WASHINGTON — The estimated development costs for the F-35 Joint Strike Fighter’s ongoing modernization program grew by $1.9 billion since 2019, and the effort is expected to extend though 2027, the U.S. Government Accountability Office said in a new March 18 report.

Over the course of the F-35′s Block 4 modernization program, which officially began in 2018, the Pentagon will upgrade the Lockheed Martin-produced jet’s hardware and software, adding new weapons capabilities and computing systems.

But due to challenges in developing and testing some of those new technologies, as well as continued schedule delays, the U.S. Defense Department’s most recent estimate in 2020 shows that Block 4 development is now projected to reach $14.4 billion, the GAO said.

Meanwhile, the modernization effort — which was initially expected to wrap up in 2026 — is now scheduled to conclude one year later, though the GAO noted that the schedule is based on “estimates formulated at the start of the Block 4 effort” and not Lockheed’s demonstrated performance. Therefore, it is possible “the scheduled 2027 completion date is not achievable,” the watchdog stated.

The GAO listed a number of reasons for the $1.9 billion in cost growth. A cost increase for flight tests resulted in an additional $705 million charge; overhead and administrative costs ballooned by $471 million; and the cost of a new training lab added $336 million. A package of computing system upgrades, known as Technology Refresh 3, experienced a $296 million cost increase.

The news of the cost increases comes weeks after Air Force Chief of Staff Gen. CQ Brown acknowledged the service is conducting a tactical aircraft study to explore whether it should buy fewer F-35s. Currently, the U.S. Air Force is the F-35′s largest customer, with 1,763 F-35As included in the service’s program of record.

House Armed Services Committee Chairman Adam Smith, D-Wash., indicated his own frustration with F-35 program costs during a March 5 event where he said the nation should “stop throwing money down that particular rathole.”

In a statement, F-35 program executive Lt. Gen. Eric Fick said the government continues to make progress on Block 4. “Program risks still exist, but are well understood and actively managed,” he said.

The Pentagon previously issued a seven-year cost estimate for the Block 4 effort, claiming in 2018 that the program would cost $10.6 billion from fiscal 2018 to fiscal 2024. The department did not provide data on fiscal 2025 to fiscal 2026, when the program was originally slated to end.

The new $14.4 billion cost estimate covers all years associated with the program, including Block 4-related costs dating back as far as FY13. “However, over half of the increase since we reported last year — $1.9 billion — is net cost growth within various aspects of the Block 4 development program” and cannot be attributed to prior years’ costs or planned expenses during fiscal 2025 to fiscal 2026, the GAO said.

Valerie Insinna is Defense News' air warfare reporter. She previously worked the Navy/congressional beats for Defense Daily, which followed almost three years as a staff writer for National Defense Magazine. Prior to that, she worked as an editorial assistant for the Tokyo Shimbun’s Washington bureau.

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