Lockheed Martin [LMT] said on March 6 that it has resumed flight acceptance testing in Fort Worth, Texas of the company’s F-35 fighter.
“We resumed F-35 production flight operations today following an F135 engine mitigation action,” the company said. “Safety remains our top priority as we continue to produce the world’s most advanced fighter aircraft.”
Last week, the F-35 Joint Program Office (JPO) said that it had directed a fleetwide retrofit for F-35As, Bs, and Cs
to mitigate any future “harmonic resonance”–a higher than normal engine vibration that may increase stress on the fighter’s Raytheon Technologies‘ Pratt & Whitney [RTX] F135 engine (Defense Daily, March 2). It appears that the “harmonic resonance” has affected a low number of fighters with fairly new F135s.
The JPO said last week that “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 retrofit can be performed at the operational level and can be completed in four to eight hours,” the F-35 program said.
The F-35 JPO, Lockheed Martin, and Pratt & Whitney have not identified the retrofit.
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.
The F-35 program said last week that it had issued a Time Compliance Technical Directive (TCTD) to direct the completion of a retrofit procedure for all F-35s within 90 days to mitigate for harmonic resonance.
The F-35 JPO said that it is investigating the root cause of the “harmonic resonance” to determine “where these system sensitivities intersect with the excitation frequencies.”
Naval Air Systems Command has been investigating the Dec. 15 mishap.