F-35 Joint Strike Fighter: More Actions Needed to Explain Cost Growth and Support Engine Modernization Decision
Fast Facts
The Department of Defense's most expensive weapon system—the F-35 aircraft—is now more than a decade behind schedule and $183 billion over original cost estimates.
This program is weighing options to upgrade its engine and cooling system. But it hasn't taken some important steps, such as fully assessing the costs and technical risks of the different options. DOD also doesn't plan to manage the upgrades as a separate acquisition program. This would limit Congress's insight into possible future cost increases.
We recommended ways to address these and other issues, including actions DOD should take and Congress should consider.
The F-35
Highlights
What GAO Found
The F-35 program continues to experience schedule delays, cost growth, and late deliveries. Program delays in completing the F-35 simulator continue to prevent the Department of Defense (DOD) from completing the testing required to demonstrate that the F-35 is ready for full manufacturing rates, even though the program is already producing over 125 aircraft per year.
The F-35 program's total procurement costs have increased by $13.4 billion since the last cost estimate in 2019. This is, in part, due to DOD spreading out aircraft purchases and adding years to its delivery schedule. Contractors also continue to have challenges with delivering aircraft and engines on time, but they are working to address these issues.
Further, DOD is 5 years into a development effort to modernize the F-35's capabilities. This effort, known as Block 4, is experiencing developmental delays for important technology updates. Block 4 costs also grew to $16.5 billion, an increase of more than $1 billion since GAO last reported.
F-35 Block 4 Modernization Cost Increases since 2018 (then-year dollars in billions)
The program's cost reporting mechanisms do not fully explain the reasons for cost growth. For example, DOD's reports to Congress on Block 4 cost growth do not distinguish higher-than-expected costs for previously planned Block 4 capabilities from growth due to adding new capabilities. Consequently, Congress does not have a complete picture of escalating F-35 modernization costs.
The program is exploring options for modernizing the F-35's engine and thermal management system that is used to cool aircraft subsystems that generate heat. The program plans to manage this multi-billion dollar effort under the existing program, which is scheduled to transition to sustainment soon and that would limit congressional oversight. The cooling system is overtasked, requiring the engine to operate beyond its design parameters. The extra heat is increasing the wear on the engine, reducing its life, and adding $38 billion in maintenance costs.
The program assessed some engine and cooling improvement options, but it has not fully defined the requirements for how much future cooling the aircraft will need. By obtaining this key information, DOD and the services would be more informed about the future performance, cost, and technical implications.
Why GAO Did This Study
The F-35 Lightning II Joint Strike Fighter program is DOD's most expensive weapon system program. DOD estimates it will cost nearly $1.7 trillion to buy, operate, and sustain the aircraft and systems over its lifetime. DOD is also assessing options for modernizing its engine.
Congress included provisions in three statutes for GAO to review the F-35 program and a Senate report included another. This report (1) identifies the F-35's progress toward full-rate production, (2) assesses DOD's F-35 modernization effort (known as Block 4), and (3) assesses DOD's approach for modernizing its engine and thermal management system. GAO reviewed program, DOD, and contractor documentation on these topics and interviewed program, DOD, and contractor representatives. GAO assessed the program's progress against its own plans. GAO also applied its cost estimating and technology readiness leading practices, as appropriate.
Recommendations
Congress should consider directing the F-35 program to manage the engine modernization as a separate program. GAO added this matter for Congress because DOD has not committed to a separate engine program consistent with GAO's recommendation. GAO made seven total recommendations to DOD, including that it improve its reporting on Block 4 cost growth and define engine requirements. Overall, DOD concurred with three, partially concurred with three, and did not concur with one recommendation. GAO believes all the recommendations are warranted.
