We Finally Know Why An F-35 Burned On The Runway

We are finally getting information as to what caused the F-35A engine fire that has left the entire program progressing at a snail's pace due to strict flight restrictions.

According to the Air Force Times, the cause of the fire that engulfed a nearly new F-35A on the runway at Eglin AFB had its genesis in a fairly benign flight maneuver performed weeks earlier:

The issue began three weeks before the fire when a pilot took the aircraft up and executed a two-second maneuver involving adding Gs, roll rate and yaw to the plane at the same time.

Although that move was ""well within the envelope of the airplane," Bogdan said, those two-seconds led to the engine rubbing against a rubber piece at a much higher rate — and nearly double the temperature — than it was designed to do. In turn, that led to what Bogdan called "microcracks" that went unnoticed until the day of the fire.

"Over the next three weeks of that airplane flying, those microcracks started growing in what we call 'high cycle fatigue,'" Bogdan explained. "And eventually on the day this happened, that fan-blade system just cracked too much, the whole circular part of that engine — through centrifugal force — stretched out and became a spear; that spear went up through the left aft fuselage of the fuel tank and it was the fuel tank that caused the fire."

At this point, it is not clear exactly what modification will be needed to keep this violent event from happening again. Does the F-35's engine need a new bushing made up of more temperature resistant material, its fan disk replaced, or is there a larger structural redesign looming?

Whatever modifications end up being needed to solve the problem, Pratt & Whitney, the engine's manufacturer and the only engine manufacturer for the giant Joint Strike Fighter program, says they will be paying for it, not the U.S. taxpayer.

We Finally Know Why An F-35 Burned On The Runway

What Pratt & Whitney will not be paying for is the building backlog of test points that have to be accomplished before the jet reaches a rudimentary initial operating capability (IOC) with the Marine Corps. Many have seen the F-35B's IOC date of next year as more of a promotional tool than a real milestone that marks the U.S. Department of Defense finally having a highly usable multi-role fighter for the many billions of dollars that have already been spent on the fledgling program.

Regardless of when the F-35B is slated as "operational," the more time the DoD's ever growing fleet of F-35s stay grounded the more they will cost, and the bigger risk the fragile program will have at losing orders. Keeping the production numbers, both for foreign and domestic consumption, where they are is key to keeping the entire program from entering into a death spiral, where each jet's cost will increase due to smaller economies of scale and the amortization of developing the already very expensive jet being spread over less airframes.

We Finally Know Why An F-35 Burned On The Runway

Currently, the JSF program says they are a couple months behind the flight test schedule (that has already been lengthened multiple times), and some of this can be made up with increased sortie rates if all the F-35s are returned to service without flight restrictions. Any more time that the jets are kept under strict flight rules beyond the end of this month will affect the program's already reformed goals.

Another engine-related issue that has come to the surface has to do with the quality of the titanium used in the Pratt & Whitney F-135 engine production. Apparently, a Pratt & Whitney subcontractor, A&P Alloys, supplied the company with questionable titanium stocks, from which components of the F-35's engine are machined. This has caused a pause in production dating back to last May as Pratt & Whitney investigated the issue. It is not clear how this gap in production will affect future F-35 deliveries.

It is claimed that the suspect metal has nothing to do with the current engine woes, and the parts that it does affect are being replaced by Pratt & Whitney. Yet this event, and the engine fire that followed it, does highlight the vulnerability of using a single engine source for an aircraft that the US and many of its allies are hanging their future of air supremacy on.

The titanium supply question also underlines a looming issue as to how America will satisfy its demand for titanium if the trade situation with Russia continues to deteriorate. We have dealt with this problem during the Cold War via clandestine means, but at the time, titanium had nowhere near the demand as it does today, with key aerospace designs such as the 787 and Super Hornet and F-35, along with many other aircraft and missiles, relying on the robust and light material.

We Finally Know Why An F-35 Burned On The Runway

Photos via USAF

Rogoway is a defense journalist and photographer who maintains the website Foxtrot Alpha for Jalopnik.com You can reach Tyler with story ideas or direct comments regarding this or any other defense topic via the email address Tyler@Jalopnik.com