The F-35 Joint Program Office (JPO) and Lockheed Martin [LMT]-led industry team plan to undertake mitigation measures to assure that harmonic resonance–a higher than normal engine vibration–does not affect the operation of the fighter’s F135 engine by Raytheon Technologies‘ [RTX] Pratt & Whitney.

So far, harmonic resonance appears to have been a rare occurrence in a relatively low number of fighters with fairly new F135s.

“Following an aircraft inspection in December, 2022, a safety concern was identified with the F135 engine,” the F-35 program said in a Feb. 24 statement.

“Since December, Pratt & Whitney along with engineers from the JPO, Lockheed Martin, Naval Air Systems Command (NAVAIR) and the Air Force Lifecycle Management Center (AFLCMC) have worked tirelessly to understand and develop mitigations for a rare system phenomenon involving harmonic resonance to develop a path forward for safe operation of the F135 in flight,” the F-35 JPO said. “The actions the government and industry team are taking will ensure incorporation of mitigation measures that will fully address/resolve this rare phenomenon in impacted F135 engines. At this time, the JPO has authorized the resumption of engine deliveries to the production line.”

On Feb. 24, the F-35 JPO, Lockheed Martin, and Pratt & Whitney did not identify the “mitigations” to alleviate engine harmonic resonance.

“The government is currently working to provide instructions to the fleet and to Lockheed Martin to enable safe resumption of flight operations of impacted aircraft and new production aircraft,” the F-35 program said on Feb. 24.

In December, Lockheed Martin and Pratt & Whitney halted deliveries of the F-35 and the F135 engine after the pilot of a Lockheed Martin-owned F-35B ejected on the runway at Naval Air Station Joint Reserve Base Fort Worth, Texas on Dec. 15 (Defense Daily, Jan. 3).

NAVAIR has been investigating the mishap, but that evaluation appears to be winding down, and F-35 flight acceptance testing may resume in the next several weeks.