchanger, which
is intended to reduce the OEA concentration to non-hazardous levels.''
The commenter says that although this is a particular solution to the
hazard, it should not be seen as the only solution. The term
``hazardous'' is open to interpretation; thus, this discussion is
considered as too design specific.
FAA Reply: We agree with the commenter that there are a number of
different means of addressing any hazards associated with the OEA.
These special conditions are applicable to the applicant's proposal for
certification of their FRM design. The description of the particular
design feature noted by the commenter was not intended to limit other
means of compliance should another applicant propose an FRM. We will
evaluate each FRM based on the proposed design. No changes were made as
a result of these comments.
Comment: The commenter requests that the first paragraph of this
discussion be replaced with the following: ``The FRM produces both
nitrogen-enriched air (NEA) and oxygen-enriched air (OEA). The OEA
generated by the FRM could result in a fire hazard if not disposed of
properly. Compliance with existing requirements of Sec. 25.863 are
sufficient to address potential leakage of OEA due to failures and safe
disposal of the OEA during normal operation.'' The commenter requests
this change to make OEA leakage compliance requirements consistent with
those applicable for other flammable leakage zone items.
FAA Reply: We concur with the commenter that certification of the
FRM will require the applicant to evaluate installation of equipment in
a flammable fluid leakage zone for compliance with Sec. 25.863.
However, compliance with Sec. 25.901 is required to ensure that no
single failure or malfunction, or probable combination of failures,
will jeopardize the safe operation of the airplane. Depending on where
the OEA is discharged, other part 25 regulations might apply. No
changes were made as a result of these comments.
Applicability
Comment: The commenter notes that the airplane applicability is not
consistent. Furthermore, the commenter says Sec. 25.981(c), amendment
25-102, is only applicable to new type designs, and therefore these
special conditions should apply to new type designs and may extend to
newly built airplanes. If the special conditions were proposed for
other Boeing Model airplanes (737, 777, etc.), the commenter believes
the standards established for the 747 airplanes should also be
applicable for these models.
FAA Reply: We concur with the commenter that the airplane
applicability was inconsistent in certain sections of the proposed
special conditions in that these sections excluded the 747-100B and
747-300 series airplanes. We have corrected the applicable sections of
the final special conditions to show the applicability as Boeing Model
747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F series
airplanes. The applicant has voluntarily proposed to show compliance
with amendment 25-102 plus the additional requirements of the special
conditions for an inerting system for the affected Boeing Model 747
series airplanes. As stated earlier, these special conditions will be
the baseline for the other airplane models for which the applicant
plans to seek approval of an FRM. No changes were made as a result of
this comment.
Special Conditions
I. Definitions
Comment: The commenter requests the definition for flammable be
revised to read as follows:
Flammable. With respect to a fluid or gas, flammable means
susceptible to igniting readily or to exploding (14 CFR Part 1,
Definitions). A non-flammable ullage is one where the gas mixture is
too lean or too rich to burn and/or is inert per the definition of
inert below. For the purposes of these special conditions, a fuel
tank is considered flammable when the ullage is not inert and the
fuel vapor concentration is within the flammable range for the fuel
type being used. The fuel vapor concentration of the ullage in a
fuel tank shall be determined based on the average fuel temperature
within the tank. This vapor concentration shall be assumed to exist
throughout all bays of the tank. An exception to this shall be
utilized when one or more major portion of the tank is exposed to
grossly dissimilar heating conditions. In this situation, the vapor
concentration of this major portion shall be determined
independently based upon the fuel temperature of this portion.
The commenter requests this change because the wording, as proposed
in the notice, is inconsistent with the modeling methods required in
appendix 2 of the special conditions. The development of the concept of
assessing average fleet flammability exposure using a Monte Carlo
analysis was based on the use of an average bulk fuel temperature of
the entire center wing fuel tank. This is the parameter that was
defined in conjunction with the conclusion that achieving a 3 percent
average fleet flammability exposure criteria would be considered
equivalent to providing similar characteristics to the type
certificated model's unheated aluminum wing tanks when the same fuel is
used in the calculation, as required by Sec. 25.981(c). None of the
Monte Carlo analytical modeling to date by the FAA, the two ARAC
studies, or the Boeing Company have been based on individual tank
compartment fuel temperatures. Each of these analyses has been based on
the average temperature of the fuel and applying the flammability
exposure based on that fuel temperature to all bays. The commenter
references FAA Report DOT/FAA/AR-TN99/65 for supporting test data.
FAA Reply: We concur, in part, with the commenter. As stated
earlier, we have modified the definition of flammable to ``With respect
to a fluid or gas, flammable means susceptible to igniting readily or
to exploding (14 CFR part 1, Definitions). A non-flammable ullage is
one where the gas mixture is
[[Page 7819]]
too lean or too rich to burn and/or is inert per the definition of
inert below.''
To ensure that flammability of individual bays is accounted for in
the Monte Carlo analysis, we have added clarification in appendix 2
that reads:
For the purposes of these special conditions, a fuel tank is
considered flammable when the ullage is not inert and the fuel vapor
concentration is within the flammable range for the fuel type being
used. The fuel vapor concentration of the ullage in a fuel tank
shall be determined based on the bulk average fuel temperature
within the tank. This vapor concentration must be assumed to exist
throughout all bays of the tank. For those airplanes with fuel tanks
having different flammability exposure within different compartments
of the tank, the flammability of the compartments must be analyzed
individually in the Monte Carlo analysis. The highest flammability
exposure must be used in the analysis. For example, the center wing
fuel tank in some designs extends into the wing and has portions of
the tank that are cooled by outside air, and other portions of the
tank that are insulated from outside air. Therefore, the fuel
temperature is different than the portion of the fuel tank in the
wing.
Comment: One commenter says use of the term ``employee'' in the
definition for ``hazardous atmosphere'' is questionable. The commenter
considers it more appropriate to extend the definition to cover the
risk to maintenance personnel, passengers, flightcrew, etc.
FAA Reply: We concur with the commenter and have revised the
definition of ``hazardous atmosphere'' to address any person(s).
Comment: A commenter requests clarification of the definition of
inert (what is the percentage at sea level to meet the 12 percent or
less oxygen limit at 10,000 feet?). The commenter also asks if the NEA
supply can keep up with demand through 10,000 feet. The commenter says
the altitude should be 15,000 feet because TWA 800 exploded at 13,500
feet. The commenter also says there is conjecture that the oxygen
concentration in the fuel tank ullage will have to be less than 10
percent at sea level to keep the oxygen level below 12 percent at
10,000 feet.
FAA Reply: We do not concur. The definition of inert is based on
FAA testing as explained previously. No changes were made as a result
of these comments.
Comment: In reference to the definition of a Monte Carlo analysis,
the commenter notes that the FAA used the ARAC analysis in the model as
the means of compliance with the special conditions. The commenter says
this analysis did not include transport effects, which they believe
should be included, as well as flammability effects on center wing tank
heated unusable (empty, 0 quantity indication) fuel. They say the fuel
temperature within a specific compartment of the tank could be within
the flammable range for the fuel type being used if the tank was empty
and heat sources were next to the compartment.
