[Federal Register: October 11, 2007 (Volume 72, Number 196)]
[Rules and Regulations]
[Page 57844-57848]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr11oc07-3]
-----------------------------------------------------------------------
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. NM366 Special Conditions No. 25-348-SC]
Special Conditions: Boeing Model 787-8 Airplane; Composite Wing
and Fuel Tank Structure--Fire Protection Requirements
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
-----------------------------------------------------------------------
SUMMARY: These special conditions are issued for the Boeing Model 787-8
airplane. This airplane will have novel or unusual design features when
compared to the state of technology envisioned in the airworthiness
standards for transport category airplanes. These novel or unusual
design features are associated with composite materials chosen for the
construction of the fuel tank skin and structure. For these design
features, the applicable airworthiness regulations do not contain
adequate or appropriate safety standards for wing and fuel tank
structure with respect to postcrash fire safety. These special
conditions contain the additional safety standards that the
Administrator considers necessary to
[[Page 57845]]
establish a level of safety equivalent to that established by the
existing standards. We will issue additional special conditions for
other novel or unusual design features of the Boeing Model 787-8
airplanes.
DATES: Effective Date: November 13, 2007.
FOR FURTHER INFORMATION CONTACT: Mike Dostert, FAA, Propulsion/
Mechanical Systems, ANM-112, Transport Airplane Directorate, Aircraft
Certification Service, 1601 Lind Avenue, SW., Renton, Washington 98057-
3356; telephone (425) 227-2132; facsimile (425) 227-1320.
SUPPLEMENTARY INFORMATION:
Background
On March 28, 2003, Boeing applied for an FAA type certificate for
its new Boeing Model 787-8 passenger airplane. The Boeing Model 787-8
airplane will be an all-new, two-engine jet transport airplane with a
two-aisle cabin. The maximum takeoff weight will be 476,000 pounds,
with a maximum passenger count of 381 passengers.
Type Certification Basis
Under provisions of Title 14 Code of Federal Regulations (CFR)
21.17, Boeing must show that Boeing Model 787-8 airplanes (hereafter
referred to as ``the 787'') meet the applicable provisions of 14 CFR
part 25, as amended by Amendments 25-1 through 25-117, except
Sec. Sec. 25.809(a) and 25.812, which will remain at Amendment 25-115.
If the Administrator finds that the applicable airworthiness
regulations do not contain adequate or appropriate safety standards for
the 787 because of a novel or unusual design feature, special
conditions are prescribed under provisions of 14 CFR 21.16.
In addition to the applicable airworthiness regulations and special
conditions, the 787 must comply with the fuel vent and exhaust emission
requirements of 14 CFR part 34 and the noise certification requirements
of 14 CFR part 36. The FAA must also issue a finding of regulatory
adequacy under section 611 of Public Law 92-574, the ``Noise Control
Act of 1972.''
The FAA issues special conditions, as defined in 14 CFR 11.19,
under Sec. 11.38, and they become part of the type certification basis
under Sec. 21.17(a)(2).
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, the special conditions would also
apply to the other model under Sec. 21.101.
Novel or Unusual Design Features
The 787 will incorporate a number of novel or unusual design
features. Because of rapid improvements in airplane technology, the
applicable airworthiness regulations do not contain adequate or
appropriate safety standards for these design features. These special
conditions for the 787 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.
The 787 will be the first large transport category airplane not
built mainly with aluminum materials for the fuel tank structure.
Instead it will use chiefly composite materials for the structural
elements and skin of the wings and fuel tanks. Conventional airplanes
with aluminum skin and structure provide a well understood level of
safety during postcrash fires with respect to fuel tanks. This is based
on service history and extensive full-scale fire testing. Composites
may or may not have capabilities equivalent to aluminum, and current
regulations do not provide objective performance requirements for wing
and fuel tank structure with respect to postcrash fire safety. Use of
composite structure is new and novel compared to the designs envisioned
when the applicable regulations were written. Because of this, Boeing
must present additional confirmation by test and analysis that the 787
provides an acceptable level of safety with respect to the performance
of the wings and fuel tanks during an external fuel-fed fire.
