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An F135 engine "blisk," a fan disk with integrated blades, like the one shown here, was to blame for the June 23, 2014, F-35 fire at Eglin AFB, Fla., officials announced on July 14, 2014. Staff photo by John A. Tirpak.

Too-tight tolerances between an F135 engine “blisk”—a fan disk with integrated blades—and the surrounding engine casing caused friction that started an F-35 fire at Eglin AFB, Fla., on June 23, Pentagon officials said Monday. The incident led to the grounding of all of the Defense Department’s F-35 strike fighters. The event was an isolated incident, rather than a fleet-wide flaw, Pentagon acquisition, technology, and logistics chief Frank Kendall told reporters at a Farnborough air show press conference. It was “not a systemic, major design problem,” he said, suggesting the grounding could be lifted at any moment. Program Executive Officer Lt. Gen. Christopher Bogdan told reporters the piece in question is the third-stage integrally bladed rotor. All 98 in-service F135 engines have been checked, and no others have the same problem, he said. Some “rubbing” between the spinning blades and the casing lining is acceptable, Bogdan explained, but in this case the fit was too tight, causing “micro-cracking” in the solid-piece blades and creating excessive heat build-up. This caused the engine to “come apart,” he said. “We have … a body of evidence now that we think is ample … to fully understand what happened,” he asserted. (DOD release).