FAA Reply: We do not concur. As explained earlier, we excluded both
of the phenomena (mass loading and fuel vaporization and condensation)
that are part of the definition of transport effects, because they were
not considered by ARAC when they established the flammability
requirements. If they had included these effects in the wing tank
flammability exposure calculation, the wing tank flammability exposure
benchmark value would have been significantly lower, which could result
in more restrictive requirements for center wing tank flammability
exposure. No changes were made as a result of these comments.
Comment: Two commenters request clarification of the definition of
operational time. One commenter proposes the definition be revised to
read as follows for consistency with AC 25.981-2 and the Monte Carlo
analysis: This commenter says the current definition would not result
in a clearly defined number of flights per day for use in the Monte
Carlo analysis and would basically define the daily operational time as
one continuous period of time.
``Operational Time. For the purpose of these special conditions,
the time from the start of preparing the airplane for flight (that
is, starting and connecting the auxiliary or ground power unit to
the aircraft electrical system) through the actual flight and
landing, and through the time to disembark any payload, passengers
and crew.''
FAA Reply: We concur in part. Because the definition of operational
time in these special conditions is not consistent with the definition
in 14 CFR part 1, Definitions, we have replaced ``operational time''
with the term ``flammability exposure evaluation time (FEET).'' We have
revised the definition to read as follows:
Flammability Exposure Evaluation Time (FEET). For the purpose of
these special conditions, the time from the start of preparing the
airplane for flight, through the flight and landing, until all
payload is unloaded and all passengers and crew have disembarked. In
the Monte Carlo program, the flight time is randomly selected from
the Mission Range Distribution (Table 3), the pre-flight times are
provided as a function of the flight time, and the post-flight time
is a constant 30 minutes.
Comment: This commenter believes additional definitions need to be
added such as operational time, fleet average, etc., for clarification.
FAA Reply: We concur in part. The definition of operational time is
already addressed in Special Condition I. Definitions, and we have
added additional definitions for clarification as needed.
II. System Performance and Reliability
Comment: Several commenters request clarification of paragraph II
(a)(2). One commenter assumes that the FRM can be non-operational for
1.8 percent of the airplane operational life. This commenter says
elsewhere in the special conditions more stringent requirements are
implied (for example ``shortest practical MMEL relief''), which is
inconsistent. The commenter considers the 1.8 percent requirement to be
sufficient. Another commenter requests explanation of the percentage
figures quoted in paragraphs II (a), (b), and (c).
FAA Reply: The 1.8 percent maximum contribution requirement for an
inoperative FRM is for an airplane fleet, not an individual airplane.
The special conditions limit the maximum fleet average flammability
exposure to 3 percent. The performance or reliability contributions can
be up to 1.8 percent, as long as the overall fleet average flammability
exposure does not exceed a total of 3 percent. The contribution for FRM
performance would be limited to 1.2 percent if the reliability
contribution were 1.8 percent. The 3 percent warm day requirement is a
separate performance requirement that must be met for warm day ground,
takeoff, and climb flight profiles and therefore does not include the
contribution for reliability of the system. All of these requirements
establish the minimum safety standards. No changes were made as a
result of these comments.
Comment: The commenter refers to the statement in paragraph II (c)
that ``the applicant must provide data from ground testing and flight
testing'' to show compliance with paragraphs II (a), (b), and (c)(2).
The commenter believes that the means of compliance should be left to
the applicant. The paragraph should therefore read, ``The applicant
must provide appropriate data * * *''
Comment: Another commenter also requests a change to paragraph
II(c). This commenter suggests the following: ``The applicant must
provide data from analysis and/or testing.'' The commenter says use of
analysis and/or testing is consistent with normal processes used to
demonstrate compliance with part 25 requirements.
[[Page 7820]]
FAA Reply: We do not concur with the commenters. The wording of the
special condition is consistent with other regulations where test data
is needed to demonstrate compliance. Analysis alone is not considered
adequate for demonstrating compliance with the special condition
requirements because with this new technology there is not a sufficient
experience base from which to derive a reliable analysis. No changes
were made as a result of these comments.
Comment: One commenter requests clarification why paragraph II (c)
has been included in the requirements listed under paragraphs II
(c)(1), II (d), and III (a).
FAA Reply: We infer from the comment that the reference to
paragraph II (c) should be removed from paragraphs II (c)(1), II (d),
and III (a) and we concur. We have therefore revised the special
conditions to change the reference in the noted paragraphs to paragraph
II (c)(2).
Comment: The commenter requests that the four elements involved
with the fleet average flammability exposure, as referenced in
``Inerting System Indications,'' be included in paragraph II (e).
FAA Reply: We do not concur. The special conditions do not dictate
a specific design, but rather state that indication and/or maintenance
checks will be required to ensure that the performance and reliability
of the FRM meets the special conditions requirements. No changes were
made as a result of this comment.
Comment: The commenter recommends that paragraph II (f) be expanded
to state that appropriate markings are required for all inerted fuel
tanks, tanks adjacent to inerted fuel tanks, and all fuel tanks
communicating with the inerted tanks via plumbing. The plumbing
includes, but is not limited to, vent system, fuel feed system, refuel
system, transfer system and cross-feed system plumbing. NEA could enter
adjacent fuel tanks via structural leaks. It could also enter other
fuel tanks through plumbing, if valves are operated or fail in the open
position. The hazardous markings should also be stenciled on the
external upper and lower surfaces of the inerted tank to ensure
maintenance personnel are aware of the possible contents of the fuel
tank.
FAA Reply: We concur in part. We revised paragraph II (f) to
clarify that any fuel tank with an FRM must be marked as required, as
well as any confined spaces or enclosed areas that could contain NEA
under normal conditions or failure conditions. The special condition
already requires the applicant to mark access doors and panels to any
fuel tank that communicates with an inerted tank.
Comment: Two commenters say that in paragraph II (g) it is not
clear which ``normal'' operating conditions the FAA is referring to,
and if this requirement is intended to address any FRM failures, or
only hazards related to the oxygen-enriched air. Both consider the
criteria specified in this paragraph to be inadequate. One commenter
says the FRM installation must be shown to comply with the safety
requirements of Sec. 25.1309 (demonstrate that an inverse relationship
exists between the probability of an event, failure condition, and its
severity). The second commenter requests that paragraph II (g) be
revised to read: ``Oxygen-enriched air produced by the nitrogen
generation system must not create a hazard during all FRS operating
conditions and it must be established that no single failure or
malfunction or probable combination of failures will jeopardize the
safe operation of the airplane.''
Comment: Another commenter requests paragraph II (g) be revised to
read: ``Oxygen-enriched air produced by the nitrogen generation system
must not create a hazard during normal operating conditions (refer to
14 CFR 25.863).'' The commenter requests this change to make OEA
leakage compliance requirements consistent with those applicable for
other flammable leakage zone items.