Although the FAA has previously approved fuel tanks made of
composite materials that are located in the horizontal stabilizer of
some airplanes, the composite wing structure of the 787 will introduce
a new fuel tank construction into service. Advisory Circular (AC) 20-
107A, Composite Aircraft Structure, under the topic of flammability,
states: ``The existing requirements for flammability and fire
protection of aircraft structure attempt to minimize the hazard to the
occupants in the event ignition of flammable fluids or vapors occurs.
The use of composite structure should not decrease this existing level
of safety.'' The relevance to the wing structure is that postcrash fire
passenger survivability is dependent on the time available for
passenger evacuation before fuel tank breach or structural failure.
Structural failure can be a result of degradation in load-carrying
capability in the upper or lower wing surface caused by a fuel-fed
ground fire. Structural failure can also be a result of over-
pressurization caused by ignition of fuel vapors in the fuel tank.
The FAA has historically developed rules with the assumption that
the material of construction for wing and fuselage would be aluminum.
As a representative case, Sec. 25.963 was developed because of a large
fuel-fed fire following the failures of fuel tank access doors caused
by uncontained engine failures. During the subsequent Aviation
Rulemaking Advisory Committee (ARAC) harmonization process with the
JAA,\1\ the structures group tried to harmonize the requirements of
Sec. 25.963 for impact and fire resistance of fuel tank access panels.
Both authorities recognized that existing aluminum wing structure
provided an acceptable level of safety. Further rulemaking has not yet
been pursued.
---------------------------------------------------------------------------
\1\ The JAA is the Joint Aviation Authority of Europe and the
JAR is its Joint Aviation Requirements, the equivalent of our
Federal Aviation Regulations. In 2003, the European Aviation Safety
Agency (EASA) was formed, and EASA is now the principal aviation
regulatory agency in Europe. We intend to work with EASA to ensure
that our rules are also harmonized with its Certification
Specifications (CS). But since these efforts in developing
harmonization of Sec. 25.963 occurred before EASA was formed, it
was the JAA that was involved with them.
---------------------------------------------------------------------------
As with previous Boeing airplane designs with underwing mounted
engines, the wing tanks and center tanks are located in proximity to
the passengers and near the engines. Experience indicates postcrash
survivability is greatly influenced by the size and intensity of any
fire that occurs. The ability of aluminum wing surfaces wetted by fuel
on their interior surface to withstand postcrash fire conditions has
been shown by tests conducted at the FAA Technical Center. These tests
have verified adequate dissipation of heat across wetted aluminum fuel
tank surfaces so that localized hot spots do not occur, thus minimizing
the threat of explosion. This inherent capability of aluminum to
dissipate heat also allows the wing lower surface to retain its load
carrying characteristics during a fuel-fed ground fire. It
significantly delays wing collapse or burn-through for a time interval
that usually exceeds evacuation times. In addition, as an aluminum fuel
tank is heated with significant quantities of fuel inside, fuel vapor
accumulates in the ullage space, exceeding the upper flammability limit
relatively quickly and thus reducing the threat of a fuel tank
explosion prior to fuel tank burn-through. Service history of
conventional aluminum airplanes has shown that fuel tank explosions
caused
[[Page 57846]]
by ground fires have been rare on airplanes configured with flame
arrestors in the fuel tank vent lines. Fuel tanks constructed with
composite materials may or may not have equivalent capability.
Current regulations were developed and have evolved under the
assumption that wing construction would be of aluminum materials, which
provide inherent properties. Current regulations may not be adequate
when applied to airplanes constructed of different materials.
Aluminum has the following properties with respect to fuel tanks
and fuel-fed external fires.
Aluminum is highly thermally conductive. It readily
transmits the heat of a fuel-fed external fire to fuel in the tank.
This has the benefit of rapidly driving the fuel tank ullage to exceed
the upper flammability limit prior to burn-through of the fuel tank
skin or heating of the wing upper surface above the auto-ignition
temperature. This greatly reduces the threat of fuel tank explosion.
Aluminum panels at thicknesses previously used in wing
lower surfaces of large transport category airplanes have been fire
resistant as defined in 14 CFR part 1 and AC 20-135.
