Strong winds whipping through the exhaust of a F-35A Joint Strike Fighter starting its engine at Mountain Home Air Force Base, Idaho, resulted in the engine catching fire last year, according to an investigation report.
The Accident Investigation Board report, released Wednesday by Air Education and Training Command, concluded the Sept. 23, 2016, mishap was caused when a tailwind "forced hot air into the inlet of the Integrated Power Pack," an internal system where the conventional auxiliary power unit and an emergency power unit combine.
The forced air "led to a series of events resulting in insufficient torque applied to the [mishap aircraft] engine during start, and thus the engine rotation speed slowed," the report said.
"At the same time, fuel continued to be supplied to the engine at an increasing rate, which enabled an uncontained engine fire. The fire came out the engine exhaust and was carried along the outer surfaces of the [mishap aircraft] by the tailwind, causing significant damage," it said.
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Col. Dale Hetke, the investigation board president, noted the F-35A community had only "vague awareness of the issue."
If the pilot had turned off the engines more quickly, the fire might not have spread, Hetke said in his opinion summary. "It stands to reason that if the engine switch had been moved to OFF at the first indication of fire, fuel would have been shut off to the engine nearly immediately and the fire would not have burned as long," he said.
The cost of the damage was estimated at more than $17 million.
Investigators said operations were conducted and systems operated "within the designated limits," explaining this wasn't a design flaw in the F-35.
The F-35A involved in the incident is assigned to the 61st Fighter Squadron, part of the 56th Fighter Wing at Luke Air Force Base, Ariz., but was temporarily stationed at Mountain Home at the time of the fire.
No one was hurt during the event. The pilot, assigned to the 944th Operations Group at Luke, safely exited the aircraft, and maintenance crew members put out the fire.
In January, the Air Force's F-35 Integration Officer at the Pentagon speculated the fire was likely caused by pooled fuel in the tailpipe.
"The initial feedback from this is it was not an engine fire; [investigators] are calling it a tailpipe fire," Brig. Gen. Scott Pleus, a former F-16 Fighting Falcon pilot, told Military.com at the time. Pleus has since moved on to a new position at Air Combat Command at Langley Air Force Base, Virginia.
The incident "is still obviously a problem but, at the same time, it had nothing to do with the engine problem [the F-35] had before," Pleus said, referring to the Lightning II's engine difficulties in 2014.
"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," he said.