UPDATE – The US Federal Aviation Administration (FAA) has set design requirements for one of the unique features of the Airbus A321XLR: the Rear Center Tank (RCT). Reuters reports today that the FAA has formulated special conditions in an advisory circular as part of the public consultation procedure for the A321XLR. FAA sets design requirements for Airbus A321XLR tank.

The Rear Center Tank has been the topic of much debate after some – notably Boeing – raised concerns about its safety. Thanks to the 12.900 liters in the RCT, the A321XLR has a range of 4.700 nautical miles/8.700 kilometers. The concerns focus on the safety of the tank, which is positioned just aft of the main landing gear in the belly hold. Critics said that the close proximity to the landing gear could pose a risk in case of a gear failure and penetration. The RCT’s position right under the cabin floor is another concern. After the FAA published the Notice of Proposed Special Conditions for the A321XLR in April, Boeing filed four comments about the safety of the tank. These have been reviewed and resulted into the Advisory Circular, which will become rule from January 5.

‘This tank design was not envisaged by FAA’

The FAA Advisory Circular of December 6 offers more insight into the debate about safety. In the document, the FAA says that “the fuel tank will be “integral” to the airplane, in that its walls will be part of the airplane structure. The exterior skin of the airplane fuselage will constitute part of the walls of the fuel tank, and these areas will lack the thermal/acoustic insulation that usually lines the exterior skin of an airplane fuselage. This design was not envisaged by the FAA’s regulatory requirements for insulation installations on transport-category airplanes. Regulation 14 CFR 25.856 (b) requires all thermal/acoustic insulation in the lower half of the airplane fuselage and their installation to comply with the flame penetration resistance (…) to raise the level of post-crash fire safety on transport category airplanes.”

As a consequence, “the FAA will require that the lower half of the airplane fuselage, spanning the longitudinal area of the tank, be resistant to fire penetration. “Resistant to fire penetration” will, for this special condition, mean that this area provides fire penetration resistance equivalent to the resistance which would be provided if the fuselage were lined with thermal/acoustic insulation that meets the flame penetration resistance test requirements.”

According to the FAA, Airbus has stated that “its design does not allow for compliant thermal/acoustic insulation to be placed beneath the cabin floor. This large volume of unheated liquid (fuel), directly below the floor of the passenger cabin, would, without mitigation, create a ‘cold feet’ effect for the passengers above it. Therefore, Airbus will install insulation panels between the fuel tank and the cabin floor, for comfort reasons. (…) Airbus states that it is technically not feasible to install thermal/acoustic insulation that complies with regulation §25.856(b), due to the lack of space in this area and the need to keep nearby decompression panels free of blockages and ensure adequate ventilation.”

The Rear Center Tank, seen here during the assembly of the first A321XLR. (Airbus)

In one of its comments, Boeing requested that the special conditions require the RCT fire penetration resistance capability to be equivalent to the capability provided by the wing box area, where additional fuel tanks are usually positioned. Boeing said that “the FAA’s proposed standard of fire penetration resistance equivalent to that of a fuselage lined with thermal/acoustic insulation that meets the flame penetration resistance test requirements does not address hazards associated with fuel tanks and is not applicable to the wing box area.” The FAA rejected this suggestion, stating that “these special conditions are not intended to ensure the RCT is constructed to provide a fire penetration resistance capability that is similar to that of the wing box area.”

Constant dialogue

Airbus has been in constant dialogue with the European regulatory agency EASA about the RCT for over three years, but neither the airframer nor the agency is willing to say if this dialogue has resulted in design changes or other modifications to the tank. Head of Program, Gary O’Donnell, said on the occasion of the XLR’s first flight on June 15 that the RCT will be closely monitored during the flight test program, which currently includes three test aircraft. Earlier this year, Airbus pushed the certification of the XLR out by some months until early 2024 following “certification requirements” from regulators. One of its main customers, Air Lease Corporation (ALC) Chairman Steven Udvar-Hazy, told AirInsight in June that the lessor has also suggested various modifications to Airbus to make the tank safer.

One of the options to improve the protection of the tank is by adding a Kevlar lining around the aluminum tank. The weight penalty of this lightweight material could be limited, which is crucial because every extra kilogram will eat into the maximum range of the A321XLR. This is not in the interest of Airbus, as the extended range is the type’s unique selling point to which Boeing has no answer with the MAX family. The A321XLR has won over 500 orders. Entry into service is not expected before the second quarter of 2024.

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Active as a journalist since 1987, with a background in newspapers, magazines, and a regional news station, Richard has been covering commercial aviation on a freelance basis since late 2016.
In 2022, he has gone full-time freelance. Richard has been contributing to AirInsight since December 2018. He is also writing for Airliner World and Aviation News. From January 2023, he will add a part-time role with Dutch website and magazine Luchtvaartnieuws. Twitter: @rschuur_aero.

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