The heat absorption capacity of aluminum and fuel will
prevent burn-through or wing collapse for a time interval that will
generally exceed the passenger evacuation time.
The extensive use of composite materials in the design of the 787
wing and fuel tank structure is considered a major change from
conventional and traditional methods of construction. This will be the
first large transport category airplane to be certificated with this
level of composite material for these purposes. The applicable
airworthiness regulations do not contain specific standards for
postcrash fire safety performance of wing and fuel tank skin or
structure.
Discussion of Special Conditions
In order to provide the same level of safety as exists with
conventional airplane construction, Boeing must demonstrate that the
787 has sufficient postcrash survivability to enable occupants to
safely evacuate in the event that the wings are exposed to a large
fuel-fed fire. Factors in fuel tank survivability are the structural
integrity of the wing and tank, flammability of the tank, burn-through
resistance of the wing skin, and the presence of auto-ignition threats
during exposure to a fire. The FAA assessed postcrash survival time
during the adoption of Amendment 25-111 for fuselage burn-through
protection. Studies conducted by and on behalf of the FAA indicated
that, following a survivable accident, prevention of fuselage burn-
through for approximately 5 minutes can significantly enhance
survivability. (See report numbers DOT/FAA/AR-99/57 and DOT/FAA/AR-02/
49.) There is little benefit in requiring the design to prevent wing
skin burn-through beyond five minutes, due to the effects of the fuel
fire itself on the rest of the airplane. That assessment was carried
out based on accidents involving airplanes with conventional fuel
tanks, and considering the ability of ground personnel to rescue
occupants. In addition, AC 20-135 indicates that, when aluminum is used
for fuel tanks, the tank should withstand the effects of fire for 5
minutes without failure. Therefore, to be consistent with existing
capability and related requirements, the 787 fuel tanks must be capable
of resisting a postcrash fire for at least 5 minutes. In demonstrating
compliance, Boeing must address a range of fuel loads from minimum to
maximum, as well as any other critical fuel load.
Discussion of Comments
Notice of Proposed Special Conditions No. 25-07-03-SC for the 787
was published in the Federal Register on April 9, 2007 (72 FR 17441).
Two comments were received from the Air Line Pilots Association,
International (ALPA), two from Airbus, and several from members of the
public.
Comment 1--Air Line Pilots Association (ALPA). The Air Line Pilots
Association, International questioned whether the 787 will be required
to comply with any and all rules related to fuel tank inerting/
flammability requirements of 14 CFR parts 25 and 121 and the guidance
in Advisory Circular 25.981-2A.
FAA Response. The 787 will be required to meet the current
requirements for the certification basis of the airplane that include
fuel vapor flammability standards, and we will be proposing additional
requirements within special conditions for a nitrogen inerting system.
The certification basis for the 787 includes Amendment 25-102, which
includes the Sec. 25.981(c) requirement for minimization of fuel tank
flammability. In the preamble to Amendment 25-102 we described the
intended level of flammability to be equivalent to an unheated aluminum
wing fuel tank. The composite fuel tank structure of the 787 does not
inherently meet this flammability standard because of the difference in
thermal conductivity between composite materials and aluminum. Boeing
has proposed a design that includes a nitrogen inerting system to meet
the flammability standard. Because of this novel and unique feature
that provides nitrogen enriched air to all fuel tanks, we will be
publishing proposed special conditions for public comment.
We have made no changes to these special conditions as a result of
this comment.
Comment 2--ALPA. ALPA also commented that it is important to
determine the characteristics of composites after prolonged exposure to
moisture of any kind (humidity, liquid, deicing fluid, fuel etc.) and
stated that the FAA must conduct or endorse research to determine
whether composite materials are susceptible to absorbing liquids during
prolonged exposure. The commenter also stated that research must be
done to determine effects of water (or other liquid) intrusion on the
aircraft weight, controllability, flammability, and survivability.
