Document ID: FAA-2013-0908-0001
Agency: faa
Document Type: Proposed Rule
Title: Special Conditions: Airbus, Model A350-900 Series Airplane; Airplane Level of Safety Provided by Composite Fuel Tank Structure: Post-Crash Fire Survivability
Posted Date: 2014-01-08T05:00Z

[Federal Register Volume 79, Number 5 (Wednesday, January 8, 2014)]
[Proposed Rules]
[Pages 1334-1336]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2014-00102]

 ========================================================================
 Proposed Rules
                                                 Federal Register
 ________________________________________________________________________
 
 This section of the FEDERAL REGISTER contains notices to the public of 
 the proposed issuance of rules and regulations. The purpose of these 
 notices is to give interested persons an opportunity to participate in 
 the rule making prior to the adoption of the final rules.
 
 ========================================================================
 

  Federal Register / Vol. 79, No. 5 / Wednesday, January 8, 2014 / 
Proposed Rules  

[[Page 1334]]

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

Docket No. FAA-2013-0908; Notice No. 25-13-24-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: Notice of proposed special conditions.

-----------------------------------------------------------------------

SUMMARY: This action proposes special conditions for Airbus Model A350-
900 series airplanes. These airplanes will have a novel or unusual 
design feature(s) 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 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.

DATES: Send your comments on or before February 24, 2014.

ADDRESSES: Send comments identified by docket number FAA-2013-0908 
using any of the following methods:
     Federal eRegulations Portal: Go to http://www.regulations.gov/ and follow the online instructions for sending 
your comments electronically.
     Mail: Send comments to Docket Operations, M-30, U.S. 
Department of Transportation (DOT), 1200 New Jersey Avenue SE., Room 
W12-140, West Building Ground Floor, Washington, DC, 20590-0001.
     Hand Delivery or Courier: Take comments to Docket 
Operations in Room W12-140 of the West Building Ground Floor at 1200 
New Jersey Avenue SE., Washington, DC, between 8 a.m. and 5 p.m., 
Monday through Friday, except federal holidays.
     Fax: Fax comments to Docket Operations at 202-493-2251.
    Privacy: The FAA will post all comments it receives, without 
change, to http://www.regulations.gov/, including any personal 
information the commenter provides. Using the search function of the 
docket Web site, anyone can find and read the electronic form of all 
comments received into any FAA docket, including the name of the 
individual sending the comment (or signing the comment for an 
association, business, labor union, etc.). DOT's complete Privacy Act 
Statement can be found in the Federal Register published on April 11, 
2000 (65 FR 19477-19478), as well as at http://DocketsInfo.dot.gov/.
    Docket: Background documents or comments received may be read at 
http://www.regulations.gov/ at any time. Follow the online 
instructions for accessing the docket or go to the Docket Operations in 
Room W12-140 of the West Building Ground Floor at 1200 New Jersey 
Avenue SE., Washington, DC, between 9 a.m. and 5 p.m., Monday through 
Friday, except federal holidays.

FOR FURTHER INFORMATION CONTACT: Doug Bryant, Propulsion/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:

Comments Invited

    We invite interested people to take part in this rulemaking by 
sending 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 will consider all comments we receive on or before the closing 
date for comments. We may change these proposed special conditions 
based on the comments we receive.

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 
June 28, 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 will accommodate 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. Airbus 
proposes the Model A350-900 series airplane to be certified for 
extended operations (ETOPS) beyond 180 minutes at entry into service 
for up to a 420-minute maximum diversion time.
    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 internal to 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

[[Page 1335]]

fire resistant as follows: ``With respect to sheet or structural 
members means the capacity to withstand the heat associated with fire 
at least as well as aluminum alloy in dimensions appropriate for the 
purpose for which they are used.'' It is noteworthy that aluminum alloy 
is identified as the performance standard for fire resistance, though 
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 promulgated rules with the assumption that 
the material of construction for wing and fuselage would be aluminum. 
As a representative case, Sec.  25.963 was promulgated 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 there should be a 
minimum requirement 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 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. Hughes 
Technical Center.\1\
---------------------------------------------------------------------------

    \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/.
---------------------------------------------------------------------------

    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.
    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 14 part 1 and AC 20-135.
     Heat capacity of aluminum and fuel will prevent burn-
through or wing collapse for a time interval that will generally exceed 
the passenger evacuation time.

Type Certification Basis

    Under Title 14, Code of Federal Regulations (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 proposed 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 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 and the FAA must issue a 
finding of regulatory adequacy under Sec.  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 features: composite fuel tanks.

Discussion

    The extensive use of composite materials in the design of the A350 
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 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.
    In order 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

[[Page 1336]]

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\
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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.

Applicability

    As discussed above, these proposed 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 proposed special 
conditions would apply to that model as well under the provisions of 
Sec.  21.101.

Conclusion

    This action affects only certain novel or unusual design features 
on the Airbus Model A350-900 series airplanes. It is not a rule of 
general applicability.

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, 44704.

The Proposed Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are proposed as part of 
the type certification basis for the Model A350-900 series airplane:
    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 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 October 22, 2013.
Stephen P. Boyd
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2014-00102 Filed 1-7-14; 8:45 am]
BILLING CODE 4910-13-P