The US Department of Defense (DoD) has cleared engine maker Pratt & Whitney (P&W) to resume deliveries of the F135 propulsion system that powers Lockheed Martin’s F-35 stealth fighter.

F135 deliveries have been on hold since December, when an F-35B short take-off and vertical landing variant of the jet crashed during flight trials in Fort Worth, Texas.

F-35B Japan USMC

Source: US Navy

An F-35B crash in December led to a flight hold for newly delivered examples of the type and airframes with fewer than 40h of operation

The Pentagon’s F-35 Joint Program Office (JPO), which oversees all F-35 deliveries to the US military and partner nations, said on 24 February that engineers from the government and industry have developed a “path forward” for safe operation of F135s.

“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,” the Pentagon says. “The JPO has authorised the resumption of engine deliveries to the production line.”

The DoD has not disclosed the nature of the problem that led to the pause.

However, the JPO calls the issue a “rare system phenomenon involving harmonic resonance”. The office adds that engineers from the US Air Force, US Navy, Lockheed and the JPO now better understand the technical issue and have developed “mitigations” to address it.

While engine deliveries from P&W to Lockheed can now resume, a flight hold remains in place on new F-35s coming off the Lockheed production line in Fort Worth. In place since December, the pause precludes mandatory pre-delivery flight tests on completed jets – meaning Lockheed has not been able to hand over finished aircraft to customers since the order took effect.

Lockheed missed its F-35 delivery target for 2022 by seven jets, which executives – including chief financial officer Jay Malave – attributed to the flight hold during the firm’s fourth-quarter earnings call on 25 January. 

The airframer delivered 141 F-35s in 2022 – and set a 2023 goal of shipping 156 of the single-engined fighters.

F-35 crash with parachute Fort Worth

Source: Houston Air Watch/Twitter: @houstonairw

A Lockheed Martin-owned F-35B fighter crashed in Fort Worth, Texas on 15 December

“We continue to work closely with the [JPO] to determine next steps for resumption of F-35 flight operations and deliveries,” Lockheed says in response to the government’s 24 February announcement.

Under a $30 billion contract with the DoD announced in December, the company is obligated to deliver the latest Block 4 jets to the USA, Finland, Poland and Belgium in F-35 Lots 15 and 16.

The Pentagon says safety of military personnel is its top priority in evaluating next steps. “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 JPO says.

The office has not revealed how many F-35s are covered under the grounding, saying only that it affects “a small number of aircraft which have been evaluated to be of higher risk”.

The current flight hold applies to newly produced F-35s and airframes with under 40h of operation.

Lockheed says it has delivered some 894 finished airframes under the global F-35 programme. Northrop Grumman, which produces the advanced fighter’s centre fuselage, says it delivered the 900th F-35 body to Lockheed on 14 February.