F-35 Engine To Undergo Fleetwide Retrofit

f135 stovl engine on test

F135 STOVL engine.

Credit: Pratt & Whitney

The F-35 Joint Program Office (JPO) has issued a directive recommending that all Pratt & Whitney F135s powering the global fleet of Lockheed Martin F-35s be retrofitted within 90 days with a fix for a vibration problem that caused an aircraft to crash on Dec. 15.

While no flight restrictions are included, the JPO Time Compliance Technical Directive (TCTD) instructs immediate compliance “for the small number of aircraft that were restricted from flight.” No details have been released about what the retrofit involves, although the JPO says it can be performed “at the operational level and can be completed in 4-8 hr.”

Pratt & Whitney developed the retrofit procedure to mitigate harmonic resonance that was uncovered as the likely cause of the December mishap, which involved an F-35B during a predelivery check flight at the company’s Fort Worth production site.

Described by the engine-maker as an extremely rare phenomenon, the vibration problem is potentially common to the F135 variant powering the F-35B short-takeoff-and-vertical-landing model as well as the versions powering the conventional-takeoff-and-landing F-35A and F-35C models. Jennifer Latka, vice president for F135 programs, says there were no differences in the build standard of the engine involved in the accident or in that of the other small population of aircraft affected. “And we can all say with certainty that it was not a manufacturing or quality issue,” she adds.

While only a small number of aircraft were affected by the harmonic resonance, “the plan is to retrofit the entire fleet, because the retrofit is inexpensive, nonintrusive and supports the JPO’s desire to maintain and manage a single configuration across the entire fleet,” the program office says. “The JPO will work with the military services and international partners to ensure understanding of the TCTD. The safety of flight crews is the JPO’s primary concern.”

The JPO said Feb. 24 that it had cleared Pratt & Whitney to resume delivering engines, but Lockheed Martin has yet to resume deliveries of new F-35s. More than 850 F-35s have so far been delivered globally.

Guy Norris

Guy is a Senior Editor for Aviation Week, covering technology and propulsion. He is based in Colorado Springs.