FAA Reply: We concur, in part, with the commenters. The intent of
this requirement is to address any hazards associated with both normal
operating and failure conditions and not just when the FRM is
operating. This intent was not clear in the original proposal. We have
revised paragraph II (g) to state that, ``Any FRM failures, or failures
that could affect the FRM, with potential catastrophic consequences
must not result from a single failure or a combination of failures not
shown to be extremely improbable.'' Note that approval of the FRM
design will require the applicant to evaluate installation of equipment
in a flammable fluid leakage zone for compliance with Sec. 25.863.
However, compliance with the existing general requirements of Sec.
25.901 is required to ensure that no single failure or malfunction or
probable combination of failures will jeopardize the safe operation of
the airplane.
III. Maintenance
Comment: The commenter requests paragraph III (a) be changed to:
``Maintenance and/or inspection tasks needed to identify items without
failure indication, so that FRM reliability does not fall below the
values assumed in the Monte-Carlo analysis, must be identified as
Airworthiness Limitations.'' The requirement to identify Airworthiness
Limitations for all maintenance and/or inspection tasks is
unprecedented in part 25 certification and would impose an unjustified
burden on operators. The application of this special condition wording
to other parts of the fuel system would, in essence, require an
Airworthiness Limitation to inspect the flight deck lights for basic
indications such as pump low pressure lights and status messages. It is
the commenter's position that identifying Airworthiness Limitations
only for items without failure indication will ensure that the desired
inspections to identify latent failures are accomplished, without an
impractical burden on the operators.
FAA Reply: We concur, in part, with the commenter. Paragraph III
(a) is not intended to apply to all maintenance and/or inspection
tasks, just those necessary to identify failures related to FRM
performance and reliability requirements. No changes were made as a
result of these comments.
Comment: The commenter requests that paragraph III(c)(1) be changed
to: ``Develop and introduce an event monitoring and reporting system
acceptable to the primary certification authority.'' The commenter
requests this change because the proposed requirement to track
inoperative time would result in the introduction of new recordkeeping
processes, which, in turn, will result in a significant increase in the
maintenance and operational burden on the operators. The commenter
accepts that the FRM system reliability should be initially monitored,
but the requirement should allow the flexibility for existing operator
and reliability reporting systems to be used to evaluate actual in-
service system reliability, at practical costs.
FAA Reply: We do not concur. We believe the applicant will be able
to gather the required data from operators using existing reporting
systems that are currently in use for airplane maintenance,
reliability, and warranty claims. We anticipate the operators would
provide this information to the applicant through existing business
arrangements. No changes were made as a result of these comments.
Comment: One commenter believes initiation of component and/or
system modification should also be included in paragraph III (c)(4) for
correcting failures of the FRM that increase the fleet flammability
exposure. Another commenter says paragraph III (c)(4) is not clear as
to whether this statement
[[Page 7821]]
refers to the 3 percent flammability requirement of paragraph II (a) or
II (b), or both. This commenter believes paragraph III (c)(4) should
specifically address the requirements of both paragraphs II (a) and II
(b) of the special conditions.
FAA Reply: We concur with the commenters that paragraph III (c)(4)
needs clarification. We have revised this paragraph to read: ``Develop
service instructions or revise the applicable airplane manual, per a
schedule agreed to by the FAA, to correct any failures of the FRM that
occur in service that could increase the fleet average or warm day
flammability exposure of the tank to more than the exposure
requirements of paragraphs II (a) and II (b) of these special
conditions.''
Comment: The commenter requests that an additional requirement be
added that would instruct an applicant to provide training material to
the industry to incorporate any new design system. This would include
any specific dangers and safety factors. The amendment of all technical
documentation, including Airplane Maintenance Manual (AMM), Airplane
Flight Manual (AFM), etc., is not enough.
FAA Reply: We do not concur with the commenter. The applicant must
provide service bulletins that will instruct the operators how to
properly install an FRM, which should include any specific dangers or
safety factors that need to be considered during installation. The
applicant is also responsible for providing any materials necessary to
ensure an operator knows how to properly operate and maintain the
system. Training is outside the scope of these special conditions. No
changes were made as a result of this comment.
Appendix 1: Monte Carlo Analysis
Comment: The commenter requests the following note be added to
paragraph (b)(3): ``Note: localized concentrations above the inert
level are allowed provided the volume of the non-inert region would not
produce a hazardous condition.'' The commenter says the fresh air drawn
into the fuel tank through the vent during descent will not be
flammable and will not cause the tank to become flammable during
descent. The commenter believes that counting these non-hazardous
periods as ``flammable'' would increase the system size, weight, and
associated costs with no benefit.
FAA Reply: We agree that a note paragraph would be appropriate and
have added the following to paragraph (b)(3): ``Note: localized
concentrations above the inert level as a result of fresh air that is
drawn into the fuel tank through vents during descent would not be
considered as flammable.''
Comment: The commenter requests the following change to paragraph
(b)(5): ``Proposed MMEL/MEL dispatch periods including action to be
taken when dispatching with the FRM inoperative.'' The commenter says
the MMEL process is outside the scope of the special conditions. The
specific MMEL time should be based on fleet data for similar systems,
not a prescriptive mandate of 60 hours. The actual inoperative MMEL
interval and corresponding fleet exposure used in the Monte Carlo
analysis is one of a number of items whose inoperative interval would
be substantiated as part of achieving part 25 certification. During any
part 25 certification project, providing acceptable substantiating data
to the FAA for assumptions and analytical processes is the
responsibility of the applicant.
FAA Reply: The establishment of an MMEL dispatch interval will be
achieved through the certification process, whereby the Flight
Operations Evaluation Board (FOEB) will review the applicable data
submitted by the applicant to determine if the proposed dispatch
interval is appropriate. However, the special conditions include the
requirement in appendix 1, paragraph (b)(5), to allow the applicant to
use an inoperative FRM interval that is shorter than the maximum
proposed interval of ten days, if they can substantiate that the 3
percent flammability requirement can be met when operating with an
inoperative FRM. Otherwise, 60 flight hours must be used in the
analysis for a proposed 10-day MMEL dispatch interval. No changes were
made as a result of these comments.
Comment: The commenter contends that in paragraph (b)(5) it should
be noted that the assumed 60 flight hours for a 10-day MMEL is the
``average'' MMEL/MEL dispatch inoperative period.
FAA Reply: We recognize that not all MMEL inoperative periods will
typically occupy the full allowed MMEL dispatch interval. To account
for this, the special conditions require an average 60 flight hours to
be used in the Monte Carlo analysis for a 10-day MMEL dispatch
interval. This is based on using an average airplane utilization of 12
hours per day, and an average of one-half the proposed 10-day MMEL
dispatch interval. No changes were made as a result of this comment.
Appendix 2: Atmosphere
Comment: The commenter says that oxygen monitoring would eliminate
the need to compute the transitional temperature, as required in this
section of appendix 2. This is because the oxygen monitoring system
measures the temperature in the tanks and uses that temperature in the
calculations to determine the oxygen percentage present.