FAA Response. The FAA concurs with the concerns of the commenter
and has discussed these items with the applicant. The existing
airworthiness regulations for certification require that all parts and
components be qualified for all foreseeable environmental conditions as
installed on the airplane. Therefore, as part of the material
certification and approval, the composite material is required to be
subjected to accelerated environmental exposure to all liquids
anticipated to be in contact with the material for the life of the
aircraft. This includes but is not limited to water, salt spray, fuel,
hydraulic fluid, and de-icing fluids. Any material effects due to this
exposure testing will have to be considered in showing the material's
ability to perform its intended function, including consideration for
the life and performance of the material. These environmental
qualifications are required by existing airworthiness regulations and
are therefore not required to be included in the special conditions for
composite structure. We have made no changes to these special
conditions as a result of this comment.
Comment 3--Airbus. Airbus noted a reference in the proposed special
conditions to testing conducted at the FAA Technical Center that
demonstrated aluminum fuel tank performance under postcrash fire
conditions. The commenter requested access to the documentation for
review of the test data to understand the applied conditions and
parameters of the test.
FAA Response. The noted reports are available to the public via the
FAA
[[Page 57847]]
Technical Center Website for Fire Safety at http://www.fire.tc.faa.gov/.
The document we were referring to in the proposed special conditions
was document FAA-RD-75-119, Investigation of Aircraft Fuel Tank
Explosions and Nitrogen Inerting Requirements During Ground Fires. We
have made no changes to these special conditions as a result of this
comment.
Comment 4--Airbus. Airbus also requested clarification of the
following statement on page 17443 of the Federal Register, under the
heading ``Discussion of Proposed Special Conditions:'' * * * AC 20-135
indicates that, when aluminum is used for fuel tanks, the tank should
withstand the effects of fire for 5 minutes without failure.'' Airbus
said this statement needed clarification, because the actual language
in the AC discusses fire resistance of a number of elements, but does
not consider the fuel tank as a whole.
FAA Response. The commenter is correct that AC 20-135 does not
specifically refer to demonstrating that the fuel tank as a whole is
fire resistant. In the past fuel tanks have typically been constructed
of aluminum, which is considered to be fire resistant. AC 20-135
provides general guidance on how materials can be shown to be fire
resistant if they can withstand the effects of fire for 5 minutes.
These special conditions require that the fuel tank be shown to meet
fire resistance standards and one means of showing a material meets
these standards is described in the AC. Since the fuel tank is
constructed of composite materials, we consider the guidance in the AC
to be applicable to the fuel tank as a whole. We've made no change to
these special conditions as a result of this comment.
The following four comments, received from the public, were outside
the scope of these special conditions.
Comment 5. One commenter requested that the FAA and foreign
authorities pursue rulemaking activities to develop specific rules
related to use of composite materials for basic airframe structure.
FAA Response. Although this comment does not address the context of
these special conditions, we agree that current transport category
rules do not adequately address the unique aspects of composite
structure. These special conditions, and others for the 787 and other
certification projects involving composite structure, are the first
steps in establishing new airworthiness standards. We anticipate that
these special conditions will be followed by rulemaking activity to
establish similar standards in the applicable sub-parts of part 25. The
FAA cannot comment on the position of other foreign authorities in this
regard. No change to the special conditions is required.
Comment 6. This commenter also requested that the scope of the
special conditions be expanded to include evaluation of the fuselage,
wing, and fuel tank to simulate actual survivable crash conditions
during a fuel fed fire with respect to fire, smoke, and toxicity and
passenger survivability. The commenter requested that the special
conditions address fire, smoke, and toxicity environments within the
fuselage interior during an external fuel fed fire.
FAA Response. While we agree with the commenter that these are
important considerations, the FAA has determined that this comment is
outside the scope of these special conditions because they are limited
to performance of the wing and fuel tank structure during a postcrash
ground fire. The performance of the fuselage barrel and interiors
during a fuel-fed fire is already addressed by existing regulations
(reference 14 CFR 25.853, 25.855, and 25.856 and Appendix F for current
standards for airplane interior fire safety). We have determined that
existing regulations for a fuel-fed external fire are adequate to
address cabin interiors, including those issues suggested by the
commenter, and special conditions are not warranted. In addition, while
full scale fire tests of the wing and fuselage were considered by the
FAA, we determined that requiring a large scale fire test could be
overly prescriptive. The means of complying with the objectives of
these special conditions will be reviewed and approved by the FAA. In
addition, although the performance standards for the wing and fuselage
were developed independently, they have a common objective of
preserving the current level of safety provided by aluminum airplanes.
