FAA Proposes Safety Requirements for Airbus A321XLR Amid External Fuel Fire Concerns
The FAA is addressing concerns that passengers and crew members would not be able to safely evacuate an Airbus A321XLR.
By Rytis Beresnevicius
The Federal Aviation Administration (FAA) has issued a notice of proposed special conditions addressing the integral rear center tank (RCT) of the A321XLR. According to the regulator, the aircraft will have an unusual design feature compared to the airworthiness standards for commercial aircraft.
Concerning the integrated fuel tank
The FAA detailed that Airbus has proposed that the A321XLR have an integral RCT that would be located in the fuselage rather than the wings of the aircraft. Its position would be aft of the center wing fuel tank and behind the wheel bay, partially replacing the aft cargo compartment of the A321neo.
The top of the tank would be directly below the floor of the cabin, while the fuel tank’s walls would be part of the aircraft’s structure. However, the exterior skin would also be part of the walls of the fuel tank, which the FAA pointed out are typically separate boundaries on other fuselage fuel tanks.
“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’.”
As such, the airworthiness standards applicable to the A321XLR do not contain specific post-crash fire-safety performance standards of the fuel tank skin or structure. Furthermore, the FAA did not consider an integral RCT when drawing up requirements related to passenger protection when fuel tanks are exposed to external fuel-fed fires.
Novel design
The regulator noted that it considered designs widely used in commercial aircraft, including auxiliary fuel tanks, when creating airworthiness standards. The additional tanks are located in the center wing and within cargo holds, and they are referred to as auxiliary center tanks (ACT).
However, the A321XLR will have the integral RCT and optional ACTs. Still, the FAA is concerned about the RCT’s exposure to the direct effects of post-crash ground fire since its walls are part of the external fuselage skin. Furthermore, the unusual configuration will result in the integral RCT not using insulation, usually lining the fuselage of a commercial aircraft.
To address safety concerns, the FAA issued a set of special conditions, following a public consultation, requiring Airbus to ensure that the lower half of the A321XLR’s fuselage, spanning the longitudinal location of the RCT, would be resistant to penetration from an external fuel-fed fire. The newly proposed standards address a different flammability aspect of the integral RCT.
Passenger and crew survivability
The FAA noted that post-crash-fire passenger and crew survivability depends on the time available to evacuate an aircraft before a fuel-tank breach or structural failure. The latter can happen because of either a fuel-fed ground fire or over-pressurization caused by the ignition of fuel vapors inside the fuel tank.
The regulator’s experience has shown that survivability is greatly affected by the size and intensity of a fire. The main tanks can dissipate heat across wetted aluminum fuel tank surfaces, preventing localized hot spots from occurring, for example.
“The Model A321neo XLR integral RCT may or may not have equivalent capability of past designs approved with existing regulations, due to the RCT design and location being integral with the fuselage.”
According to the FAA, while there are airworthiness standards to address the fire safety performance of the fuel tanks and fuselage of the A321XLR, they do not directly affect the performance of the fuel tank skin or fuselage during a post-crash fire. The regulator highlighted two Part 25 sections, which address ignition sources but do not solve problems related to a post-crash external ground fire.
Special conditions proposed
As a result, the regulator said it was obliged to propose special conditions that would prevent fuel vapor ignition during an external fuel-fed ground fire. These conditions include considering the potential for hot surface ignition created by an external flame.
“Factors influencing occupant survival time when a fuel tank is exposed to a ground-fed fire are the structural integrity of the tank; burn-through resistance; flammability of the tank; and the presence of auto-ignition threats during exposure to a fire.”
If Airbus proves that the integral RCT’s design prevents fuel vapors from igniting inside the fuel tank for at least five minutes, the FAA will be satisfied with the tank’s integrity. The regulator said that five minutes would be consistent with its studies, which showed that the time period would enable passengers and crew members to evacuate the aircraft.
In addition, the FAA suggested that the European plane maker could consider a flammability reduction system or ignition mitigation system to comply with its requirements. However, the regulator emphasized that Airbus must meet the special conditions the FAA proposed on May 7.
The regulator added that the intent was for the applicant – Airbus – to show that ignition sources do not occur due to the external heat applied to the integral RCT from an external fire. As such, Airbus would have to prove that the A321XLR has sufficient post-crash fire safety performance of the fuel tank skin or structure to enable passengers and/or crew to evacuate safely if an integral RCT is exposed to an external fuel-fed ground fire.
