A “quality nonconformance” in a fueldraulic tube is “the most probable cause” for an operational failure that prompted the U.S. Marine Corps to ground its young fleet of F-35Bs since Jan. 16, according to officials at Pratt & Whitney.

Though the tubes are manufactured by Stratoflex, the F135 power plant for the single-engine, stealthy fighter is managed by Pratt & Whitney. The problem was most likely introduced “during the production of the fueldraulic tube,” says Matthew Bates, a spokesman for Pratt & Whitney.

The Marine Corps grounded all 25 flyable F-35Bs delivered from prime contractor Lockheed Martin after a fueldraulic line was found to be “detached” when a pilot aborted a conventional takeoff in an F-35B after experiencing a failure. This has brought a halt to flight testing for the short-takeoff-and-vertical-landing (Stovl) fighter as well as pilot training, both of which are critical for the Marine Corps to declare initial operational capability with the aircraft.

Engine experts are now performing X-ray imaging inspections of tubes in the fleet “in order to ensure their integrity,” Bates says. “We anticipate a return to flight for the Stovl variant soon.”

Only the U.S. and the U.K. are currently slated to purchase this variant.