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 F135engine, 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  &  Whitney is installing a retrofit in its F135 engine to repair the problem that prevented the F-35 Joint Strike Fighter from making its international debut last summer. It expects to complete the fix fleetwide by early next year.

An engine fire in an F-35A that was preparing to take off for a training mission at Eglin Air Force Base, Fla., last June caused the Pentagon to ground the entire fleet, preventing the F-35’s planned international debut the following month at both the Royal International Air Tattoo and the Farnborough Airshow in the UK. The new time and place of that debut remain unannounced. Speaking with reporters in Washington, D.C., on March 24, show organizers said they do not expect the F-35 to appear at the Paris Air Show in June.

Investigators determined that the F135 fire started as a result of excessive rubbing of a polyimide plate seal between the second- and third-stage integrally bladed rotors in the engine’s compressor section. The rubbing caused excessive heating and led to the failure of the third-stage fan rotor.

Once the root cause of the engine fire was identified, Pratt & Whitney developed two fixes. The first, “controlled rub-in” procedure involves flying the F-35 in a series of planned maneuvers so that seal, which is designed to be abraded into a groove, or trench, wears in a controlled manner. The second fix was to create a “pre-trench” in the seal during the manufacturing process and retrofit the part in fighters that have been already delivered.

“ We basically said, instead of rubbing it in, let’s just cut it out,” Mark Buongiorno, Pratt  &  Whitney’s F135 program vice president, told  AIN . “We basically machined that trench in as part of the manufacturing process as opposed to letting the engine do it itself. That fix has been validated. We demonstrated that there was an insignificant loss in performance because of it and we are in the process of retrofitting the fleet to that pre-trench configuration.”

Interviewed during Pratt & Whitney’s media day in Hartford, Conn., on April 2, program officials said the retrofit has been installed on all but one of 17 system development and demonstration (SDD) fighters; the exception being an F-35 that was unavailable because it was undergoing climatic testing at Eglin AFB. The engine manufacturer is rolling out the retrofit to the remainder of the fleet using its field technicians. As of late January, F-35 manufacturer Lockheed Martin said it had delivered 131 fighters to the Department of Defense.