The Pentagon has modified all F-35 Lightning II fighters, but has yet to find the cause of "harmonic resonance" in the F135 engines

At the end of last year, the Pentagon halted flights and deliveries of the F-35 fifth-generation fighter jet as well as the F135 engines. The reason was the so-called "harmonic resonance" that caused the F-35 to crash during a landing near the Lockheed Martin plant in Texas. Six months later, all American fighters were modified, but the cause of the "harmonic resonance" is still unknown.

Here's What We Know

In simple terms, "harmonic resonance" is a vibration that destroys the engine's fuel tube. The F-35 crash in Texas is not the first manifestation of the problem. Vibration caused the fighter jet to fail back in March 2020, almost three years before the incident near the Lockheed Martin plant.

The F-35 program office and Pratt & Whitney found a temporary solution. The F135 propulsion systems have been given a "hole" that "reduces the likelihood of losing engine control" in the event of a fuel tube failure due to vibration. Despite this, so far the US Department of Defence has not been able to find out what exactly is causing the "harmonic resonance".

By early May, all US F-35s had received "upgraded" engines. The government and industry team continue to work on handing over all the fighters whose deliveries have been delayed because of the engine vibration problem.

On February 17, 2023, Pratt & Whitney received approval to resume deliveries of the F135 engines. Lockheed Martin was able to resume deliveries of the F-35 Lightning II about a month later. The backlog is expected to be fully cleared by early summer.

As the Pentagon continues to investigate engine problems, the number of US F-35s ready for service remains below the target of 70% for the F-35A and below 75% for the B/C models. In April, the combat readiness rate for all US F-35s was 56%.

Source: Defence One