[Federal Register: June 15, 2005 (Volume 70, Number 114)]
[Proposed Rules]
[Page 34702-34714]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr15jn05-26]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. NM309; Notice No. 25-05-06-SC]
Proposed Special Conditions: Boeing Model 737-200/200C/300/400/
500/600/700/700C/800/900 Series Airplanes; Flammability Reduction Means
(Fuel Tank Inerting)
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Notice of proposed special conditions.
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SUMMARY: The Federal Aviation Administration (FAA) proposes special
conditions for the Boeing Model 737-200/200C/300/400/500/600/700/700C/
800/900 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 proposed 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: Comments must be received on or before July 15, 2005.
ADDRESSES: Comments on this proposal may be mailed in duplicate to:
Federal Aviation Administration, Transport Airplane Directorate, Attn:
Rules Docket (ANM-113), Docket No. NM309, 1601 Lind Avenue SW., Renton,
Washington, 98055-4056; or delivered in duplicate to the Transport
Airplane Directorate at the above address. Comments must be marked:
Docket No. NM309. Comments may be inspected in the Rules Docket
weekdays, except Federal holidays, between 7:30 a.m. and 4 p.m.
FOR FURTHER INFORMATION CONTACT: Mike Dostert, Propulsion and
Mechanical Systems Branch, FAA,
[[Page 34703]]
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:
Comments Invited
The FAA invites interested persons to participate in this
rulemaking by submitting written comments, data, or views. The most
helpful comments reference a specific portion of the special
conditions, explain the reason for any recommended change, and include
supporting data. We ask that you send us two copies of written
comments.
We will file in the docket all comments we receive, as well as a
report summarizing each substantive public contact with FAA personnel
concerning these special conditions. The docket is available for public
inspection before and after the comment closing date. If you wish to
review the docket in person, go to the address in the ADDRESSES section
of this preamble between 9 a.m. and 5 p.m., Monday through Friday,
except Federal holidays.
We will consider all comments we receive on or before the closing
date for comments. We will consider comments filed late if it is
possible to do so without incurring expense or delay. We may change
these proposed special conditions in light of the comments we receive.
If you want the FAA to acknowledge receipt of your comments on this
proposal, include with your comments a pre-addressed, stamped postcard
on which the docket number appears. We will stamp the date on the
postcard and mail it back to you.
Background
Boeing Commercial Airplanes intends to modify the Model 737 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 proposed special conditions
address novel design features. This document proposes the same special
conditions that were published in the Federal Register [Docket No.
NM270; Special Conditions No. 25-285-SC] for incorporation of an FRM on
Boeing Model 747-100/200B/200F/200C/SR/SP/100B/300/100B SUD/400/400D/
400F series airplanes (70 FR 7800, January 24, 2005).
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 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;
[[Page 34704]]
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 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
[[Page 34705]]
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 September 23, 2005 (737Classics) and December 2, 2005 (737NG),
Boeing Commercial Airplanes applied for a change to Type Certificate
A16WE to modify Model 737-200/200C/300/400/500/600/700/700C/800/900
series airplanes to incorporate a new FRM that inerts the center fuel
tanks with NEA. These airplanes, approved under Type Certificate No.
A16WE, are two-engine transport airplanes with a passenger capacity up
to 189, depending on the submodel. These airplanes have an approximate
maximum gross weight of 174,700 lbs with an operating range up to 3,380
miles.
Type Certification Basis
Under the provisions of Sec. 21.101, Boeing Commercial Airplanes
must show that the Model 737-200/200C/300/400/500/600/700/700C/800/900
series airplanes, as changed, continue to meet the applicable
provisions of the regulations incorporated by reference in Type
Certificate No. A16WE, 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 A16WE include 14 CFR part 25, dated
February 1, 1965, as amended by Amendments 25-1 through 25-94, except
for proposed special conditions and exceptions noted in Type
Certificate Data Sheet A16WE.
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 737-200/200C/300/400/500/600/700/700C/
800/900 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 737-200/200C/300/400/500/600/700/
700C/800/900 series airplanes because of a novel or unusual design
feature, proposed special conditions are prescribed under the
provisions of Sec. 21.16.
In addition to the applicable airworthiness regulations and
proposed special conditions, the Model 737-200/200C/300/400/500/600/
700/700C/800/900 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.
[[Page 34706]]
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, these proposed 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 737-
200/200C/300/400/500/600/700/700C/800/900 series airplanes. Boeing also
plans to seek approval of this system on Boeing Model 747, 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 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 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. These proposed 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 proposed
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 proposed 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 proposed special conditions. Therefore,
the effect of the definition of ``inert'' within these proposed 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
proposed special conditions (12 percent or less at sea level) to be
considered inert.
