Yet Another F-35 Jet Caught on Fire While Starting Its Engine

This post originally ran on War Is Boring.

A single high-tech F-35A Joint Strike Fighter costs nearly $150 million to buy, and all together should comprise the bulk of the US Air Force's combat power for decades. But one very serious and now recurring problem is the plane's propensity to burst into flames.

One F-35A did just that on Sept. 23 while starting its engine at Mountain Home Air Force Base in Idaho, according to Defense News. The pilot escaped the plane and no one was seriously injured, according to the paper.

It's unclear what caused the fire or whether the jet was damaged.

"The pilot had to egress the aircraft during engine start due to a fire from the aft section of the aircraft," Air Force spokesman Capt. Mark Graff told Defense News. "The fire was extinguished quickly."

The F-35A pilot from the 61st Fighter Squadron and seven other airmen were moved to Mountain Home's medical center after the fire "for standard evaluation," Graff added in an email to the newspaper.

Seven of the twin-tail, single-engine F-35As from the squadron—normally based in Arizona—have been at Mountain Home with American and Australian pilots since September 10. The purpose is to develop training procedures on "finding and killing surface-to-air threats," Lt. Col. Michael Gette of the 61st Fighter Squadron told an Air Force news broadcast.

The fire occurred nearly eight weeks after the Air Force declared the F-35A ready for combat.

While the source of the accident is unknown, we do know that the jet's Pratt & Whitney F135 turbofan engine has caught on fire before. In June 2014, an F-35A at Eglin Air Force Base in Florida caught on fire during takeoff.

Specifically, one of the engine's rotating fan blades failed after repeatedly rubbing against a rubber-like polyimide seal inside the rotor assembly. The rubbing is normal, except the blade rubbed too hard, generating lots of heat and leading the blade to break off and slash through fuel lines and the fuel tank, starting the blaze.

The pilot jumped out and firefighting crews raced to the scene. The blaze was out within minutes but the damage was done—two-thirds of the jet had been barbecued at a cost of $50 million in repairs.

The Pentagon grounded all of its F-35s—in three version for the Air Force, Navy and Marines—for several weeks.

Engineers had warned of the fire hazard as far back as 2007, according to 2015 Air Force Accident Investigation Board. In 2013, tests proved the danger existed. "Engine live fire tests in FY13 and prior live fire test data and analyses demonstrated vulnerability to engine fire, either caused by cascading effects or direct damage to engine fuel lines," the report stated.

This month, the Air Force grounded 15 F-35As after it discovered that insulation surrounding the jets' coolant lines were deteriorating.

Over time, the insulation could mix with the fuel, eventually leading to "structural damage to the fuel tanks," according to an Air Force statement.

This isn't a design flaw with the plane, but a specific problem with materials in the insulation provided by a subcontractor, according to the Air Force. The 15 grounded F-35As are a minority of the total operational Joint Strike Fighters in the Air Force's inventory. Forty-two others with the issue are on the production line.

It's not clear if the F-35A which caught on fire at Mountain Home had this problem, either.

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