The F-35 Joint Strike Fighter program is scaling back its ambitions for a slate of upgrades called Block 4 , the Government Accountability Office said Wednesday, as the aircraft struggles with production delays, cost overruns and supply chain snarls.

The downgrading of plans for F-35 upgrades, revealed in GAO’s report “F-35 Program: Actions Needed to Address Late Deliveries and Improve Future Development,” is the latest challenge for the vital — as well as troubled and expensive — program.

“The F-35 remains critical to our national defense, as well as that of our partners and allies, and is expected to retain critical roles for decades to come,” the report said. “After nearly 20 years of aircraft production, however, the F-35 program continues to overpromise and underdeliver.”

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