WASHINGTON — The U.S. regulator has authorized specific requirements for the A321XLR following concerns that a new type of fuel tank used at the Airbus' latest-generation long-haul narrow-body jet could pose a fire risk.

In an internal document issued by the U.S. Federal Aviation Administration, the regulator stated that it would mandate that the lower half of the A321XLR fuselage, which spans the longitudinal region of the tank, be resistant to fire penetration.

Airbus designed the new tank to extend the flight range of the A321XLR, the latest and longest-range variant of the A320neo Family. The new aircraft has an additional fuel tank that is installed under the fuselage rather than the wings. Unlike the design of other passenger planes, it is located in an area of the lower fuselage that partly replaces the rear cargo container. FAA's document sheds further light on the specifications of the new tank.

According to industry sources, question marks in regulators' minds over how to deal with this novel fuel tank put the aircraft's delivery schedule in an ambiguous state. Airbus plans to begin delivering the aircraft to its customers from the second quarter of 2024.

"While discussions with the airworthiness authorities are still ongoing, we are not in a position to comment during the public consultation period," an Airbus spokesperson said.

The Agency said the current directives do not propose requirements for post-crash fire protections in this type of design and special provisions are needed in accordance with the existing safety standards.

In May, the regulatory assessment forced Airbus to push back the A321XLR's entry of service from late 2023 to early 2024.

Experts say potential design changes to be mandated by regulators could reduce the plane's introduced range, which is the manufacturer's main selling point.

Airbus A321XLR made its maiden flight on June 15. 2022. The aircraft has a flight range of 4700 NM with a 180-220 seat two-class cabin configuration. The A321XLR can enable airlines to launch new long-haul routes such as New York - Rome, London - Vancouver, Delhi - London, Sydney - Kuala Lumpur, with a single-aisle jet rather than a widebody aircraft 

The aircraft registered orders over 500 copies from 22 customers, including American Airlines, United Airlines, Air Canada and Qantas. Lebanon's  Middle East Airlines (MEA) is the launch customer of the type.