Skip to main content

F-35 Aircraft: DOD Should Assess and Update Its Engine Sustainment Strategy to Support Desired Outcomes

GAO-22-104678 Published: Jul 19, 2022. Publicly Released: Jul 19, 2022.
Jump To:

Fast Facts

The F-35 is DOD’s most ambitious and costly weapon system. But DOD’s strategy for maintaining the aircraft’s highly advanced engine has shortcomings.

DOD's strategy allows 6% of F-35s to be unavailable for missions at any given time due to engine issues. But the number of F-35s that this leaves available for operations isn't what the military services consider to be sufficient. Also, DOD currently exceeds the 6% limit—in part because its strategy doesn't ensure enough spare engine parts are available. As a result, 9% of F-35s were not operational in mid-2020.

Our recommendations address this and other issues we found.

The F-35

fighter jet on the tarmac

Skip to Highlights


What GAO Found

The Department of Defense's (DOD) F-35 engine sustainment strategy does not meet the desired outcomes of the military services. DOD's current strategy, if implemented as planned, allows for 6 percent of its F-35 aircraft to be non-mission capable, or unable to perform assigned missions, due to engine issues. The military services desire outcomes similar to their other tactical fighter aircraft, which since 2017, have generally experienced 1 percent or less of aircraft being unable to operate due to engine issues, according to officials. Until DOD assesses its F-35 sustainment strategy, including its goals, and reaches agreement on any needed changes, the program will fall short of the desired outcomes of users.

DOD has not met several key performance goals for sustaining the F-35 engine. First, DOD only met its 6 percent or less non-mission capable due to engine issues goal in one month from January 2021 through February 2022. As a result, the number of F-35 aircraft unable to fly due to the lack of an operating engine has been increasing since January 2020 with a slight decrease from July 2021 through February 2022. Second, DOD has met three of five of its reliability and maintainability goals—metrics aimed at ensuring that the F-35 engine will be available for operations as opposed to out of service for maintenance. The goals that DOD has not met have resulted in higher-levels of maintenance.

F-35 Aircraft without an Operating Engine, January 2020–February 2022

F-35 Aircraft without an Operating Engine, January 2020–February 2022

DOD has developed and is implementing corrective-action plans since fall 2020 to improve the capacity of its engine-repair maintenance depots. DOD's plans have resulted in improvements, such as reducing the time to repair a key module of the engine from 207 days in October 2020 to 119 days in January 2022. However, DOD's plans are highly dependent on assumptions about obtaining funding and its ability to address future risks. One area that has not been fully addressed is an agreement between DOD and the prime contractor on a model for forecasting spare parts needs. The prime contractor and DOD are using different data inputs to estimate spare parts needs. Until addressed, the program risks future parts shortages that could affect its ability to repair engines and ensure F-35 aircraft have operating engines.

Why GAO Did This Study

The F-35 aircraft, with its advanced capabilities, represents a growing portion of DOD's tactical aviation fleet—with over 400 aircraft in use to date. DOD plans to procure about 2,000 more F-35s with estimated life-cycle costs of the program exceeding $1.7 trillion; $1.3 trillion of those costs are associated with sustaining the aircraft. GAO previously reported that challenges sustaining the F-35 engine may pose its greatest sustainment risk over the next 10 years.

House Report 116-442 included a provision for GAO to review F-35 engine sustainment challenges.

This report evaluates the extent to which (1) the F-35's engine sustainment strategy has been aligned with military service desired outcomes; (2) DOD has met performance goals for sustaining the F-35 engine; and (3) DOD has developed and implemented plans to address any challenges. GAO reviewed program documentation, analyzed sustainment data, and interviewed officials from DOD and the prime contractor for the F-35 engine, Pratt & Whitney.


GAO is making two recommendations to DOD: assess and make changes to the F-35 engine sustainment strategy; and develop a shared model for forecasting spare parts needs with the engine's prime contractor. DOD concurred with both recommendations.

Recommendations for Executive Action

Agency Affected Recommendation Status
Department of Defense The Secretary of Defense should ensure that the F-35 Joint Program Office, in collaboration with the military services, assesses and updates the F-35 engine sustainment strategy, including its goals and the necessary actions to achieve its goals—such as the required number of spare engines and modules and the levels of maintenance and capacity needed to repair the modules. The assessment and any corresponding decisions and actions should be documented and take into consideration engine sustainment costs and modernization plans. (Recommendation 1)
DOD concurred with this recommendation. In September 2023, the Joint Program Office stated that the services had developed a new engine sustainment strategy that would be incorporated into the next sustainment contract in December 2025. In the meantime, the Air Force has decided to purchase its own spare parts for its engines beginning in 2024 to help it better manage its fleet. Until we are able to review the new sustainment contract with the updated sustainment strategy, this recommendation will remain open.
Department of Defense The Secretary of Defense should ensure that the F-35 Joint Program Office collaborates with the military services and Pratt & Whitney on developing a shared model for spare part forecasts, reaches agreement with the military services and Pratt & Whitney on a model for spare parts forecasting, and documents that agreement to ensure common understanding of the model. (Recommendation 2)
In September 2023, the F-35 Joint Program Office stated that the Propulsion team had developed a 5-gated modeling system at the modular level in conjunction with Pratt & Whitney. This model output is used to develop a joint coordinated spare parts model with final decision authority at the Joint Program Office. In addition, funding requirements from this model are fed into the annual cost estimate for future procurement actions. To date, two successful joint modeling efforts have been completed successfully. The Department will continue to refine this modeling methodology moving forward and expects that the output will be included in the December 2025 contract requirements list. We will continue to monitor these actions and until we can review the model and its outputs, this recommendation will remain open.

Full Report

Office of Public Affairs


AircraftAircraft enginesAircraft maintenanceFighter aircraftMilitary aircraftMilitary readinessPerformance goalsWeapon systemsMilitary forcesSpare parts