US Air Force: F-35 Engine Core Upgrade to Face Critical Design Review in August

DAYTON, Ohio — A major planned upgrade for the F-35 fighter's engine is expected to reach a significant 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 (ECU) is "proceeding well."

Upgrade Details

The ECU is a major effort to modify the F-35 Joint Strike Fighter's existing F135 engines, manufactured by Pratt & Whitney. The upgrade aims to provide:

• Increased thrust
• Greater cooling power

These attributes 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.

Critical Design Review

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

Timeline

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.

When Pratt announced in July 2024 that ECU had passed its preliminary design review, the company stated that the critical design review was expected for mid-2025. However, the Air Force Life Cycle Management Center (AFLCMC) 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.

GAO Findings

The Government Accountability Office (GAO) said in a September 2025 report that while the preliminary review validated ECU's design met expectations, it found "major risks," including:

• 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
• Immature test plans

GAO said in the report that production on the engine upgrade is expected to begin in 2031.

Budget Request

In its 2027 budget request, the Air Force is seeking a major infusion of cash to ensure it meets its production targets despite the slip in the design review schedule. Budget documents show the service is requesting $317.5 million for the entire F-35 propulsion enterprise, up from approximately $121 million in 2026. Much of this funding is allocated 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."

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