Pooled fuel in the tailpipe of an F-35 Joint Strike Fighter likely caused it to catch fire recently at Mountain Home Air Force Base, Idaho, an Air Force general involved with the program said.

“The initial feedback from this is it was not an engine fire; [investigators] are calling it a tailpipe fire,” said Brig. Gen. Scott Pleus, a former F-16 Fighting Falcon pilot who directs the F-35 Joint Strike Fighter program’s integration office for the Air Force.

“A tailpipe fire is a result of some form of fuel getting ignited in the back-end of the airplane that is not necessarily in the engine, so this was not an engine problem. There was some excess fuel that pooled in the back of the airplane and then ignited” before takeoff, he said in an interview with Military.com on Wednesday.

The F-35A, assigned to the 56th Fighter Wing, was preparing for a training mission at Mountain Home when it caught fire just before takeoff Sept. 23, according to the service. But official details into the investigation have yet to be released.

“An engine fire is obviously a fire that starts in the engine,” Pleus said. This “is still obviously a problem but, at the same time, it had nothing to do with the engine problem [the F-35] had before, nor the [polyalphaolefin cooling] lines.”

Pleus was referring to the Lightning II’s engine difficulties in 2014, and recent insulation issues.

“In the F-35 program, I think the largest ‘setback’ we had was the fire at Eglin Air Force Base, [Florida], where we had the engine fire on takeoff, and the reason why I would call that a setback is because we found an engineering problem with the engine itself. ”

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The rear of an F-35A caught fire while preparing to take off from the base that summer. As a result, the U.S. military services stopped flights of the Lockheed Martin Corp.-manufactured fighter jet.

“In my world, that’s a pretty big setback,” Pleus said, but “all of the training kept that pilot alive. We found the root cause to the problem in the engine … we got a solution, we fixed it, implemented it fleetwide, and [we have] not had any issues [like that] since then.”

The recent tubing insulation flaw basically came down to human error, Pleus said.

The service on Sept. 16 ordered a temporary stand-down of 13 out of 104 F-35s in the fleet “due to the discovery of peeling and crumbling insulation in avionics cooling lines inside the fuel tanks,” according to a statement at the time. Two additional aircraft, belonging to Norway and stationed at Luke Air Force Base, Arizona, were also affected.

The issue, discovered during depot servicing, affected a total of 57 aircraft.

The subcontractor — still not identified by the service — had nothing to do with the aircraft’s design. Simply put: “They put the wrong insulation in,” Pleus said.

“I almost look at this as more of a success,” he added, “because we proved something that they never thought that they would have to deal with and they were able to fix it right.”

The F-35s involved returned to the skies in early November.