After reviewing this comment, we have determined that no change to the
special conditions is required.
Comment 7. This commenter has noted that burn-through tests at the
component level do not address high lateral fire burning rates or fire
and smoke ingress into the cabin. The commenter suggested testing
should be expanded to include a full scale fire test of a fuselage
barrel section with all exits opened and slides deployed throughout the
test.
FAA Response. The FAA has determined that the requirements for the
smoke, toxicity, and fire resistance of the fuselage materials are
adequately addressed by the current regulations and, therefore,
inclusion in these special conditions is unwarranted. The intent and
scope of these special conditions was to ensure that the wing and fuel
tank structure will not pose an additional hazard to passengers and
crew during postcrash fire scenarios because of the introduction of
composite materials. Cabin safety special conditions have been
developed and published for comment in Special Conditions No. 25-07-09-
SC, Docket No. NM373, published April 26, 2007 (72 FR 20774). Those
special conditions require that the 787 provide the same level of in-
flight survivability as a conventional aluminum fuselage airplane. This
includes its thermal/acoustic insulation meeting requirements of Sec.
25.856(a). Those special conditions state that resistance to flame
propagation must be shown, and all products of combustion that may
result must be evaluated for toxicity and found acceptable.
We have made no changes to these special conditions as a result of
this comment.
Comment 8. Another commenter provided extensive background
information on the current level of safety provided by the
crashworthiness of aluminum transport category airframes. This
commenter expressed concern that the introduction of a composite
fuselage will reduce the crashworthiness of transport airplanes. The
commenter further requested that we impose a fuselage drop test for the
787 to ensure that the current level of safety provided by an aluminum
fuselage is provided by the composite materials used in the
construction of the 787 fuselage.
FAA Response: We would like to note that the scope of these special
conditions is limited to the fire safety provisions of the fuel tanks
and wing structure during a fuel-fed ground fire. These special
conditions are not intended to address the structural crashworthiness
of the airframe. We have considered the impact of composites on
airframe crashworthiness and have proposed Special Conditions 25-07-05-
SC, published on June 11, 2007, in the Federal Register (72 FR 32021).
As stated in those special conditions, ``The Boeing Model 787-8 must
provide an equivalent level of occupant safety and survivability to
that provided by previously certificated wide-body transports of
similar size under foreseeable survivable impact events for the
following four criteria. In order to demonstrate an equivalent level of
occupant safety and survivability, the applicant must demonstrate that
the
[[Page 57848]]
Model 787-8 meets the following criteria for a range of airplane
vertical descent velocities up to 30 ft/sec * * *'' The FAA considers
that proposed Special Conditions 25-07-05-SC adequately addresses the
commenter's concerns for crashworthiness and we note that the commenter
had opportunity to submit comments to that proposal as well. We have
made no changes to these special conditions as a result of this
comment.
Applicability
As discussed above, these special conditions are applicable to the
787. Should Boeing apply at a later date for a change to the type
certificate to include another model on the same type certificate
incorporating the same novel or unusual design features, these special
conditions would apply to that model as well.
Conclusion
This action affects only certain novel or unusual design features
of the 787. It is not a rule of general applicability.
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 the Boeing Model 787-8 airplane.
In addition to complying with 14 CFR part 25 regulations
governing the fire-safety performance of the fuel tanks, wings, and
nacelle, the Boeing Model 787-8 must demonstrate acceptable
postcrash survivability in the event the wings are exposed to a
large fuel-fed ground fire. Boeing must demonstrate that the wing
and fuel tank design can endure an external fuel-fed pool fire for
at least 5 minutes. This shall be demonstrated for minimum fuel
loads (not less than reserve fuel levels) and maximum fuel loads
(maximum range fuel quantities), and other identified critical fuel
loads. Considerations shall include fuel tank flammability, burn-
through resistance, wing structural strength retention properties,
and auto-ignition threats during a ground fire event for the
required time duration.
Issued in Renton, Washington, on September 28, 2007.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. E7-20031 Filed 10-10-07; 8:45 am]
BILLING CODE 4910-13-P