WASHINGTON — U.S. Air Force investigators have found that last September's F-35A mishap at Mountain Home Air Force Base, Idaho, was indeed an uncontained engine fire — albeit one started because of tailwinds present during engine start, not deficiencies with the aircraft's Pratt and Whitney F135 engine.

According to a U.S. Air Force accident investigation board, or AIB, report signed May 9 by the board's president and obtained by Defense News, the engine fire started after tailwinds forced hot air into the inlet of the jet's integrated power pack. A chain of factors, such as insufficient torque and slow engine rotation speed, caused the F-35 to continuously supply fuel to its engine at an increased rate.

"During this mishap, however, the fire became uncontained due to the increased amount of fuel added while the engine rotation speed was slowing," the report stated. "Once the uncontained fire started coming out of the aircraft exhaust, the tailwind carried it rapidly along the exterior surfaces of the jet."

The pilot escaped from the aircraft but sustained burns to his head, neck and face.

The service is still evaluating how much it will cost to repair the F-35A involved in the mishap, which was assigned to the 61st Fighter Squadron at Luke Air Force Base, Arizona, and involved in a training flight at Mountain Home when the fire occurred. However, damage to the plane is estimated to amount to at least $17 million.

Ultimately, the mishap had little effect on F-35 training and operations. After the fire broke out on Sept. 23, the U.S. Air Force announced that it had no plans of grounding its F-35As — a sign that the service believed the incident arose because of weather or human factors rather than a design flaw. The F-35 joint program executive office has not announced modifications to the jet as a result of the event.

The AIB report noted that aircraft systems, including the F135 engine, performed as designed. Still, investigators stated that more could have been done to prevent the mishap, especially in the realm of educating pilots.

For instance, the service did not include information about how tailwinds contribute to integrated power pack failures on its engine start checklist. Pilots were also not given any additional training on concerns related to tailwinds and were not aware of warning signs that the F-35 exhibits when tailwinds cause issues with the jet’s power and propulsion systems.

"[Integrated power pack] and engine start issues with a tailwind were known prior to the incident. However, the publications were written and communicated in such a way that the F-35A pilot community had only vague awareness of the issue. This vague awareness led to inadequate training for engine starts with a tailwind," Col. Dale Hetke, the AIB’s president, wrote in his statement of opinion on the investigation.

The F-35 is renowned for its complex software and sensor fusion capabilities, but Hetke noted that the level of automation present in the aircraft led to some complacency within the pilot community.

"The F-35A engine start process is heavily automated, which drove a perception among pilots [that] the aircraft handled virtually all of the starting procedures, and so long as the dials were ‘green’ there was no problems," the AIB report stated.

During the incident, the fire spread along about two-thirds of the aircraft’s aft surfaces, with most burn damage occurring on the top, side and underneath portion in the center of the jet, the report said.

Because the aircraft’s weapon bay doors are open during engine start, the fire spread to aircraft surfaces, panels, cables and components. The aircraft’s landing gear was damaged, and its integrated power pack will need to be replaced.

Investigators noted that the engine’s exhaust nozzle section was engulfed in flames but did not spell out how much of the engine will need to be repaired or replaced.

About Valerie Insinna

Valerie Insinna is Defense News' air warfare reporter. She previously worked the Navy/congressional beats for Defense Daily, which followed almost three years as a staff writer for National Defense Magazine. Prior to that, she worked as an editorial assistant for the Tokyo Shimbun’s Washington bureau.

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