FAA Reply: From the comment, we infer that the commenter is
questioning why a temperature needs to be calculated for the Monte
Carlo analysis when an oxygen sensor can be used to measure temperature
in the fuel tank. Modeling the atmosphere during climb and descent
using the tables in appendix 2 is required to determine the
flammability exposure for use in the Monte Carlo analysis. It is not
related to possible design features such as an oxygen sensor. No
changes were made as a result of this comment.
Comment: The commenter would like to know who would make the
decision regarding the use of lower flash point fuels for more than
1percent of the fleet operating time. The commenter asks how this
determination will be made to apply to a particular airplane flown with
a particular defined flight profile. Another commenter believes there
should be allowance for factoring in a higher flash point for fuels if
used for more than 1 percent of the fleet operating time.
Comment: A third commenter requests that the 3rd and 4th sentences
in paragraph three of the ``Atmosphere'' discussion be changed to:
Table 2 is based on typical use of Jet A type fuel, with limited
TS-1 use. If an airplane fleet is expected to operate with low flash
point fuels (such as JP-4) more than 1 percent of its operating
time, or intermediate flash point fuels (such as TS-1) more than 10
percent of the fleet operating time, then the Monte Carlo analysis
must include fuel property variation acceptable to the FAA for these
approved fuels.
The commenter believes this change clarifies that some TS-1 fuel is
already included in the Table 2 distribution, and adds a separate usage
limit for low and intermediate flash point fuel that would require
development of new worldwide fuel type studies only if exceeded.
Currently, there are no data available to use for a statistical
distribution of non Jet-A type fuels and it is unreasonable to expect
an applicant to provide a Monte Carlo analysis incorporating a
flammability exposure dataset for these other fuels where the
appropriate data is not available. The impact on the flammability
analysis of
[[Page 7822]]
up to 10 percent use of intermediate flash point fuels would be small;
therefore, the study is not justified unless it is expected that the
use of these fuels would exceed 10 percent.
FAA Reply: We agree, in part, with the commenters. The fuel
properties tables in appendix 2 of the special conditions include a
distribution of flash points reflecting an FAA survey of jet fuels used
in both U.S. domestic and international routes. The tables therefore
include an allowance for use of lower flash points fuels. The intent of
the Monte Carlo analysis method is to provide a standardized analysis
method to compare the flammability of the fuel tank under evaluation to
the established flammability limits. The flammability limits were
established based on a Monte Carlo analysis using the flash point table
in these special conditions. To simplify the standardized analysis, we
have deleted the need to consider other fuel flash point distributions
from these special conditions.
Appendix 2: Oxygen Evolution
Comment: The commenter asks, if 12 percent or less oxygen
percentage is tolerable at 10,000 feet (as opposed to 20.9 at sea level
before NEA is available to the fuel tank), what oxygen concentration is
needed on the ground at departure if the FRM is not fully effective
immediately after engine start? Can the available NEA high flow rate
keep up with the possible out gassing of the 30 percent oxygen level in
the fuel in order to be at an oxygen level of 12 percent or less at
10,000 feet?
FAA Reply: The flammability requirements in the special conditions
will limit the maximum oxygen concentration. We expect that if the FRM
were not designed so that the oxygen concentration of the center wing
fuel tank ullage is below 12 percent at sea level, it would not meet
these requirements. It is also not possible to meet the specific risk
requirements in the special conditions for warm day operations if the
FRM does not reduce the oxygen concentration level below 12 percent
during ground operations. The affects of oxygen evolution during climb
must be accounted for in the analysis required by these special
conditions. These special conditions do not preclude exceeding the 12
percent oxygen concentrations during transient conditions. For example,
the tank may no longer be inert during a high descent rate or during a
rapid climb where the tank could be above the 12 percent oxygen level
for short periods of time. As previously discussed, we do not believe
it is practical to require an FRM that would inert the fuel tank during
all operational conditions within the airplane operating envelope. No
changes were made as a result of these comments.
Comment: The commenter says the last sentence of this discussion
should read, ``The applicant must provide the assumptions relating to
air evolution rate'' because provision of substantiated data would not
be possible due to the uncertain manner in which air evolves from the
fuel during climb.
FAA Reply: We agree with the commenter that air evolution rates are
uncertain and can vary from flight to flight depending on the fuel load
and the conditions under which the fuel was loaded. However, we do not
agree that it will not be possible to provide data to substantiate the
air evolution rate for the center wing fuel tank. The FAA has not seen
large transients related to air evolution during airplane model testing
(FAA Report No. DOT/FAA/AR-01/63, ``Ground and Flight Testing of a
Boeing 737 Center Wing Fuel Tank Inerted With Nitrogen-Enriched Air.''
We would expect air evolution rates determined by flight testing with
typical fuel loading to be representative of those anticipated in
service, so this data should be sufficient to address the effects of
air evolution on oxygen concentrations. No changes were made as a
result of this comment.
Other
In addition to the changes to the special conditions in response to
comments, we made some changes to provide additional clarification in
certain areas. Because those changes do not change the intent of the
special conditions, they are not included in the discussion of
comments.
Applicability
As discussed above, these special conditions are applicable to the
Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/
400F series airplanes. Should the type certificate be amended later to
include any other model that incorporates the same or similar novel or
unusual design feature, or should any other model already included on
the same type certificate be modified to incorporate the same or
similar novel or unusual design feature, the special conditions would
also apply to the other model under the provisions of Sec. 21.101.
Conclusion
This action affects only certain novel or unusual design features
on Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/
400D/400F series airplanes. It is not a rule of general applicability
and affects only the applicant who applied to the FAA for approval of
these features on the airplane.
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping
requirements.
0
The authority citation for these special conditions is as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.
The Special Conditions
0
Accordingly, pursuant to the authority delegated to me by the
Administrator, the following special conditions are issued as part of
the type certification basis for Boeing Model 747-100/200B/200F/200C/
SR/SP/100B/300/100B SUD/400/400D/400F series airplanes, modified by
Boeing Commercial Airplanes to include a flammability reduction means
(FRM) that uses a nitrogen generation system to inert the center wing
tank with nitrogen-enriched air (NEA).
Compliance with these special conditions does not relieve the
applicant from compliance with the existing certification requirements.
I. Definitions. (a) Bulk Average Fuel Temperature. The average fuel
temperature within the fuel tank, or different sections of the tank if
the tank is subdivided by baffles or compartments.
(b) Flammability Exposure Evaluation Time (FEET). For the purpose
of these special conditions, the time from the start of preparing the
airplane for flight, through the flight and landing, until all payload
is unloaded and all passengers and crew have disembarked. In the Monte
Carlo program, the flight time is randomly selected from the Mission
Range Distribution (Table 3), the pre-flight times are provided as a
function of the flight time, and the post-flight time is a constant 30
minutes.
