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​Two F-35 strike fighters fly above Eglin AFB, Fla. Air Force file photo.

Investigators think they understand why an F135 engine problem caused an F-35 strike fighter to burn in June, slowing the test and training effort, program manager Lt. Gen. Christopher Bogdan said Wednesday. Speaking at the COMDEF 2014 conference in Washington, D.C., Bogdan said the natural “flex” of the mishap engine under high-G conditions caused fan blades to “rub” and “dig too deep” in the rubber-like seal around the powerplant. From 138 possible causes, “we’ve narrowed it down to four” and the root cause could be some combination of them all, Bogdan said. Steps to address the issue are underway, including a “burn in” technique requiring two sorties per engine that could prevent the problem on other jets. Bogdan expects a permanent fix in late October, but “I need 21 test airplanes” back to full capability “by the end of the month or else there will” be delays to the flight test program. The engine problem has created a 30-45 day “headwind,” but Bogdan believes he can make up some of that. Pratt & Whitney, maker of the strike fighter’s F135 engine, has been “very, very good” about solving the issue, Bogdan said. “They’ve put their A-team on this” and agreed—before the likely root causes were identified—to bear the cost of retrofitting the 156 F135 engines now extant. Pentagon acquisition, technology, and logistics chief Frank Kendall told reporters at the conference earlier in the day he expects a fix soon and at “a relatively minor cost.” Bogdan said the mishap jet may be used for “battle damage repair” training. The problem has nothing to do with a separate titanium supplier issue causing Pratt to replace parts in other F135s, Bogdan said.