Notice of proposed special conditions.
CFR Part: "14 CFR Part 25"
Citation: "79 FR 1334"
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
ADDRESSES: Send comments identified by docket number
* 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,
* Hand Delivery or Courier: Take comments to Docket Operations in Room W12-140 of the West Building Ground Floor at
* Fax: Fax comments to Docket Operations at 202-493-2251.
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
FOR FURTHER INFORMATION CONTACT:
SUPPLEMENTARY INFORMATION: 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.
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
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
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,
As with previous
FOOTNOTE 1 Hill, R., and Johnson, G.R., "Investigation of Aircraft Fuel Tank Explosions and Nitrogen Inerting Requirements During Ground Fires," FAA Report DOT/
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,
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
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
The FAA issues special conditions, as defined in 14 CFR 11.19, under
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
Novel or Unusual Design Features
The Airbus Model A350-900 series airplane will incorporate the following novel or unusual design features: composite fuel tanks.
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,
FOOTNOTE 2 Cherry, R. and Warren, K. "Fuselage Burnthrough Protection for Increased Postcrash Occupant Survivability: Safety Benefit Analysis Based on Past Accidents, "FAA Report DOT/
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,
As discussed above, these proposed special conditions apply to Airbus Model A350-900 series airplanes. Should
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.
[FR Doc. 2014-00102 Filed 1-7-14;
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