(c) Flammable. With respect to a fluid or gas, flammable means
susceptible to igniting readily or to exploding (14 CFR part 1,
Definitions). A non-flammable ullage is one where the gas mixture is
too lean or too rich to burn and/or is inert per the definition below.
(d) Flash Point. The flash point of a flammable fluid is the lowest
temperature at which the application of a flame to a heated sample
causes the vapor to ignite momentarily, or ``flash.'' The test for jet
fuel is defined in ASTM Specification D56, ``Standard Test Method for
Flash Point by Tag Close Cup Tester.''
(e) Hazardous Atmosphere. An atmosphere that may expose any
person(s) to the risk of death,
[[Page 7823]]
incapacitation, impairment of ability to self-rescue (escape unaided
from a space), injury, or acute illness.
(f) Inert. For the purpose of these special conditions, the tank is
considered inert when the bulk average oxygen concentration within each
compartment of the tank is 12 percent or less at sea level up to 10,000
feet, then linearly increasing from 12 percent at 10,000 feet to 14.5
percent at 40,000 feet and extrapolated linearly above that altitude.
(g) Inerting. A process where a noncombustible gas is introduced
into the ullage of a fuel tank to displace sufficient oxygen so that
the ullage becomes inert.
(h) Monte Carlo Analysis. An analytical tool that provides a means
to assess the degree of fleet average and warm day flammability
exposure time for a fuel tank. See appendices 1 and 2 of these special
conditions for specific requirements for conducting the Monte Carlo
analysis.
(i) Transport Effects. Transport effects are the effects on fuel
vapor concentration caused by low fuel conditions, fuel condensation,
and vaporization.
(j) Ullage, or Ullage Space. The volume within the fuel tank not
occupied by liquid fuel at the time interval under evaluation.
II. System Performance and Reliability. The FRM, for the airplane
model under evaluation, must comply with the following performance and
reliability requirements:
(a) The applicant must submit a Monte Carlo analysis, as defined in
appendices 1 and 2 of these special conditions, that--
(1) Demonstrates that the overall fleet average flammability
exposure of each fuel tank with an FRM installed is equal to or less
than 3 percent of the FEET; and
(2) Demonstrates that neither the performance (when the FRM is
operational) nor reliability (including all periods when the FRM is
inoperative) contributions to the overall fleet average flammability
exposure of a tank with an FRM installed is more than 1.8 percent (this
will establish appropriate maintenance inspection procedures and
intervals as required in paragraph III (a) of these special
conditions).
(3) Identifies critical features of the fuel tank system to prevent
an auxiliary fuel tank installation from increasing the flammability
exposure of the center wing tank above that permitted under paragraphs
II (a)(1) and (2) of these special conditions and to prevent
degradation of the performance and reliability of the FRM.
(b) The applicant must submit a Monte Carlo analysis that
demonstrates that the FRM, when functional, reduces the overall
flammability exposure of each fuel tank with an FRM installed for warm
day ground, takeoff, and climb phases to a level equal to or less than
3 percent of the FEET in each of these phases for the following
conditions--
(1) The analysis must use the subset of 80 [deg]F and warmer days
from the Monte Carlo analyses done for overall performance; and
(2) The flammability exposure must be calculated by comparing the
time during ground, takeoff, and climb phases for which the tank was
flammable and not inert, with the total time for the ground, takeoff,
and climb phases.
(c) The applicant must provide data from ground testing and flight
testing that--
(1) Validate the inputs to the Monte Carlo analysis needed to show
compliance with (or meet the requirements of) paragraphs II (a), (b),
and (c)(2) of these special conditions; and
(2) Substantiate that the NEA distribution is effective at inerting
all portions of the tank where the inerting system is needed to show
compliance with these paragraphs.
(d) The applicant must validate that the FRM meets the requirements
of paragraphs II (a), (b), and (c)(2) of these special conditions, with
any combination of engine model, engine thrust rating, fuel type, and
relevant pneumatic system configuration approved for the airplane.
(e) Sufficient accessibility for maintenance personnel, or the
flightcrew, must be provided to FRM status indications necessary to
meet the reliability requirements of paragraph II (a) of these special
conditions.
(f) The access doors and panels to the fuel tanks with an FRM
(including any tanks that communicate with an inerted tank via a vent
system), and to any other confined spaces or enclosed areas that could
contain NEA under normal conditions or failure conditions, must be
permanently stenciled, marked, or placarded as appropriate to warn
maintenance crews of the possible presence of a potentially hazardous
atmosphere. The proposal for markings does not alter the existing
requirements that must be addressed when entering airplane fuel tanks.
(g) Any FRM failures, or failures that could affect the FRM, with
potential catastrophic consequences must not result from a single
failure or a combination of failures not shown to be extremely
improbable.
III. Maintenance. (a) Airworthiness Limitations must be identified
for all critical features identified under paragraph II (a)(3) and for
all maintenance and/or inspection tasks required to identify failures
of components within the FRM that are needed to meet paragraphs II (a),
(b), and (c)(2) of these special conditions.
(b) The applicant must provide the maintenance procedures that will
be necessary and present a design review that identifies any hazardous
aspects to be considered during maintenance of the FRM that will be
included in the instructions for continued airworthiness (ICA) or
appropriate maintenance documents.
(c) To ensure that the effects of component failures on FRM
reliability are adequately assessed on an on-going basis, the applicant
must--
(1) Demonstrate effective means to ensure collection of FRM
reliability data. The means must provide data affecting FRM
availablity, such as component failures, and the FRM inoperative
intervals due to dispatch under the MMEL;
(2) Provide a report to the FAA on a quarterly basis for the first
five years after service introduction. After that period, continued
quarterly reporting may be replaced with other reliability tracking
methods found acceptable to the FAA or eliminated if it is established
that the reliability of the FRM meets, and will continue to meet, the
exposure requirements of paragraphs II (a) and (b) of these special
conditions;
(3) Provide a report to the validating authorities for a period of
at least two years following introduction to service; and
(4) Develop service instructions or revise the applicable airplane
manual, per a schedule agreed on by the FAA, to correct any failures of
the FRM that occur in service that could increase the fleet average or
warm day flammability exposure of the tank to more than the exposure
requirements of paragraphs II (a) and (b) of these special conditions.
Appendix 1
Monte Carlo Analysis
(a) A Monte Carlo analysis must be conducted for the fuel tank
under evaluation to determine fleet average and warm day
flammability exposure for the airplane and fuel type under
evaluation. The analysis must include the parameters defined in
appendices 1 and 2 of these special conditions. The airplane
specific parameters and assumptions used in the Monte Carlo analysis
must include:
(1) FRM Performance--as defined by system performance.
[[Page 7824]]
(2) Cruise Altitude--as defined by airplane performance.
(3) Cruise Ambient Temperature--as defined in appendix 2 of
these special conditions.
(4) Overnight Temperature Drop--as defined in appendix 2 of
these special conditions.
(5) Fuel Flash Point and Upper and Lower Flammability Limits--as
defined in appendix 2 of these special conditions.
(6) Fuel Burn--as defined by airplane performance.
