An F-35 Lightning II performs a maneuver Sept. 12, 2016 over Luke Air Force Base, Ariz.

An F-35 Lightning II performs a maneuver Sept. 12, 2016 over Luke Air Force Base, Ariz. U.S. Air Force / Senior Airman Devante Williams

F-35 Program Completes Band-Aid Fix for Engine

All jets had received the modification earlier this month, but the Pentagon is still trying to figure out why a mysterious vibration is happening.

The Pentagon has finished modifying its F-35 jets to prevent catastrophic engine damage, and expects by June to take delivery of all the aircraft and engines delayed by a December crash.

The F-35 Joint Program Office paused flight operations and the acceptance of new jets after a December crash in Fort Worth, Texas, and a subsequent investigation revealed a vibration, or  “harmonic resonance,” that causes the jet’s fuel tube to fracture. A viral video of the crash showed an F-35B bouncing off the ground and spinning around before the pilot safely ejected. Defense News reported that an F-35 engine also failed during a pre-delivery test due to a similar vibration problem in March 2020—almost three years before the December mishap.

Earlier this year, the program began fitting F135 engines with an “orifice” that “reduces the impact to loss of engine control” if the fuel tube “fractures as a result of the harmonic phenomenon,” F-35 Program Executive Officer Lt. Gen. Michael Schmidt told lawmakers at a March 29 hearing of the House Armed Services tactical air and land forces subcommittee.

The Pentagon is still trying to figure out why the vibration occurs and how to prevent it, Schmidt said.

All U.S. F-35s had received the fix by early May, JPO spokesperson Russ Goemaere said in a statement. 

“The hardware mitigation is fully implemented on all F-35 aircraft presented for government acceptance. The majority of backlogged aircraft are accepted to date. The collective government and industry team will continue to work through the remaining backlog to deliver high quality aircraft,” Goemaere said. 

The program expects to have the backlog fully cleared by early June “pending completion of acceptance flights and artifact review for government acceptance,” Goemaere said. 

Installing the fix is a “simple process,” Schmidt testified at the March hearing. 

Pratt & Whitney resumed engine deliveries on Feb. 17, and Lockheed Martin resumed aircraft deliveries on March 14. 

“This government and industry collaboration will ensure incorporation of multi-stepped mitigation efforts that will fully address this rare phenomenon in F135 engines. Meanwhile, root cause identification and analysis of this failure mechanism will continue,” Schmidt said.

As the Pentagon continues to investigate the F135 engine problems, the number of U.S. F-35s ready for duty remains lower than the goal of 70 percent for F-35As and 75 percent for the B and C models. In April, the mission-capable rate for all U.S. F-35s was 56 percent, Schmidt said in an April statement. 

“Our deployed/combat coded fleet significantly exceeds this average,” he said. 

JPO declined to explain why readiness is down “due to operational security concerns,” but said it has “dedicated degrader action cells” assigned to each of the main problems reducing F-35 mission capability. 

Schmidt said the program aims to increase mission capability and bring the average up to 64 percent by next April. 

The Air Force intends to upgrade its existing Pratt engines through a program called engine core upgrade, or ECU, having rejected a proposal to pursue a new engine. Service officials have said the upgrade will restore engine life and prevent degradation for all three F-35 variants “at the lowest cost.”