Key Points – The F-35’s Engine Core Upgrade (ECU), a critical component of the broader Block 4 modernization, has been delayed by at least a year, pushing its key Critical Design Review to mid-2026.
-This vital upgrade by Pratt & Whitney is needed to provide the increased electrical power and cooling required for the F-35’s advanced new sensors and electronic warfare systems.
-The delay raises concerns about a potential ripple effect on the entire Block 4 timeline, which is already facing challenges with its TR-3 hardware and software integration, potentially pushing back the fielding of a fully capable, modernized F-35 fleet past 2029.
More Power, More Problems: The Hurdles Facing the F-35’s Modernization
The F-35 is currently undergoing a series of upgrades to fit the new F-35 Block 4 standard.
Under these comprehensive upgrade programs, the fighter is expected to see an increase in its overall performance and reliability, ensuring that the aircraft remains in service for years to come. However, recent reports emerged stating that the program may be facing even more delays due to problems implementing the planned engine upgrades. These delays could set back the entire program by at least a year.
The F-35 is Delayed… Again…
The problem arises with the ECU (Engine Core Upgrade). Intended to make the engines more fuel efficient and increase output, the program faces several challenges. The most immediate is the schedule delay in the Critical Design Review, which could ripple through the rest of the development timeline and potentially push back the 2029 target for operational deployment. Both the F-35 Joint Program Office (JPO) and Pratt have been silent on the matter, refusing to comment on what exactly is causing delays, raising questions as to when the project will actually be ready.
Some analysts suggests that engineering and technical challenges are the reason behind this delay. Coordinating the ECU with the Power Thermal Management Upgrade (PTMU) and Block 4 upgrades requires close collaboration between multiple contractors and the F-35 JPO. Cost management is also a critical issue. According to reports, officials in the JPO and the Pentagon are skeptical about the F-35 Block 4 upgrade due to fears of cost overruns and other uncertainties. Additionally, retrofitting the existing fleet with the upgraded engine presents logistical and financial challenges, even as new F-35s are built with the ECU.
This is not the first time that the F-35 Block 4 has been delayed. Last year, reports emerged that the Block 4 was facing delays due to a series of hardware and software difficulties. The upgraded software and hardware (collectively referred to as TR-3) was supposed to provide a fairly comprehensive upgrade to its internal electronics and improve situational awareness among other things. However, TR-3 faced delays due to integration challenges, forcing the DoD to temporarily halt procurement of F-35s until TR-3 was ready. It is likely that the ECU is facing similar problems right now.
Why the Pentagon Wants the Block 4
The need for an engine upgrade stems from the evolving mission profile of the F-35. Originally designed with a specific set of performance parameters, the aircraft has since been tasked with more complex operations and equipped with increasingly sophisticated systems.
These enhancements, particularly those planned under Block 4, require significantly more electrical power, improved thermal management, and greater engine durability
. The current F135 engine, while highly capable, is being pushed beyond its original design limits. A 2023 report by the Government Accountability Office highlighted that the existing Power and Thermal Management System (PTMS) is overburdened, forcing the engine to operate under conditions that accelerate wear and reduce its lifespan. This has led to increased maintenance costs and operational inefficiencies.
In response, the Pentagon chose to pursue an upgrade to the existing F135 engine rather than develop a completely new adaptive engine. This decision was driven by considerations of cost, integration complexity, and timeline.
The ECU is designed to improve thrust and fuel efficiency, enhance thermal management, increase electrical power generation, and extend engine life while reducing maintenance demands. The program passed its Preliminary Design Review in July 2023, confirming that it was on track. However, by mid-2025, the Critical Design Review, a key milestone that finalizes the design, had been delayed by a year and is now expected in mid-2026.
In September 2024, Pratt & Whitney was awarded a $1.31 billion contract to continue developing the ECU. This cost-plus-incentive fee contract covers a wide range of activities, including design and analysis, rig and engine testing, developmental hardware production, air system integration, airworthiness evaluation, and product support. If the program proceeds as planned, the first operational F-35s equipped with the upgraded engine could enter service by 2029. However, the delay in the Critical Design Review has introduced uncertainty into this timeline.
Too many Upgrades at the Same Time?
The ECU is closely linked to the Block 4 upgrade, which introduces advanced sensors, electronic warfare capabilities, and AI-driven systems. These enhancements significantly increase the aircraft’s power and cooling requirements. To support this, the Pentagon is also pursuing the PTMU. This involves replacing or enhancing the current PTMS, which is responsible for cooling avionics and other onboard systems.
The PTMU is being developed in parallel with the ECU and is critical for ensuring the F-35 can operate effectively with Block 4 systems. Lockheed Martin, the F-35’s prime contractor, has been designated the Lead Systems Integrator for the PTMU. A competition is currently underway between Honeywell, the current PTMS provider, and RTX (formerly Raytheon Technologies) to develop the new system.
Despite the recent delay, Pratt & Whitney remains committed to delivering the ECU. Jill Albertelli, president of Pratt’s military engines division, emphasized that the program is still progressing and that the design requirements are locked in, allowing for continued development.
Given the rising costs and technical hurdles, the Pentagon is also reevaluating the Block 4 upgrade to better align with what industry can realistically deliver. This reevaluation may help streamline the integration of the ECU and PTMU, ensuring a more cohesive modernization effort.
About the Author:
Isaac Seitz, a Defense Columnist, graduated from Patrick Henry College’s Strategic Intelligence and National Security program. He has also studied Russian at Middlebury Language Schools and has worked as an intelligence Analyst in the private sector.
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