Special Conditions: Airbus Model A321neo Extra-Long Range (XLR) Airplane; Cabin Evacuation-Protection From Fuel Tank Explosion Due to External Fuel-Fed Ground Fire

<RULE> DEPARTMENT OF TRANSPORTATION <SUBAGY>Federal Aviation Administration</SUBAGY> <CFR>14 CFR Part 25</CFR> <DEPDOC>[Docket No. FAA-2023-2412; Special Conditions No. 25-868-SC]</DEPDOC> <SUBJECT>Special Conditions: Airbus Model A321neo Extra-Long Range (XLR) Airplane; Cabin Evacuation—Protection From Fuel Tank Explosion Due to External Fuel-Fed Ground Fire</SUBJECT> <HD SOURCE="HED">AGENCY:</HD> Federal Aviation Administration (FAA), DOT. <HD SOURCE="HED">ACTION:</HD> Final special conditions. <SUM> <HD SOURCE="HED">SUMMARY:</HD> These special conditions are issued for the Airbus Model A321neo XLR airplane. This airplane will have a novel or unusual design feature when compared to the technology envisaged by the airworthiness standards for transport category airplanes. This design feature is an integral rear center tank (RCT). The applicable airworthiness regulations do not contain adequate or appropriate safety standards for fire-safety performance of fuel-tank skin or structure in a post-crash external fuel-fed ground fire. 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. </SUM> <EFFDATE> <HD SOURCE="HED">DATES:</HD> Effective July 18, 2024. </EFFDATE> <FURINF> <HD SOURCE="HED">FOR FURTHER INFORMATION CONTACT:</HD> Douglas Bryant, Engine and Propulsion Section, AIR-625, Technical Policy Branch, Policy and Standards Division, Aircraft Certification Service, Federal Aviation Administration, 2200 South 216th Street, Des Moines, Washington 98198; telephone and fax 206-231-3166; email <E T="03">douglas.n.bryant@faa.gov.</E> </FURINF> <SUPLINF> <HD SOURCE="HED">SUPPLEMENTARY INFORMATION:</HD> <HD SOURCE="HD1">Background</HD> On September 16, 2019, Airbus applied for an amendment to Type Certificate No. A28NM to include the new Model A321neo XLR series airplane. The Airbus Model A321neo XLR series airplane, which is a derivative of the Model A321neo Airbus Cabin Flex (ACF) currently approved under Type Certificate No. A28NM, is a twin-engine transport category aircraft that seats up to 244 passengers and has a maximum takeoff weight of 222,667 lbs. <HD SOURCE="HD1">Type Certification Basis</HD> Under the provisions of title 14, Code of Federal Regulations (14 CFR) 21.101, Airbus must show that the Model A321neo XLR series airplane meets the applicable provisions of the regulations listed in Type Certificate No. A28NM, or the applicable regulations in effect on the date of application for the change, except for earlier amendments as agreed upon by the FAA. If the Administrator finds that the applicable airworthiness regulations ( <E T="03">e.g.,</E> 14 CFR part 25) do not contain adequate or appropriate safety standards for the Airbus Model A321neo XLR series airplane because of a novel or unusual design feature, special conditions are prescribed under the provisions of § 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 novel or unusual design feature, or should any other model already included on the same type certificate be modified to incorporate the same novel or unusual design feature, these special conditions would also apply to the other model under § 21.101. In addition to the applicable airworthiness regulations and special conditions, the Airbus Model A321neo XLR series airplane must comply with the fuel venting and exhaust emission requirements of 14 CFR part 34 and the noise certification requirements of 14 CFR part 36. The FAA issues special conditions, as defined in 14 CFR 11.19, in accordance with § 11.38, and they become part of the type certification basis under § 21.101. <HD SOURCE="HD1">Novel or Unusual Design Features</HD> The Airbus Model A321neo XLR series airplane will incorporate the following novel or unusual design feature: An integral RCT. <HD SOURCE="HD1">Discussion</HD> The Airbus Model A321neo XLR series airplane incorporates an integral RCT. This tank is a “center” fuel tank, that would, if approved, be located in the airplane fuselage rather than in its wings. The tank is a “rear” tank, that would be located aft of the center wing fuel tank and behind the wheel bay; it would be in an area of the lower section of the fuselage, partially replacing the aft cargo compartment of the airplane from which this model is derived. The top of the tank would be directly below the floor of the passenger cabin. The fuel tank would be “integral” to the airplane, in that its walls would be part of the airplane structure. The exterior skin of the airplane