Investigators have narrowed their focus to the third stage turbine of the F135 engine as the likely source of a fire that erupted June 23 as an F-35A fighter was preparing for takeoff at Eglin AFB, Florida, last month.

The June 23 fire, which occurred at 9:15 a.m. local time, first prompted "safety pauses" to flights for the fleet directed by local commanders, followed by a more serious fleetwide grounding order issued the evening of July 3, just before the U.S. Independence Day holiday.

In addition to the flight grounding, "We cannot run any engines," including those in testing at the contractor’s facility, as a result of the fire, U.S. Air Force Lt. Gen. Christopher Bogdan told Aviation Week during a July 3 interview. He declined to discuss the investigation or assumptions about root cause. Accident investigators have sequestered the charred F-35A and any related foreign object debris at a hangar at Eglin for review.

The third stage turbine is the second stage in the low-pressure turbine section. It is common to all F135 variants – the A, B and C F-35 aircraft.

The low-pressure section of the F135 has bedeviled the program before. The fleet was grounded from Feb. 21-March1, 2013, due to the discovery of a 0.6-in. crack on a third-stage low-pressure turbine blade on AF-2, an F-35A used for testing, at Edwards AFB, California. Officials at F135 maker Pratt & Whitney attributed that issue to a one-off manufacturing problem and not related to high-cycle fatigue.

Subsequently, in December, F135 ground test engine FX648 "blew" during trials at Pratt’s West Palm Beach facility as it reached 77% of its expected life. The culprit was a fan crack in the front stage of the engine; it occurred while the engine was operating in conventional mode. The fan is comprised of bladed disks — called blisks — that are machined in a solid piece. In this case, the blisks in the first stage were hollow and susceptible to cracking; they were made hollow earlier in the program during a weight-reduction push. Now, however, Pratt is going back to a solid design, adding about 6 lb. to the weight of the engine.

Prior to the blown engine last year, Pratt was already working on a redesign to incorporate the solid blisks. Bogdan says the Joint Program Office is reviewing fixes to the redesign now and they are limited to the compressor section, making retrofit in the field easier than if it has been a more widespread problem. "We were using hollow blades, which were very hard to manufacture and cost a lot of money," Bogdan said. "Much of the design of that section of the engine was completed. So all that they needed to do was to take the information that they learned from the engine incident — which was some stresses on the blades that they has not previously used in the modeling — put them in the modeling and then pop out the design."

Bogdan said there is no reason to believe this incident and the fire occurred as a result of related problems.

Even earlier redesigns also resulted from problems with test engines in advance of integration onto BF-1 – the first Marine version – in 2007 and 2008.

The timing of the fire – likely the first F-35 Class A mishap, costing more than $2 million – could not be thornier as four F-35Bs are stuck on the ground stateside. They were slated to have already made a historic crossing of the Atlantic Ocean in preparation for flying displays at the Royal International Air Tattoo, set to begin July 11, followed by Farnborough air show the following week.

Air worthiness authorities in the Air Force, Navy and United Kingdom have been doing back-to-back reviews ever since to see if the aircraft can be cleared for their journey to RAF Fairford, where they will be based for the deployment, but it is unclear when they could transit. The U.S. Marines are pushing hard to get clearance for the deployment. "It’s not off. We are still planning to support the two air shows," says Capt. Rich Ulsh, a Marine Corps spokesman.

The Joint Program Office and Pratt & Whitney did not respond to queries about the focus on the F135 third stage.

"We are working closely with the Air Force Safety Investigation Board to determine root cause and to inspect all engines in the fleet. Safety is our top priority," says Matthew Bates, a spokesman for Pratt. "Since the incident is the subject of an investigation it is inappropriate to comment further."