US F-35 audits: modernization and delivery status

What the audit found

GAO has reviewed the F-35 programme each year since 2001. A 2016 review called for Block 4 modernization to have its own baseline and regular cost, schedule and performance reporting ¹.

In 2019 GAO found that four of eight reliability and maintainability targets were not being met. It reported that the programme planned to start modernization work without a complete business case or baseline cost and schedule estimate ².

The 2024 acquisition review reported that every engine and 91 percent of aircraft delivered in 2023 were late. It linked modernization delay to Technology Refresh 3 hardware and software and recorded unresolved decisions on engine and thermal-management work ³.

The official response

Defense did not concur with GAO's 2016 recommendation to manage Block 4 as a separate acquisition programme. Congress later required baseline reporting, and in March 2024 Defense directed that Block 4 be treated as a major subprogramme. GAO then closed the recommendation as implemented ¹.

For the six recommendations made in 2025, Defense concurred with four and partly concurred with two. It described planned reviews of production capacity, incentive fees, development methods and acquisition documentation ⁵.

Parliament

In the US system, Congress required recurring GAO review of the F-35 programme. Congress responded to the 2016 oversight issue by requiring baseline information before new Block 4 development contracts and annual updates to defence committees ¹.

GAO's 2024 and 2025 reports were produced under congressional review provisions. They supplied Congress with cost, schedule, production and recommendation-status information ^3,5.

Follow-up audits and recommendations

The April 2024 sustainment review estimated planned fleet sustainment at $1.58 trillion in 2023 dollars, up from $1.1 trillion in 2018. Thirty of 43 sustainment recommendations made since 2014 were then unimplemented ⁴.

The September 2025 review found Block 4 costs more than $6 billion above the original estimate and completion at least five years later. All 110 aircraft delivered in 2024 were late by an average of 238 days ⁵.

As of July 2026, GAO listed all six 2025 recommendations as open. Defense had described work under way, but GAO had not closed the production-capacity, variance tracking, incentive, acquisition-pathway or development-practice actions ⁵.

Courts, tribunals and investigations

No court judgment deciding the GAO programme findings was identified in the official sources reviewed. These reports concern acquisition oversight, production, modernization and sustainment rather than a judicial determination ^1,5.

GAO's findings and open recommendations do not state that the department or either named contractor committed a crime ^3,5.

Where it stands now

The programme entered full-rate production in March 2024 after completing its final simulated tests. Production and modernization continued, with delivery and software issues still under review ³.

The 2025 recommendation page records planned Defense actions into 2027 and 2028. At the 20 October 2026 check, GAO still marked all six recommendations open ⁵.

Timeline