PARIS—A key milestone for the Pentagon’s program to upgrade the F-35 engine has been pushed back a year, raising questions about the overall timeline of the effort.
The engine upgrade will reach “critical design review,” a milestone that essentially closes the design phase of the program, by “mid-next year,” according to Jill Albertelli, president of military engines for Pratt & Whitney—a year later than Pratt previously estimated.
The upgraded engines were supposed to hit the fleet by 2029, but Albertelli declined to confirm whether they will be ready by then, deferring specifics to the F-35 Joint Program Office. JPO did not respond in time for publication.
“We were working the timeline with the JPO because many things have to come in place: obviously I have to design, develop the engine, test it, deliver the hardware, all of that. But you also have certification with the jet, so we’re working very closely with Lockheed and the JPO on that timing. So we’ll see how that ends up,” Albertelli told Defense One on the sidelines of the Paris Air Show.
Pratt declined to say why the delay is happening.
If the Engine Core Upgrade arrives later than expected, it wouldn’t hold up deliveries of F-35s, but it could affect the eventual retrofits of the jets. The upgrade was designed to improve the engine’s performance and provide the necessary power for a slew of improvements to the plane, known as Block 4. In 2023, the Pentagon decided to move ahead with the upgrade instead of developing a new, adaptive engine for the jet. Last July, ECU passed the preliminary design review.
The jet’s cooling system, called PTMS, also needs to be upgraded to handle more cooling beyond Block 4. Lockheed Martin has launched a competition between Honeywell, which makes the F-35’s current Power and Thermal Management System, and RTX, which is offering a new cooling system. The system picked won’t change the design of ECU, Albertelli said.
As the timeline for ECU remains murky, the Pentagon is also revising plans for Block 4 as a whole. Pentagon officials are “reimagining Block 4” to examine what industry can deliver since costs have skyrocketed and delays have plagued the effort.
The “reimagine” won’t change much for Pratt, Albertelli said, since the specific requirements for the engine upgrade have already been locked in with JPO.
Having those requirements “allows us to design and continue to move forward. We still leave some flexibility in there if there are some slight modifications during the time period. A normal engine development program would do that,” she said.
Beyond ECU, Pratt is pushing forward with its adaptive engine program, called Next Generation Adaptive Propulsion. The Air Force is funding two companies through the development phase, Pratt and General Electric, and one will eventually build an adaptive engine for the service’s new sixth-gen F-47.
Albertelli didn’t disclose a specific timeline for their NGAP offering, but said the effort is going “extremely well” and that Pratt is “on the timeline of the U.S. Air Force.”
“It is 100% digital, so it’s more efficient, it’s collaborative design, and my customer is in a collaborative digital environment along that journey with me. So that means there’s no surprises along the way,” she said.
Pratt and GE both won a $3.5 billion contract in January to continue NGAP development, with work set to complete in 2032. However, the Pentagon seems to be slowing funding for the effort in initial 2026 budget documents, decreasing from $507 million in the 2025 budget to $330 million in the 2026 request.
When asked about the proposed decrease in funding, Albertelli said it’s still “very early” in the budget process.
“What hasn’t changed is the threat of the adversary, and I think that that concern alone is just, there’s a lot of conversations, a lot of briefings that I’ve been doing on the Hill as well to make sure people understand all the information around it. We are full steam ahead at this point, and we are on contract for that as well,” she said.
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