An “improperly crimped” fueldraulics line is the probable cause of a fuel leak that led the U.S. military to suspend flight operations of the F-35B short takeoff/vertical landing (Stovl) variant of the Joint Strike Fighter, which was grounded on January 18. The F-35 Joint Program Office (JPO) announced the probable-cause finding late Monday afternoon and said steps are being taken to return the aircraft to flight. The flight suspension affected 25 of the Stovl variants.

“The root cause investigation ruled out any design or maintenance issues,” the JPO said. “Evidence revealed a quality discrepancy from the company that produces the fueldraulics line. The investigation determined the line was improperly crimped.” An audit of quality control records revealed six additional “non-compliant” units, which were removed from F-35Bs and returned to engine manufacturer Pratt & Whitney. The Stratoflex division of Parker Aerospace manufactures the fueldraulics line under subcontract to Pratt & Whitney.

The JPO said that Stratoflex, Pratt & Whitney and Rolls-Royce, which provides the integrated engine lift system for vertical thrust, “have instituted corrective actions to improve their quality control processes and ensure part integrity.”

The fueldraulics line involved is specific to the F-35B and drives the actuator movement of the Stovl variant’s vectoring exhaust system. It uses fuel as the operating fluid instead of traditional hydraulic fluid to save weight. On January 16, a pilot beginning a conventional takeoff in an F-35B at Eglin Air Force Base, Fla., aborted the takeoff after receiving an engine warning in the cockpit. The initial inspection found a detached fueldraulic line in the aft portion of the engine compartment.

A military and contractor engineering team “determined the most probable cause of the leak is a quality nonconformance introduced during the production of the fueldraulic tube, manufactured by Stratoflex,” Pratt & Whitney said in a statement on Monday. “We have begun the process of removing the suspect fueldraulics tubes from the Stovl aircraft, and we are performing additional X-ray imaging inspections on the tubes to ensure their integrity. The team continues to work diligently toward completing the investigation and implementing corrective actions with the supplier. We anticipate a return to flight for the Stovl variant soon.”