[Federal Register Volume 79, Number 162 (Thursday, August 21, 2014)]
[Rules and Regulations]
[Pages 49429-49431]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2014-19823]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. FAA-2013-0908; Special Conditions No. 25-538-SC]
Special Conditions: Airbus Model A350-900 Series Airplane;
Airplane Level of Safety Provided by Composite Fuel-Tank Structure:
Post-Crash Fire Survivability
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final special conditions.
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SUMMARY: These special conditions are issued for Airbus Model A350-900
series airplanes. These airplanes will have a novel or unusual design
feature associated with the post-crash fire survivability of composite
fuel tanks. The applicable airworthiness regulations do not contain
adequate or appropriate safety standards for this design feature. These
special conditions contain the additional safety standards that the
Administrator considers necessary to establish a level of safety
equivalent to that established by the existing airworthiness standards.
DATES: Effective date: September 22, 2014.
FOR FURTHER INFORMATION CONTACT: Doug Bryant, Propulsion and Mechanical
Systems, ANM-112, Transport Airplane Directorate, Aircraft
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone (425) 227-2384; facsimile (425) 227-1320.
SUPPLEMENTARY INFORMATION:
Background
On August 25, 2008, Airbus applied for a type certificate for their
new Model A350-900 series airplane. Later, Airbus requested, and the
FAA approved, an extension to the application for FAA type
certification to November 15, 2009. The Model A350-900 series airplane
has a conventional layout with twin wing-mounted Rolls-Royce Trent XWB
engines. It features a twin-aisle, 9-abreast, economy-class layout, and
accommodates side-by-side placement of LD-3 containers in the cargo
compartment. The basic Model A350-900 series airplane configuration
accommodates 315 passengers in a standard two-class arrangement. The
design cruise speed is Mach 0.85 with a maximum take-off weight of
602,000 lbs.
The Model A350-900 series airplane will be the second large,
transport-category airplane certificated with composite wing and fuel-
tank structure that may be exposed to the direct effects of post-crash
ground, or under-wing, fuel-fed fires. Although the FAA has previously
approved fuel tanks made of composite materials located in the
horizontal stabilizer of some airplanes, the composite wing structure
of the Model A350-900 series airplane will incorporate 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.
Pertinent to the wing structure, post-crash-fire passenger
survivability is dependent on the time available for passenger
evacuation prior to 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 inside the fuel tank.
The inherent capability of aluminum to resist fire has been
considered by the FAA in development of the current regulations. Title
14, Code of Federal Regulations (14 CFR) part 25 Chapter 1, Section
1.1, General Definitions, defines ``fire resistant'' to mean, with
respect to sheet or structural members, the capacity to withstand heat
associated with fire at least as well as aluminum alloy does in
dimensions appropriate for the purpose for which those materials are
used.
Note that aluminum alloy is identified as the performance standard
for fire resistance, although no thickness or heat intensities are
defined. Based on the performance of aluminum alloy, the definition of
``fire resistance'' was later defined, for testing of other materials
in AC 20-135, as the capability to withstand a 2000 [deg]F flame for
five minutes.
The FAA has historically issued rules with the assumption that the
material of construction for wing and fuselage would be aluminum. As a
representative case, 14 CFR 25.963 was issued as a result 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, the
structures group attempted to harmonize Sec. 25.963 regarding the
impact-and-fire resistance of the fuel-tank access panels. Discussions
between the FAA and the European Aviation Safety Agency (EASA),
formerly the European Joint Aviation Authorities (JAA), ensued
regarding the need for fire resistance of the fuel-tank access panels.
The EASA position was that the FAA requirement for the access panels to
be fire resistant, when the surrounding wing structure was not required
to be fire resistant, was inconsistent, and that the access panels only
needed to be as fire resistant as the surrounding tank structure. The
FAA position stated that the fuel-tank access-panel fire-resistance
requirement should be retained, and that, long-term, a minimum
requirement should be created for the wing skin itself. Both
authorities recognized that existing aluminum wing structure provided
an acceptable level of safety. Further rulemaking has not yet been
pursued.
As with previous Airbus airplane designs with under-wing-mounted
engines, the wing tanks and center tanks are located in proximity to
the passengers and near the engines. Past experience indicates that
post-crash 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 post-
crash fire conditions, has been demonstrated by tests conducted at the
FAA William J.
[[Page 49430]]
Hughes Technical Center.\1\ Results of 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, and significantly delay
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 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.