(7) Fuel Quantity--as defined by airplane performance.
(8) Fuel Transfer--as defined by airplane performance.
(9) Fueling Duration--as defined by airplane performance.
(10) Ground Temperature--as defined in appendix 2 of these
special conditions.
(11) Mach Number--as defined by airplane performance.
(12) Mission Distribution--the applicant must use the mission
distribution defined in appendix 2 of these special conditions or
may request FAA approval of alternate data from the service history
of the Model 747.
(13) Oxygen Evolution--as defined by airplane performance and as
discussed in appendix 2 of these special conditions.
(14) Maximum Airplane Range--as defined by airplane performance.
(15) Tank Thermal Characteristics--as defined by airplane
performance.
(16) Descent Profile Distribution--the applicant must use either
a fixed 2500 feet per minute descent rate or provide alternate data
from the service history of the Model 747.
(b) The assumptions for the analysis must include--
(1) FRM performance throughout the flammability exposure
evaluation time;
(2) Vent losses due to crosswind effects and airplane
performance;
(3) Any time periods when the system is operating properly but
fails to inert the tank;
Note: localized concentrations above the inert level as a result of
fresh air that is drawn into the fuel tank through vents during
descent would not be considered as flammable.
(4) Expected system reliability;
(5) The MMEL/MEL dispatch inoperative period assumed in the
reliability analysis (60 flight hours must be used for a 10-day MMEL
dispatch limit unless an alternative period has been approved by the
FAA), including action to be taken when dispatching with the FRM
inoperative (Note: The actual MMEL dispatch inoperative period data
must be included in the engineering reporting requirement of
paragraph III(c)(1) of these special conditions.);
(6) Possible time periods of system inoperability due to latent
or known failures, including airplane system shut-downs and failures
that could cause the FRM to shut down or become inoperative; and
(7) Affects of failures of the FRM that could increase the
flammability of the fuel tank.
(c) The Monte Carlo analysis, including a description of any
variation assumed in the parameters (as identified under paragraph
(a) of this appendix) that affect flammability exposure, and
substantiating data must be submitted to the FAA for approval.
Appendix 2
I. Monte Carlo Model. (a) The FAA has developed a Monte Carlo
model that can be used to develop a specific analysis model for the
Boeing 747 to calculate fleet average and warm day flammability
exposure for a fuel tank in an airplane. Use of the program requires
the user to enter the airplane performance data specific to the
airplane model being evaluated, such as maximum range, cruise mach
number, typical step climb altitudes, tank thermal characteristics
specified as exponential heating/cooling time constants, and
equilibrium temperatures for various fuel tank conditions. The
general methodology for conducting a Monte Carlo model is described
in AC 25.981-2.
(b) The FAA model, or one with modifications approved by the
FAA, must be used as the means of compliance with these special
conditions. The accepted model can be downloaded from the Web site
http://qps.airweb.faa.gov/sfar88flamex. On this Web site, the model
is located under the page ``Flam Ex Resources,'' and is titled
``Monte Carlo Model Version 6a.'' The ``6a'' represents Version 6A.
Only version 6A or later of this model can be used. The following
procedures, input variables, and data tables must be used in the
analysis if the applicant develops a unique model to determine fleet
average flammability exposure for a specific airplane type.
II. Monte Carlo Variables and Data Tables. (a) Fleet average
flammability exposure is the percent of the mission time the fuel
tank ullage is flammable for a fleet of an airplane type operating
over the range of actual or expected missions and in a world-wide
range of environmental conditions and fuel properties. Variables
used to calculate fleet average flammability exposure must include
atmosphere, mission length (as defined in Special Condition I.
Definitions, as FEET), fuel flash point, thermal characteristics of
the fuel tank, overnight temperature drop, and oxygen evolution from
the fuel into the ullage. Transport effects, including mass loading,
flammability lag time, and condensation of vapors due to cold
surfaces, are not to be allowed as parameters in the analysis.
(b) For the purposes of these special conditions, a fuel tank is
considered flammable when the ullage is not inert and the fuel vapor
concentration is within the flammable range for the fuel type being
used. The fuel vapor concentration of the ullage in a fuel tank must
be determined based on the bulk average fuel temperature within the
tank. This vapor concentration must be assumed to exist throughout
all bays of the tank. For those airplanes with fuel tanks having
different flammability exposure within different compartments of the
tank, where mixing of the vapor or NEA does not occur, the Monte
Carlo analysis must be conducted for the compartment of the tank
with the highest flammability. The compartment with the highest
flammability exposure for each flight phase must be used in the
analysis to establish the fleet average flammability exposure. For
example, the center wing fuel tank in some designs extends into the
wing and has compartments of the tank that are cooled by outside
air, and other compartments of the tank that are insulated from
outside air. Therefore, the fuel temperature and flammability is
significantly different between these compartments of the fuel tank.
(c) Atmosphere. (1) To predict flammability exposure during a
given flight, the variation of ground ambient temperatures, cruise
ambient temperatures, and a method to compute the transition from
ground to cruise and back again must be used. The variation of the
ground and cruise ambient temperatures and the flash point of the
fuel is defined by a Gaussian curve, given by the 50 percent value
and a 1 standard deviation value.
(2) The ground and cruise temperatures are linked by a set of
assumptions on the atmosphere. The temperature varies with altitude
following the International Standard Atmosphere (ISA) rate of change
from the ground temperature until the cruise temperature for the
flight is reached. Above this altitude, the ambient temperature is
fixed at the cruise ambient temperature. This results in a variation
in the upper atmospheric (tropopause) temperature. For cold days, an
inversion is applied up to 10,000 feet, and then the ISA rate of
change is used.
(3) The analysis must include a minimum number of flights, and
for each flight a separate random number must be generated for each
of the three parameters (that is, ground ambient temperature, cruise
ambient temperature, and fuel flash point) using the Gaussian
distribution defined in Table 1. The applicant can verify the output
values from the Gaussian distribution using Table 2.
(d) Fuel Properties. (1) Flash point variation. The variation of
the flash point of the fuel is defined by a Gaussian curve, given by
the 50 percent value and a 1-standard deviation value.
(2) Upper and Lower Flammability Limits. The flammability
envelope of the fuel that must be used for the flammability exposure
analysis is a function of the flash point of the fuel selected by
the Monte Carlo for a given flight. The flammability envelope for
the fuel is defined by the upper flammability limit (UFL) and lower
flammability limit (LFL) as follows:
(i) LFL at sea level = flash point temperature of the fuel at
sea level minus 10 degrees F. LFL decreases from sea level value
with increasing altitude at a rate of 1 degree F per 808 ft.
(ii) UFL at sea level = flash point temperature of the fuel at
sea level plus 63.5 degrees F. UFL decreases from the sea level
value with increasing altitude at a rate of 1 degree F per 512 ft.
Note: Table 1 includes the Gaussian distribution for fuel flash
point. Table 2 also includes information to verify output values for
fuel properties. Table 2 is based on typical use of Jet A type fuel,
with limited TS-1 type fuel use.
