The F-35 Joint Program Office (JPO) has directed a fleetwide retrofit for Lockheed Martin [LMT] F-35As, Bs, and Cs to mitigate any future “harmonic resonance,” which may increase stress on the fighter’s Raytheon Technologies‘ [RTX] F135 engine.

“While only a small number of aircraft were impacted by the ‘harmonic resonance,’ the plan is to retrofit the entire fleet, because the retrofit is inexpensive, non-intrusive and supports the JPO’s desire to maintain and manage a single configuration across the entire fleet,” the F-35 JPO said on March 3. “The retrofit can be performed at the operational level and can be completed in four to eight hours.”

The “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 last week.

“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 last week. “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.”

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).

DoD’s upcoming fiscal 2024 budget request is likely to propose a course of action on an F-35 engine for future blocks of the fighter. Raytheon is trying to build the case in Congress for the company’s F135 Engine Core Upgrade, which is vying with General Electric‘s [GE] XA100. The Pentagon may decide to upgrade the F135 or build a new engine to accommodate new weapons and other upgrades envisioned for F-35 Block 4.

Regarding “harmonic resonance,” the F-35 JPO said on March 2 said that it “has issued a Time Compliance Technical Directive (TCTD) to direct the completion of a retrofit procedure to mitigate for harmonic resonance.”

“The TCTD directs immediate compliance for the small number of aircraft that were restricted from flight; completion of the procedure will allow these aircraft to resume flight operations,” the F-35 program said. “The TCTD further recommends the completion of the retrofit procedure within 90 days for all other aircraft although no flight restrictions are recommended.”

“The JPO will work with the military services and international partners to ensure understanding of the TCTD,” per the F-35 program. “The safety of flight crews is the JPO’s primary concern.”

The F-35 JPO is investigating the root cause of the “harmonic resonance” to determine “where these system sensitivities intersect with the excitation frequencies.”