FARNBOROUGH - The blade rubbing that prompted a fire in an F-35A – leading to a fleetwide grounding July 3 – took place in the third-stage fan of the integrally bladed rotor (IBR) in the low pressure section of the F135 engine, according to Lt. Gen. Christopher Bogdan, program executive officer for the U.S.–led F-35 program.

The three-stage IBR sits behind the front fan in the F135 and compresses the air before passing it into the high-pressure core. Each stage is separated by a stator and rotates within the casing, which is lined with an abradable strip to maintain tight clearances between the blade tips and the inner wall of the compressor casing. This enables tight tolerances while reducing pressure loss and some rubbing is acceptable. In the engine for AF-27, the blades were rubbing far in excess of the design, creating excessive heat and microcracking in the blades. The resulting high cycle fatigue failure forced the section to “come apart,” Bogdan says, prompting the fire June 23 at Eglin.

The pilot safely egressed and the flames were extinguished with foam.

Senior Pentagon officials say the problem thus far appears isolated; officials have inspected all 98 Pratt & Whitney F135 engines in service, Bogdan says. “All 98 of the other engines did not indicate the same phenomena as the one that failed,” he said. “We have created a body of evidence now that we think is ample enough to fully understand what happened.”

Investigators narrowed their focus on the third stage fan in the IBR about two weeks after the fire took place, Bogdan says. That was last week, well after four F-35Bs were slated to make a historic flight across the Atlantic Ocean for their international debut at the Royal International Air Tattoo at RAF Fairford last week followed by flying displays at Farnborough.

They missed RIAT, but Pentagon procurement chief Frank Kendall says they might still make an appearance at Farnborough later this week. He said their appearance is in the hands of the air worthiness authorities and “safety is first” as they consider a return to flight.

When asked about a July 14 post on the official Facebook page of NAS Patuxent River saying the aircraft could fly over as early as July 15, Bogdan and Pentagon procurement chief Kendall looked stunned. “I don’t think Facebook is an official source,” Bogdan said, adding the Air Force and Navy air worthiness authorities continue review data ahead of approval to resume flights. If they arrive in the United Kingdom in time for the show, Bogdan says the plan is to do the entire display routine showing the aircraft’s hover and some maneuvering.

“We do not intend on doing a B-52-style fly over if it gets here. We intend to do the profile that the United States Marine Corps pilots trained for four months.”

Meanwhile, “there is a growing body of evidence that this is not a systemic, major design problem,” Kendall said.

“This is not related to any incident in the past,” said Pratt & Whitney President Paul Adams.

Pratt engineers were already in the midst of a redesign to the first stage fan in the IBR, embracing a solid bladed design over the hollow design. A ground-based test engine “blew” late last year, Bogban said, owing to cracking in the hollow blades. The second and third stage fans –- the area in question for the June 23 fire -– are both constructed of solid blades. Pratt has submitted plans to Bogdan’s office for approval of plan to produce solid blades in the first stage and they are under review. This work is unrelated to the June fire incident, Adams said.