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​This Air Force illustration shows an F-35’s engine exhaust and Integrated Power Pack. A tailwind into an F-35’s engine system heated the IPP and caused a fire that damaged the aircraft Sept. 23, 2016, at Mountain Home AFB, Idaho. Air Force photo.

—Brian Everstine and John A. Tirpak

A strong tailwind caused the engine of an F-35A to catch fire last fall, prompting checklist revisions and new precautions and procedures to be put in place to change how F-35s are handled on the ground.

On Sept. 23, 2016, an F-35A assigned to the 61st Fighter Squadron and deployed to Mountain Home AFB, Idaho, caught fire on the flight line. The aircraft was damaged with an estimated cost of about $17 million, and the pilot was injured.

An Air Education Training Command Accident Investigation Board report, released Wednesday, found that the cause of the fire was large tailwind forcing excess hot air into the Integrated Power Pack; comparable to an Auxiliary Power Unit on other aircraft. This led to a series of events resulting in insufficient torque applied to the engine, slowing its rotation speed. Simultaneously, fuel continued to go to the engine, causing an uncontained engine fire, the report states.

The fire came out of the engine exhaust, with the tail wind carrying it along outer surfaces of the jet. This caused extensive damage before being extinguished about 20 seconds after the first visual indications of the fire, the report states.

The jet, tail number 12-5052, was deployed to Mountain Home from Luke AFB, Ariz., and was preparing for a training flight.

AETC found that neither publications nor guidance and training were enough to prepare airmen for this incident. While issues with tailwind and the Integrated Power Pack were known, “The publications were written and communicated in such a way that the F-35A pilot community had only vague awareness of the issue,” AIB President Col. Dale Hetke said in the report. “This vague awareness led to inadequate training for engine starts with a tailwind.”

F-35 training also resulted in “complacency and over-reliance” on automation. Evidence found that if the crew expected startup issues with a tailwind, the crew might have relied less on automation and could have identified an “abnormal engine start” earlier.

Because the fire spread with speed, the pilot had trouble following the checklist. For example, the pilot did not move the engine switch to “off” in accordance with the egress checklist. Within seconds, the fire reached the landing gear.

“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,” the report states. The pilot was under duress, however, and sustained burns to his head, neck, face, and ears.

Following the AIB report, AETC implemented procedural fixes and checklist changes to prevent similar mishaps, including a 20-knot tailwind limit for the start of an F-35 engine.

Pentagon officials with knowledge of the AIB findings said operators of all three variants of the F-35—the A, B and C communities—have been informed of the new engine start requirements and the procedures have been incorporated into the basic F-35 manual.

“It will be up to the individual services” whether they would skip the rule under extreme operational duress, said one, and each operating service will “individually assess” how to implement the rule on a crowded flightline, he added.