Lt. Gen. Christopher Bogdan spoke to the media at the ComDef conference earlier this week, confirming the cause of the issue and saying that the problems began three weeks before the fire. A pilot took the plane up and performed a maneuver that was "well within the envelope of the airplane," Bogdan told AFT. That maneuver caused the engine to rub against the rubber piece at a higher rate and temperature than it was designed for, causing the "microcracks," as Bogdan called them.
"Over the next three weeks of that airplane flying, those microcracks started growing in what we call 'high cycle fatigue,'" Bogdan told AFT. "And eventually on the day this happened, that fan-blade system just cracked too much, the whole circular part of that engine – through centrifugal force – stretched out and became a spear; that spear went up through the left aft fuselage of the fuel tank and it was the fuel tank that caused the fire."
The Air Force hasn't discovered the microcracks on any other F-35s, although Bogdan did say that there were signs of a similar, but less serious issue on other examples of the controversial fighter.
Pratt and Whitney, the manufacturer's of the F-35's F135 engine, have committed to covering all the costs relating to the engine fix, with Bogdan calling the company's response "very good."
As for how Pratt will prevent the problem in future planes, Bogdan described what sounds like a break-in period for the engines, where they'd be slowly worked up through what he called the "burn in" phase. If that doesn't work, the company may be forced to redesign the engine, with a particular focus on the fan section.
Meanwhile, Bogdan echoed Pratt's statement regarding the company's issues with titanium supplier A&P Alloy, saying the affected titanium engine part is "absolutely" unrelated to the fire.