Investigators have found six additional fueldraulic lines with manufacturing defects on the Pentagon’s young F-35B fleet, which was grounded Jan. 18 after a takeoff incident at Eglin AFB, Fla.

A “quality nonconformance” in a fueldraulic tube was “the most probable cause” for the Jan. 18 operational failure, according to officials at Pratt & Whitney.

Though the tubes are manufactured by Stratoflex, a division of Parker Aerospace, 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 six additional “non-compliant” units have been removed from the aircraft and will be replaced, says Joe Dellavedova, the Pentagon’s F-35 spokesman. The fueldraulic lines carry fuel rather than traditional hydraulic fluid to enable actuator movements for the F-35B’s short-takeoff-and-vertical-landing (stovl) vectoring exhaust system.

The lines are part of the F135 power plant, which includes a unique lift fan for the stovl version. Rolls-Royce provides the lift fan.

At issue were tubes that were manufactured to the design specifications but were not crimped properly, Bates says. Dellavedova says the companies are taking steps to improve quality control in building the fueldraulic lines.

Inspectors will examine the lines on all of the 25 grounded Marine Corps F-35Bs.

The Marine Corps grounded all the 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 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.”

The Marine Corps and U.K. are slated to buy the B model and Italy has shown interest in the variant as well.