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\1\ Hill, R., and Johnson, G.R., ``Investigation of Aircraft
Fuel Tank Explosions and Nitrogen Inerting Requirements During
Ground Fires,'' FAA Report DOT/FAA/RD-75-119, October 1975.
Available via the FAA Technical Center Web site for Fire Safety at
http://www.fire.tc.faa.gov/.
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Due to the inherent properties provided by aluminum skin and
structure, current regulations may not be adequate as they were
developed, and have evolved under the assumption that wing construction
would be of aluminum materials. Inherent properties of aluminum, with
respect to fuel tanks and fuel-fed fires, are as follows:
Aluminum is highly thermally conductive and 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, thus greatly reducing 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 1.1 and AC 20-135.
Heat capacity of aluminum and fuel prevents burn-through
or wing collapse for a time interval that generally exceeds the
passenger evacuation time.
Type Certification Basis
Under 14 CFR 21.17, Airbus must show that the Model A350-900 series
airplane meets the applicable provisions of 14 CFR part 25, as amended
by Amendments 25-1 through 25-129.
If the Administrator finds that the applicable airworthiness
regulations (i.e., 14 CFR part 25) do not contain adequate or
appropriate safety standards for the Model A350-900 series airplane
because of a novel or unusual design feature, special conditions are
prescribed under Sec. 21.16.
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.
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).
In addition to the applicable airworthiness regulations and special
conditions, the Model A350-900 series airplane 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 issue
a finding of regulatory adequacy under section 611 of Public Law 92-
574, the ``Noise Control Act of 1972.''
Novel or Unusual Design Features
The Airbus Model A350-900 series airplane will incorporate the
following novel or unusual design feature: Composite fuel tanks.
Discussion
The extensive use of composite materials in the design of the A350-
900 airplane wing and fuel-tank structure is considered a major change
from conventional and traditional methods of construction, as this will
be only the second large, transport-category airplane design to be
certificated with this level of composite material for these purposes.
The applicable airworthiness regulations do not contain specific
standards for post-crash fire-safety performance of wing and fuel-tank
skin or structure.
To provide the same level of safety as exists with conventional
airplane construction, Airbus must demonstrate that the Model A350-900
series airplane has sufficient post-crash 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 post-crash
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.\2\
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\2\ Cherry, R. and Warren, K. ``Fuselage Burnthrough Protection
for Increased Postcrash Occupant Survivability: Safety Benefit
Analysis Based on Past Accidents, ``FAA Report DOT/FAA/AR-99/57,
September 1999 and R G W Cherry & Associates Limited, ``A Benefit
Analysis for Cabin Water Spray Systems and Enhanced Fuselage
Burnthrough Protection,'' FAA Report DOT/FAA/AR-02/49, April 7,
2003.
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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 Model A350-900 series airplane
fuel tanks must be capable of resisting a post-crash fire for at least
5 minutes. In demonstrating compliance, Airbus must address a range of
fuel loads from minimum to maximum, as well as any other critical fuel
load.
These special conditions contain the additional safety standards
that the Administrator considers necessary to establish a level of
safety equivalent to that established by the existing airworthiness
standards.
Discussion of Comments
Notice of Proposed Special Conditions No. 25-13-24-SC for Airbus
Model A350-900 series airplanes was published in the Federal Register
on January 8, 2014 (79 FR 1334). No comments were received, and the
special conditions are adopted as proposed.
Applicability
As discussed above, these special conditions apply to Airbus Model
A350-900 series airplanes. Should Airbus apply later for a change to
the type certificate to include another model incorporating the same
novel or unusual design feature, the special conditions would apply to
that model as well under the provisions of Sec. 21.101.
[[Page 49431]]
List of Subjects in 14 CFR Part 25
Aircraft, Aviation safety, Reporting and recordkeeping
requirements.
The authority citation for these special conditions is as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701, 44702 and 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 Airbus Model A350-900 series
airplanes.
In addition to complying with 14 CFR part 25 regulations governing
the fire-safety performance of the fuel tanks, wings, and nacelle, the
Airbus Model A350-900 series airplane must demonstrate acceptable post-
crash survivability in the event the wings are exposed to a large fuel-
fed ground fire. Airbus must demonstrate that the wing and fuel-tank
design can endure an external fuel-fed pool fire for at least five
minutes. This must 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
must 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 August 1, 2014.
Jeffrey E. Duven,
Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 2014-19823 Filed 8-20-14; 8:45 am]
BILLING CODE 4910-13-P