Document ID: FAA-2013-0908-0002
Agency: faa
Document Type: Rule
Title: Special Conditions: Airbus Model A350-900 Airplanes; Level of Safety Provided by Composite Fuel Tank Structure - Post-Crash Fire Survivability
Posted Date: 2014-08-21T04:00Z

[Federal Register Volume 79, Number 162 (Thursday, August 21, 2014)]
[Rules and Regulations]
[Pages 49429-49431]
From the Federal Register Online via the Government Printing 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