The microcracks in the third stage of the integrally bladed rotor that caused a catastrophic F135 engine fire in an F-35A on June 23 began forming three weeks earlier when a pilot conducted a fairly benign maneuver with a unique combination of yaw, roll and g stresses, according to the F-35’s program executive officer.

That 2-sec. maneuver prompted a "hard rub" on the titanium of the blade against the abradable strip lining the engine casing, and the engine saw temperatures of a 1,900F, well over the expected 1,000F, U.S. Air Force Lt. Gen. Christopher Bogdan told the ComDef 2014 conference Sept. 3.

The microcracks that formed that day grew over the subsequent three weeks and eventually caused the blade to break up and puncture the fuel tank.

The F-35 Joint Program Office is taking a two-pronged approach to fixing the F135 blade rubbing that caused the catastrophic fire and subsequent fleet grounding. F135 engine manufacturer Pratt & Whitney is designing a prototype of a new front section with an abradable strip that would eliminate the likelihood of the blade rubbing experienced in June. It is slated to begin testing in October, Bogdan says.

Meanwhile, program officials also are devising a separate "burn in" procedure to properly, methodically wear down that strip by executing a sequence of maneuvers in controlled flight. The hope with this latter procedure is to get the test aircraft now operating under severely restricted flight envelopes back into more rigorous flight testing without undergoing the full modification. Program officials also are exploring the metallurgy of the titanium in the blade.

The test program includes 21 aircraft. Engines also are subject to a borescope inspection for every 3 hr. of flight, another requirement hindering expeditious testing.

Pratt & Whitney will pick up the tab for fixes to the 156 engines already delivered.

"Pratt and Whitney’s reaction to this problem from my opinion has been very, very good," Bogdan said. "From a technical standpoint, they have put their A team on this. From a business standpoint, they realize this is a problem they need to solve."

Thus far, the team has narrowed to four potential root causes; Bogdan hopes the investigation will find the culprit this month.

Overall, the test program has lost about 30-45 days due to the engine mishap, though Bogdan insists the Marine Corps can still make its initial operational capability date next July.