F-35 Engine Core Upgrade to Face Critical Review in August, Flight Testing in 2030

DAYTON, Ohio—A major planned upgrade for the F-35 fighter’s engine is expected to reach a major milestone in August when it undergoes its critical design review, an Air Force official said July 27.

John Sneden, the service’s portfolio acquisition executive for propulsion, told reporters at the Life Cycle Industry Days conference here that the so-called Engine Core Upgrade is “proceeding well.”

ECU is a major effort to modify the F-35 Joint Strike Fighter’s existing F135 engines, made by Pratt & Whitney, to give them more thrust and cooling power—attributes that are needed to support a series of extensive F-35 upgrades known as Block 4, as well as future improvements. The Air Force is also pursuing an upgraded power and thermal management system to complement the ECU.

The critical design review is an important milestone in the progression of a major defense program. Acquisition experts scrutinize all elements of a system and verify its design is stable, is likely to meet performance requirements, and is on track to meet its cost goals. 

After the ECU program’s design review is finished, Sneden said, it is scheduled to undergo ground testing in late 2027. Flight tests will start around 2030, Sneden said.

When Pratt announced in July 2024 that ECU had passed its preliminary design review, the company said the critical design review was expected for mid-2025. But the Air Force Life Cycle Management Center said in a statement to Air & Space Forces Magazine that the planned date for ECU’s critical design review has not changed during the last two years, since passing its preliminary design review. AFLCMC said the Air Force did not make a similar projection to Pratt’s announcement and never expected the CDR to take place in 2025.

The Government Accountability Office said in a September 2025 report that while the preliminary review validated ECU’s design met expectations, it found “major risks” such as a lack of integrated development and test schedules between the engine and the F-35, parts that were not arriving in time to support testing, and immature test plans. GAO said in the report that production on the engine upgrade is expected to begin in 2031.

In its 2027 budget request, the Air Force is seeking a major infusion of cash to make sure it meets its production targets despite the slip in the design review schedule.. Budget documents show the service is seeking $317.5 million for the entire F-35 propulsion enterprise, up from around $121 million in 2026, much of it for the Engine Core Upgrade and the Power and Thermal Management Upgrade.

The increase is meant “to accelerate the development and delivery of F135 ECU and F-35 [power and thermal management upgrade]. Both are critical propulsion upgrades for enhanced fleet readiness,” according to budget documents. “Acceleration allows for achieving target fielding dates as well as support to the Block 4 aircraft.”