The F-35 Joint Program Office has cleared F-35 fighters for flight operations, but the US Air Force’s test jets at Eglin Air Force Base, Florida, US, will remain grounded until continuing investigations are completed.

The Eglin test F-35s are under investigation following the AF-4’s integrated power package (IPP) failure, which led to the grounding of the aircraft fleet.

A release from the F-35 Joint Program Office states, “The root cause investigation indicates that an IPP valve did not function properly.

“Monitoring of valve position is a mitigating action to allow monitored operations. A permanent resolution is in work.”

The F-35 IPP is a turbo-machine that provides power to start the engine and generates cooling for the aircraft, while AF-4 is the fourth conventional take-off and landing variant of the F-35 aircraft produced by Lockheed Martin.