[[Page 7825]]
Table 1.--Gaussian Distribution for Ground Ambient Temperature, Cruise Ambient Temperature, and Fuel Flash Point
----------------------------------------------------------------------------------------------------------------
Temperature in Deg F
-----------------------------------------------------------------------------------------------------------------
Ground ambient Cruise ambient
Parameter temperature temperature Flash point (FP)
----------------------------------------------------------------------------------------------------------------
Mean Temp................................................. 59.95 -70 120
Neg 1 std dev............................................. 20.14 8 8
Pos 1 std dev............................................. 17.28 8 8
----------------------------------------------------------------------------------------------------------------
Table 2.--Verification of Table 1
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ground Cruise Ground Cruise
% Probability of temps & flash point being below the ambient ambient Flash point ambient ambient Flash point
listed values temperature temperature Deg F temperature temperature (FP) Deg C
Deg F Deg F Deg C Deg C
--------------------------------------------------------------------------------------------------------------------------------------------------------
1...................................................... 13.1 -88.6 101.4 -10.5 -67.0 38.5
5...................................................... 26.8 -83.2 106.8 -2.9 -64.0 41.6
10..................................................... 34.1 -80.3 109.7 1.2 -62.4 43.2
15..................................................... 39.1 -78.3 111.7 3.9 -61.3 44.3
20..................................................... 43.0 -76.7 113.3 6.1 -60.4 45.1
25..................................................... 46.4 -75.4 114.6 8.0 -59.7 45.9
30..................................................... 49.4 -74.2 115.8 9.7 -59.0 46.6
35..................................................... 52.2 -73.1 116.9 11.2 -58.4 47.2
40..................................................... 54.8 -72.0 118.0 12.7 -57.8 47.8
45..................................................... 57.4 -71.0 119.0 14.1 -57.2 48.3
50..................................................... 59.9 -70.0 120.0 15.5 -56.7 48.9
55..................................................... 62.1 -69.0 121.0 16.7 -56.1 49.4
60..................................................... 64.3 -68.0 122.0 18.0 -55.5 50.0
65..................................................... 66.6 -66.9 123.1 19.2 -55.0 50.6
70..................................................... 69.0 -65.8 124.2 20.6 -54.3 51.2
75..................................................... 71.6 -64.6 125.4 22.0 -53.7 51.9
80..................................................... 74.5 -63.3 126.7 23.6 -52.9 52.6
85..................................................... 77.9 -61.7 128.3 25.5 -52.1 53.5
90..................................................... 82.1 -59.7 130.3 27.8 -51.0 54.6
95..................................................... 88.4 -56.8 133.2 31.3 -49.4 56.2
99..................................................... 100.1 -51.4 138.6 37.9 -46.3 59.2
--------------------------------------------------------------------------------------------------------------------------------------------------------
(e) Flight Mission Distribution. (1) The mission length for each
flight is determined from an equation that takes the maximum mission
length for the airplane and randomly selects multiple flight lengths
based on typical airline use.
(2) The mission length selected for a given flight is used by
the Monte Carlo model to select a 30-, 60-, or 90-minute time on the
ground prior to takeoff, and the type of flight profile to be
followed. Table 3 must be used to define the mission distribution. A
linear interpolation between the values in the table must be
assumed.
Table 3.--Mission Length Distribution Airplane Maximum Range--Nautical Miles (NM)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Flight length (NM) Airplane maximum range (NM)
--------------------------------------------------------------------------------------------------------------------------------------------------------
From To 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
--------------------------------------------------------------------------------------------------------------------------------------------------------
Distribution of mission lengths (%)
---------
0......................................... 200......................... 11.7 7.5 6.2 5.5 4.7 4.0 3.4 3.0 2.6 2.3
200....................................... 400......................... 27.3 19.9 17.0 15.2 13.2 11.4 9.7 8.5 7.5 6.7
400....................................... 600......................... 46.3 40.0 35.7 32.6 28.5 24.9 21.2 18.7 16.4 14.8
600....................................... 800......................... 10.3 11.6 11.0 10.2 9.1 8.0 6.9 6.1 5.4 4.8
800....................................... 1000........................ 4.4 8.5 8.6 8.2 7.4 6.6 5.7 5.0 4.5 4.0
1000...................................... 1200........................ 0.0 4.8 5.3 5.3 4.8 4.3 3.8 3.3 3.0 2.7
1200...................................... 1400........................ 0.0 3.6 4.4 4.5 4.2 3.8 3.3 3.0 2.7 2.4
1400...................................... 1600........................ 0.0 2.2 3.3 3.5 3.3 3.1 2.7 2.4 2.2 2.0
1600...................................... 1800........................ 0.0 1.2 2.3 2.6 2.5 2.4 2.1 1.9 1.7 1.6
1800...................................... 2000........................ 0.0 0.7 2.2 2.6 2.6 2.5 2.2 2.0 1.8 1.7
2000...................................... 2200........................ 0.0 0.0 1.6 2.1 2.2 2.1 1.9 1.7 1.6 1.4
2200...................................... 2400........................ 0.0 0.0 1.1 1.6 1.7 1.7 1.6 1.4 1.3 1.2
2400...................................... 2600........................ 0.0 0.0 0.7 1.2 1.4 1.4 1.3 1.2 1.1 1.0
2600...................................... 2800........................ 0.0 0.0 0.4 0.9 1.0 1.1 1.0 0.9 0.9 0.8
2800...................................... 3000........................ 0.0 0.0 0.2 0.6 0.7 0.8 0.7 0.7 0.6 0.6
3000...................................... 3200........................ 0.0 0.0 0.0 0.6 0.8 0.8 0.8 0.8 0.7 0.7
3200...................................... 3400........................ 0.0 0.0 0.0 0.7 1.1 1.2 1.2 1.1 1.1 1.0
3400...................................... 3600........................ 0.0 0.0 0.0 0.7 1.3 1.6 1.6 1.5 1.5 1.4
3600...................................... 3800........................ 0.0 0.0 0.0 0.9 2.2 2.7 2.8 2.7 2.6 2.5
[[Page 7826]]
3800...................................... 4000........................ 0.0 0.0 0.0 0.5 2.0 2.6 2.8 2.8 2.7 2.6
4000...................................... 4200........................ 0.0 0.0 0.0 0.0 2.1 3.0 3.2 3.3 3.2 3.1
4200...................................... 4400........................ 0.0 0.0 0.0 0.0 1.4 2.2 2.5 2.6 2.6 2.5
4400...................................... 4600........................ 0.0 0.0 0.0 0.0 1.0 2.0 2.3 2.5 2.5 2.4
4600...................................... 4800........................ 0.0 0.0 0.0 0.0 0.6 1.5 1.8 2.0 2.0 2.0
4800...................................... 5000........................ 0.0 0.0 0.0 0.0 0.2 1.0 1.4 1.5 1.6 1.5
5000...................................... 5200........................ 0.0 0.0 0.0 0.0 0.0 0.8 1.1 1.3 1.3 1.3
5200...................................... 5400........................ 0.0 0.0 0.0 0.0 0.0 0.8 1.2 1.5 1.6 1.6
5400...................................... 5600........................ 0.0 0.0 0.0 0.0 0.0 0.9 1.7 2.1 2.2 2.3
5600...................................... 5800........................ 0.0 0.0 0.0 0.0 0.0 0.6 1.6 2.2 2.4 2.5
5800...................................... 6000........................ 0.0 0.0 0.0 0.0 0.0 0.2 1.8 2.4 2.8 2.9
6000...................................... 6200........................ 0.0 0.0 0.0 0.0 0.0 0.0 1.7 2.6 3.1 3.3
6200...................................... 6400........................ 0.0 0.0 0.0 0.0 0.0 0.0 1.4 2.4 2.9 3.1
6400...................................... 6600........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.9 1.8 2.2 2.5
6600...................................... 6800........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.5 1.2 1.6 1.9
6800...................................... 7000........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.8 1.1 1.3
7000...................................... 7200........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.7 0.8