Matter for Congressional Consideration
| Matter | Status | Comments |
|---|---|---|
| Congress should consider directing the Office of the Under Secretary of Defense for Acquisition and Sustainment to ensure the F-35 engine and thermal management modernization effort is managed as a separate program, with its own distinct cost, schedule, and performance baseline. | Congress took action in Section 226 (c) of the FY24 NDAA stating that the Secretary of Defense shall designate all activities relating to the modernization, upgrade, and integration of the major subsystems of the F-35 propulsion and thermal management modernization effort collectively, as a single major subprogram of the F-35 aircraft acquisition program. This action meets the intent of our recommendation. |
Recommendations for Executive Action
| Agency Affected | Recommendation | Status |
|---|---|---|
| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should ensure the F-35 program office reports to Congress on cost differences between original estimates and actual costs for a defined group of modernization capabilities over time. (Recommendation 1) |
DOD concurred with this recommendation. In September 2025, DOD defined a set of capabilities for the Block 4 subprogram. DOD officials also reported that the Block 4 contract requires the contractor to track the cost of each Block 4 capability. If DOD uses this information to compare costs of Block 4 subprogram capabilities over time and report to Congress on the results, it would meet the intent of our recommendation. As of July 2026, the program has not reported to Congress on the cost differences of Block 4 subprogram capabilities. We will continue to monitor DOD's progress in this area.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should ensure the F-35 program conducts a technology readiness assessment to evaluate TRLs for all critical technologies and systems for engine and thermal management modernization to inform the military services' modernization decisions. (Recommendation 2) |
DOD concurred with this recommendation. In December 2023, Congress included a provision in the fiscal year 2024 National Defense Authorization Act, which directed the Program Executive Officer for F-35 to complete a cost-benefit and technical risk analysis of the aircraft modernization efforts. DOD is planning to hold a technology readiness review in December 2026, which officials state will include an evaluation of readiness for all critical technologies and systems for engine and thermal management modernization. Completion of this review will meet the intent of our recommendation. We will continue to monitor DOD's progress in this area.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should ensure the F-35 program matures all critical technologies and systems for engine and thermal management modernization to a TRL 7 prior to starting product development. (Recommendation 3) |
DOD partially concurred with this recommendation. In December 2023, Congress included a provision in the fiscal year 2024 National Defense Authorization Act that directed the Program Executive Officer for F-35 to complete a cost-benefit and technical risk analysis that determines and evaluates F-35 modernization benchmarks. DOD's Technical Risk Assessment Methodology requires that each critical technology has achieved the necessary level of technology maturity. As of July 2026, DOD is planning to hold an Engine Core Upgrade (ECU) risk reduction design review in December 2026. We will continue to monitor DOD's efforts to mature engine modernization technologies.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should ensure the F-35 program reports the full life-cycle costs, for each engine and thermal management modernization option, including integration and any necessary related aircraft upgrade costs, to inform the military services' modernization decisions. (Recommendation 4) |
DOD concurred with this recommendation. In December 2023, Congress included a provision in the fiscal year 2024 National Defense Authorization Act, which directed the Program Executive Officer for F-35 to complete a comprehensive cost-benefit analysis of the F-35 modernization efforts. As of July 2026, DOD was planning to complete a cost benefit risk analysis for engine and thermal management modernization in December 2026, which DOD states will include the full life-cycle cost for each engine and thermal management modernization option. Completing this analysis would meet the intent of our recommendation. We will continue to monitor DOD's efforts in this area.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should ensure the F-35 program obtains an independent cost estimate for all engine and thermal management modernization options, including integration and any necessary related aircraft upgrade costs, to inform the military services' modernization decisions. (Recommendation 5) |
DOD partially concurred with this recommendation. In April 2023, DOD completed an independent cost estimate for F-35 engine modernization options, but this estimate did not include costs for modernizing thermal management systems. As we noted in our report, all potential options for resolving the F-35's current and future power and cooling needs involve more than just the engine. We continue to believe our recommendation is valid. In December 2023, Congress included a provision in the fiscal year 2024 National Defense Authorization Act that directed the F-35 Program Executive Officer to obtain an independent cost estimate as part of their overall engine modernization cost-benefit analysis, to include engine subsystems. As of July 2026, DOD plans to complete an independent cost estimate in March 2027 that will include thermal management modernization options.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should direct the F-35 program office to reevaluate its comparative analysis after the military services define their power and cooling requirements, and before proceeding with development of the engine and thermal management modernization effort. (Recommendation 6) |
DOD did not concur with this recommendation. Officials stated that as approved requirements are updated, the F-35 program will reevaluate its analysis as appropriate. We continue to believe that a comparative analysis based on approved power and cooling requirements will help DOD determine what additional power and cooling is needed to support planned capabilities through 2035. As of July 2026, DOD reported that as part of their December 2026 cost-benefit and technical risk analysis, they plan to reevaluate F-35 engine and thermal management modernization options based on requirements established by the services. Completing this analysis will meet the intent of the recommendation. We will continue to monitor DOD's progress in this area.
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| Department of Defense | The Under Secretary of Defense for Acquisition and Sustainment should manage F-35 engine and thermal management modernization as a separate program, with its own distinct cost, schedule, and performance baseline. (Recommendation 7) |
DOD partially concurred with this recommendation. On March 12, 2024, the Under Secretary of Defense for Acquisition and Sustainment directed the F-35 program to designate Engine and Power Thermal Management Modernization development as a major subprogram of the overall F-35 acquisition program. Designating engine and power management modernization efforts as a major subprogram meets the intent of our recommendation by establishing a distinct cost, schedule, and performance baseline.
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