fuselage would constitute part of the walls of the fuel tank, and these areas are usually separate boundaries (not integral) on other fuselage fuel tanks. An integral fuel tank may be referred to as a conformal fuselage structural fuel tank since boundaries of the fuel tank “conform” with the airplane exterior. The integral RCT is installed in a location that may be exposed to the direct effects of post-crash ground, or pool, fuel-fed fires. An external fuel-fed ground fire or external fuel-fed pool fire is also referred to as `external ground fire'. The airworthiness standards applicable to the Model A321neo XLR do not contain specific standards for post-crash fire-safety performance of fuel-tank skin or structure. In addition, the integral RCT on the A321neo XLR was not envisaged by the FAA when promulgating requirements related to occupant protection when fuel tanks are exposed to external fuel-fed fires. The FAA considered fuel tank designs in widespread use on transport airplanes, including main fuel tanks and auxiliary fuel tanks when promulgating requirements related to occupant protection. Auxiliary fuel tanks are normally located in the center wing and within cargo holds, and in such cases are sometimes referred to as an auxiliary center tank (ACT). Airplane manufacturers commonly incorporate a center wing fuel tank as an auxiliary fuel tank to make fuel available for increasing the flight range of the airplane. Continued expansion of range performance requirements has resulted in airplane designs using other areas of the airplane to carry fuel, such as incorporating fuel tanks in the empennage and fuselage. The Airbus model A321neo XLR airplane includes a center wing fuel tank, an integral RCT and the option for additional ACTs within the fuselage. Unlike an integral RCT, a center wing fuel tank and optional ACTs are not expected by the FAA or manufacturers to be exposed to the direct effects of post-crash ground fire because the fuel tank walls are not exterior airplane skin on the center fuel tank or ACT designs. Due to its unusual configuration, the A321neo XLR's integral RCT will also not incorporate the insulation that usually lines the fuselage skin of a modern transport category airplane. Therefore, the FAA has issued, after notice and comment, a set of special conditions that address that novel or unusual aspect of the A321neo XLR's integral RCT with regard to certain of the FAA's regulatory requirements for thermal/acoustic insulation installations, specifically 14 CFR 25.856(b). Those special conditions, No. 25-825-SC, require that the lower half of the fuselage spanning the longitudinal location of the RCT resist penetration from an external fuel-fed fire, to ensure that the design provides the same level of passenger protection from such fires as do the FAA's existing regulations for such insulation. The special conditions herein address a different flammability aspect of the A321neo XLR's integral RCT. Pertinent to the fuel tank structure, post-crash-fire occupant survivability is dependent on the time available for occupant evacuation prior to fuel-tank breach or structural failure. Structural failure can be a result of degradation in load-carrying capability 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. Past experience indicates that occupant survivability following a post-crash fire is greatly influenced by the size and intensity of any fire that occurs. The ability of main fuel tanks, when they have 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. 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 aircraft's lower surface, which is also the fuel tank boundary, to retain its load-carrying characteristics during a fuel-fed ground fire, and significantly delays structural collapse or burn-through for a time interval that usually exceeds evacuation times. In addition, as an aluminum fuel tank with significant quantities of fuel inside is heated, 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. The center wing tank and optional ACTs are surrounded by fuselage structure and would not be directly exposed to a post-crash ground fire. This inherent separation is also expected to significantly delay structural collapse or burn-through and reduce the threat of explosion for a time interval that usually exceeds evacuation times. 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. The Model A321ne ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Preview showing 10k of 30k characters. Full document text is stored and available for version comparison. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━