7200...................................... 7400........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.5 0.7
7400...................................... 7600........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.5 0.6
7600...................................... 7800........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.5 0.7
7800...................................... 8000........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.6 0.8
8000...................................... 8200........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.8
8200...................................... 8400........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 1.0
8400...................................... 8600........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 1.3
8600...................................... 8800........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 1.1
8800...................................... 9000........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.8
9000...................................... 9200........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5
9200...................................... 9400........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2
9400...................................... 9600........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1
9600...................................... 9800........................ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1
9800...................................... 10000....................... 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
(f) Fuel Tank Thermal Characteristics. (1) The applicant must
account for the thermal conditions of the fuel tank both on the
ground and in flight. The Monte Carlo model, available on the
website listed above, defines the ground condition using an
equilibrium delta temperature (relative to the ambient temperature)
the tank will reach given a long enough time, with any heat inputs
from airplane sources. Values are also input to define two
exponential time constants (one for a near empty tank and one for a
near full tank) for the ground condition. These time constants
define the time for the fuel in the fuel tank to heat or cool in
response to heat input. The fuel is assumed to heat or cool
according to a normal exponential transition, governed by the
temperature difference between the current temperature and the
equilibrium temperature, given by ambient temperature plus delta
temperature. Input values for this data can be obtained from
validated thermal models of the tank based on ground and flight test
data. The inputs for the inflight condition are similar but are used
for inflight analysis.
(2) Fuel management techniques are unique to each manufacturer's
design. Variations in fuel quantity within the tank for given points
in the flight, including fuel transfer for any purpose, must be
accounted for in the model. The model uses a ``tank full'' time,
specified in minutes, that defines the time before touchdown when
the fuel tank is still full. For a center wing tank used first, this
number would be the maximum flight time, and the tank would start to
empty at takeoff. For a main tank used last, the tank will remain
full for a shorter time before touchdown and would be ``empty'' at
touchdown (that is, tank empty at 0 minutes before touchdown). For a
main tank with reserves, the term empty means at reserve level
rather than totally empty. The thermal data for tank empty would
also be for reserve level.
(3) The model also uses a ``tank empty'' time to define the time
when the tank is emptying, and the program uses a linear
interpolation between the exponential time constants for full and
empty during the time the tank is emptying. For a tank that is only
used for longrange flights, the tank would be full only on longer-
range flights and would be empty a long time before touchdown. For
short flights, it would be empty for the whole flight. For a main
tank that carried reserve fuel, it would be full for a long time and
would only be down to empty at touchdown. In this case, empty would
really be at reserve level, and the thermal constants at empty
should be those for the reserve level.
(4) The applicant, whether using the available model or using
another analysis tool, must propose means to validate thermal time
constants and equilibrium temperatures to be used in the analysis.
The applicant may propose using a more detailed thermal definition,
such as changing time constants as a function of fuel quantity,
provided the details and substantiating information are acceptable
and the Monte Carlo model program changes are validated.
(g) Overnight Temperature Drop. (1) An overnight temperature
drop must be considered in the Monte Carlo analysis as it may affect
the oxygen concentration level in the fuel tank. The overnight
temperature drop for these special conditions will be defined using:
A temperature at the beginning of the overnight period
based on the landing temperature that is a random value based on a
Gaussian distribution; and
An overnight temperature drop that is a random value
based on a Gaussian distribution.
(2) For any flight that will end with an overnight ground period
(one flight per day out of an average of ``x'' number of flights per
day, (depending on use of the particular airplane model being
evaluated), the landing outside air temperature (OAT) is to be
chosen as a random value from the following Gaussian curve:
Table 4.--Landing OAT
------------------------------------------------------------------------
Landing
Parameter temperature
[deg]F
------------------------------------------------------------------------
Mean Temp............................................. 58.68
[[Page 7827]]
neg 1 std dev......................................... 20.55
pos 1 std dev......................................... 13.21
------------------------------------------------------------------------
(3) The outside air temperature (OAT) drop for that night is to
be chosen as a random value from the following Gaussian curve:
Table 5.--OAT Drop
------------------------------------------------------------------------
OAT Drop
Parameter temperature
[deg]F
------------------------------------------------------------------------
Mean Temp............................................. 12.0
1 std dev............................................. 6.0
------------------------------------------------------------------------
(h) Oxygen Evolution. The oxygen evolution rate must be
considered in the Monte Carlo analysis if it can affect the
flammability of the fuel tank or compartment. Fuel contains
dissolved gases, and in the case of oxygen and nitrogen absorbed
from the air, the oxygen level in the fuel can exceed 30 percent,
instead of the normal 21 percent oxygen in air. Some of these gases
will be released from the fuel during the reduction of ambient
pressure experienced in the climb and cruise phases of flight. The
applicant must consider the effects of air evolution from the fuel
on the level of oxygen in the tank ullage during ground and flight
operations and address these effects on the overall performance of
the FRM. The applicant must provide the air evolution rate for the
fuel tank under evaluation, along with substantiation data.
(i) Number of Simulated Flights Required in Analysis. For the
Monte Carlo analysis to be valid for showing compliance with the
fleet average and warm day flammability exposure requirements of
these special conditions, the applicant must run the analysis for an
appropriate number of flights to ensure that the fleet average and
warm day flammability exposure for the fuel tank under evaluation
meets the flammability limits defined in Table 6.
Table 6.--Flammability Limit
------------------------------------------------------------------------
Maximum
acceptable
Number of flights in Monte Carlo analysis fuel tank
flammability
(%)
------------------------------------------------------------------------
1,000.................................................... 2.73
5,000..................................................... 2.88
10,000.................................................... 2.91
100,000................................................... 2.98
1,000,000................................................. 3.00
------------------------------------------------------------------------
Issued in Renton, Washington, on January 24, 2005.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 05-2752 Filed 2-14-05; 8:45 am]
BILLING CODE 4910-13-P