The cause of a June engine fire involving an F-35 Lightning II has been determined, according to a joint report from the Department of Defense and engine supplier Pratt and Whitney, Military Times reports.

The fire, which grounded the military's fleet of the next-generation frontline fighter, was initially blamed on "microcracks" caused by flight maneuvers. A fix has been in the works, but PW and the DOD delayed the band-aid effort until the root cause of the problem could be discovered.

According to a new report filed by the F-35's Joint Program Office, the ultimate source of the microcracks is "prolonged rubbing into the material in the stator." Carrying on, the JPO's report claims the rubbing "decomposed and superheated the titanium rotor, leading to excessive heating which started very small cracks in a titanium seal and then led to failure of the third stage fan rotor."

The JPO is now working with Pratt and Whitney to develop a solution for both the planes currently in the field and for new engines being produced.

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