Determining Flammability
The methodology for determining fuel tank flammability defined for
use in these proposed 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 proposed 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
[[Page 34707]]
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 proposed 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 proposed 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 proposed 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 proposed special
conditions, are excluded from consideration in the Monte Carlo model
used for demonstrating compliance with these proposed 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 European Aviation Safety Agency
(EASA) 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 proposed special conditions, the FAA
and EASA 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 EASA and Transport Canada, has
developed criteria within these proposed 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 proposed special conditions also include a requirement to
limit fuel tank flammability to 3 percent during ground operations, and
climb phases of 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
proposed 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
[[Page 34708]]
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, since the intent of Sec.
25.981(c)(1) is to minimize flammability, the FRM system should be
operational to the maximum extent practical. Therefore, these proposed
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 proposed 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
proposed 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.
Validation of proper function of essential features of the system would
likely be required once per day by maintenance review of indications,
reading of stored maintenance messages or functional checks (possibly
prior to the first flight of the day) to meet the reliability levels
defined in these special conditions. 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
proposed 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 proposed 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
proposed 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 proposed special conditions requires the applicant to
provide markings to emphasize the potential hazards associated with
confined spaces and areas where a hazardous atmosphere could be present
due to the addition of an FRM.
For the purposes of these proposed 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 proposed 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
[[Page 34709]]
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 describe
installation of 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 center wing fuel tank volume of the 737 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 proposed
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, proposed special conditions (per Sec. 21.16) are needed to
address the unusual design features of an FRM. These proposed 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.
Applicability
As discussed above, these proposed special conditions are
applicable to the Boeing Model 737-200/200C/300/400/500/600/700/700C/
800/900 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 proposed 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 737-200/200C/300/400/500/600/700/700C/800/900 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.
The authority citation for these proposed special conditions is as
follows:
Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.
The Proposed Special Conditions
Accordingly, the Federal Aviation Administration (FAA) proposes the
following special conditions as part of the type certification basis
for the Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900
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 proposed 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 proposed 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
[[Page 34710]]
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, incapacitation, impairment of ability
to self-rescue (escape unaided from a space), injury, or acute illness.
(f) Inert. For the purpose of these proposed 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 proposed
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 (mass loading), 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 proposed 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 proposed 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), II(a)(2), and II(b) of these proposed 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 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 and climb phases for which the tank was flammable
and not inert, with the total time for the ground 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 proposed 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 proposed 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 proposed
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
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 proposed special conditions. Airworthiness
Limitations must also be identified for the critical fuel tank system
features identified under paragraph II(a)(3).
(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 dequately 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 proposed
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
[[Page 34711]]
exposure of the tank to more than the exposure requirements of
paragraphs II(a) and (b) of these proposed 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 proposed special conditions. The
airplane specific parameters and assumptions used in the Monte Carlo
analysis must include:
(1) FRM Performance--as defined by system performance.
(2) Cruise Altitude--as defined by airplane performance.
(3) Cruise Ambient Temperature--as defined in appendix 2 of
these proposed special conditions.
(4) Overnight Temperature Drop--as defined in appendix 2 of
these proposed special conditions.
(5) Fuel Flash Point and Upper and Lower Flammability Limits--as
defined in appendix 2 of these proposed 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
proposed 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 proposed special
conditions or may request FAA approval of alternate data from the
service history of the Model 737.
(13) Oxygen Evolution--as defined by airplane performance and as
discussed in appendix 2 of these proposed 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 a
fixed 2500 feet per minute descent rate or may request FAA approval
of alternate data from the service history of the Model 737.
(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 proposed 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) Effects 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 fleet average 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 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 proposed
special conditions. The accepted model can be obtained from the
person identified in the FOR FURTHER INFORMATION CONTACT section of
this document. 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 are not to be allowed as parameters in the
analysis.
(b) For the purposes of these proposed 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. The warm day subset (see paragraph II(b)(2) of
Appendix 2 of these proposed special conditions) for ground and
climb uses a range of temperatures above 80[deg] F and is included
in the Monte Carlo model.
(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
[[Page 34712]]
(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.
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 ambient Cruise ambient Ground ambient Cruise ambient
Percent probability of temps & flash temperature temperature Flash point [deg]F temperature temperature Flash point (FP)
point being below the listed values [deg]F [deg]F [deg]C [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
[[Page 34713]]
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
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,
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 in-flight
condition are similar but are used for in-flight 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 long-range 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 proposed
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
[[Page 34714]]
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
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 proposed 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 fuel
Number of flights in Monte Carlo analysis 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 June 3, 2005.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 05-11762 Filed 6-14-05; 8:45 am]
BILLING CODE 4910-13-P