RAF FAIRFORD, England — The U.S. military is once again accepting deliveries of the newest F-35 Joint Strike Fighters from Lockheed Martin after a yearlong halt caused by problems with the fighters’ upgrades.

The first two F-35As loaded with a “truncated,” or partial, version of the Technology Refresh 3 upgrade software were delivered to the U.S. Air Force on Friday, the F-35 Joint Program Office said. One went to the Air National Guard’s 187th Fighter Wing at Dannelly Field, Alabama, the JPO said, and the other went to Nellis Air Force Base in Nevada.

But the lengthy delay in delivering F-35s loaded with TR-3 — and the fact that they won’t be able to fly combat missions until 2025 — has had ripple effects on the Air Force’s basing of the jets in Europe, a top general said Saturday.

“Don’t think that the TR-3 problems are over,” Gen. James Hecker, commander of U.S. Air Forces in Europe, told reporters at the Royal International Air Tattoo (RIAT) air show here on Saturday. “We do have a working software in TR-3 that is definitely good enough for training. … But there’s more to go.”

The TR-3 upgrades to the F-35 include better displays, computer memory and processing power. They are necessary to lay the groundwork for a broader F-35 modernization known as Block 4, which will include the ability to carry more weapons, better identify targets, and improve its electronic-warfare capabilities.

The TR-3 effort has been hampered by software problems, as well as some delays in hardware production. Lockheed Martin was originally supposed to have TR-3 ready around April 2023. But by July 2023, when the first jets earmarked for TR-3 began rolling off the line in Fort Worth, Texas, the upgrades were still incomplete. The government refused to accept deliveries of these jets, and a backlog of undelivered jets began to grow at Fort Worth.

Lockheed Martin and the JPO zeroed in on a strategy to finish an interim version of the TR-3 software that worked well enough to perform some training missions and conduct the test flights necessary for the government to accept planes.

But the F-35s with the truncated TR-3 software will not be able to conduct combat missions until 2025, F-35 program executive officer Lt. Gen. Mike Schmidt said in the JPO’s statement.

Schmidt said the F-35 program is taking a “phased approach” to the delivery of these F-35s. Jets with initial training capability will be delivered in July and August, he said, and jets with a “robust combat training capability” will come by the end of August.

“Our focus has been on providing our customers with aircraft that are stable, capable and maintainable, and this phased approach does that,” Schmidt said.

Air Force acquisition chief Andrew Hunter also said at RIAT on Saturday that the F-35 will receive additional capabilities as software modifications become available.

An F-35B flown by the Royal Air Force hovers during a display at the Royal International Air Tattoo at RAF Fairford in England on July 20, 2024. (Stephen Losey)

A Lockheed executive pledged to keep delivering subsequent upgrades to bring these F-35s up to their full potential.

“TR-3 and Block 4 represent a critical evolution in capability and their full development remains a top priority for us,” Bridget Lauderdale, Lockheed Martin’s vice president and general manager of the F-35 program, said in the JPO’s statement. “These and future software updates over the life of the program will ensure the F-35 continues to be an effective deterrent and the cornerstone of joint all-domain operations now and decades into the future.”

At least dozens of nearly-completed F-35s are now parked at Fort Worth, though the government has declined to say exactly how many. Hunter, the Air Force’s assistant secretary for acquisition, technology and logistics, was not sure how long it will take to deliver all backlogged jets but said, “It’s not overnight, that’s for sure.”

Deliveries of the Air Force’s F-35s are proceeding, Hunter said, and the military is working with partner nations to ensure those that are “critically dependent” on getting their F-35s for operational needs take priority.

But the TR-3 delay is going to have ripple effects on the Air Force’s operations, procurements and subsequent upgrades to the F-35 for years to come.

“We’ve been trying to get Block 4 of the F-35 for some time now, and it’s been delayed substantially,” Air Force Secretary Frank Kendall said at RIAT Saturday, noting that TR-3 is the foundation of the Block 4 upgrades.

The Government Accountability Office said in December 2023 that Block 4 was originally expected to be done in 2026 at a cost of $10.6 billion. But as requirements for the F-35 have evolved and Block 4′s list of added capabilities swelled, its costs rose to $16.5 billion and the schedule slipped to 2029.

Hunter said that the team of contractors working with Lockheed Martin on the F-35 — including Northrop Grumman, BAE Systems and Pratt & Whitney — have made tremendous progress over the last year on breaking down “stovepipes” that separated their development processes. The increased cooperation helped the interim version of TR-3 get done, he said, and should continue to benefit the program as Block 4 work progresses.

The TR-3 delays also were a factor in the Air Force’s decision to dial back its purchase plan for F-35As in the proposed 2025 budget, Kendall said. The service now plans to buy 42 F-35As in 2025, a reduction from its original plan to buy 48 jets next year.

And TR-3′s delay has led to a “slowdown” in the number of F-35s arriving at RAF Lakenheath, where the Air Force’s two European-based F-35 squadrons are located, Hecker said. Hecker said that fewer than a dozen F-35s intended for Lakenheath had been delayed.

He said the Air Force somewhat slowed down production of new F-35 pilots, so existing F-35 pilots and maintenance crews can keep flying and keep their readiness rating up.

As the training-only TR-3 jets start to come on board, Hecker said, the Air Force will likely send them to training bases. Combat-capable F-35s that had previously been flying training missions at those bases will then be transferred to Lakenheath, he said.

And the U.S. military is still withholding some payments to Lockheed Martin until the updates complete, Hunter said on Saturday.

“We will not pay for that which we have not yet received,” Hunter said.

Kendall said imposing further financial penalties on Lockheed Martin would likely not do much more to push the company to get TR-3 done faster.

“I don’t think Lockheed’s trying not to deliver,” Kendall said. “There’s a lot of motivation. It’s inherent in the program. They want to get this done.”

Stephen Losey is the air warfare reporter for Defense News. He previously covered leadership and personnel issues at Air Force Times, and the Pentagon, special operations and air warfare at Military.com. He has traveled to the Middle East to cover U.S. Air Force operations.

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