Flight operations of the F-35B short takeoff/vertical landing (Stovl) variant of the Joint Strike Fighter remained suspended this week as U.S. military and contractor engineering teams investigated the cause of an engine fueldraulic line failure in a test aircraft at Eglin Air Force Base, Fla. The fueldraulic line is part of the fuel-based hydraulic system that controls the actuators of the F-35B’s vectoring exhaust system. The grounding did not affect the F-35A and C models, respectively, the conventional takeoff and carrier variants.

The F-35 Joint Program Office in Arlington, Va., imposed the “precautionary” flight suspension for development and test aircraft on January 18. The Naval Air Systems Command also suspended flights of production F-35Bs at Eglin AFB, the Marine Corps Air Station at Yuma, Ariz., and at manufacturer Lockheed Martin’s production facility in Fort Worth, Texas. The suspension affected 25 F-35Bs; static ground operations continued.

Two days earlier, a pilot initiating a conventional takeoff in test aircraft BK-1 aborted the flight after receiving an engine warning in the cockpit. BK-1 is the first operational test and evaluation aircraft ordered by the UK military. Lockheed Martin formally delivered the aircraft to the UK Ministry of Defence last July in Fort Worth; it was then flown to Eglin AFB for testing.

Following the aborted takeoff, an initial inspection “discovered a detached fueldraulic line in the aft portion of the engine compartment near the bearing swivel module,” according to Pratt & Whitney, manufacturer of the aircraft’s F135 turbofan engine. Rolls-Royce provides the three-bearing swivel module, a swiveling pipe that redirects the rear thrust from the horizontal to the vertical direction, as part of its integrated lift system for the Stovl variant. The Stratoflex division of Parker Aerospace supplies the fueldraulic line.

The fueldraulic system was identified as an F-35 “vulnerability” in the 2012 annual report of the Pentagon’s Office of the Director, Operational Test and Evaluation (OT&E), submitted to Congress on January 11. The system “introduces a significant amount of fuel plumbing to the aft end of the engine and, consequently, an increased potential for fire,” according to the OT&E report. The office said fueldraulic “flow fuses” that serve to cut off fuel flow when a leak is detected were removed from the aircraft’s production design to reduce weight. It recommends that the F-35 program reconsider the removal of those fuses.