(Bloomberg) -- Only about half of the Pentagon’s fleet of F-35 fighter jets are considered mission-capable, well below the target of 65% and a state of readiness the program manager terms “unacceptable.”

As of February, the monthly average rate of mission-capable jets in the US’s fleet of more than 540 F-35s was 53.1%, according to Air Force Lieutenant General Michael Schmidt, the program manager. That means they can fly at least some of their required missions, such as combat, show-of-force flights, training and testing.

The percentage of planes capable of flying all their missions — the so-called full mission capable rate — was less than 30%, Schmidt said in written testimony prepared for a Wednesday hearing of the House Armed Service Committee’s aviation subcommittee.

“This is unacceptable and maximizing readiness is my top priority,” Schmidt said in his prepared remarks. Schmidt said his goal is to increase readiness rates by at least 10% in the next 12 months. 

“Readiness challenges remain, as indicated in multiple Government Accountability Office findings,” he said, according to the remarks.

The readiness rates marked a drop from 2020, when the fleet’s average full mission-capable rate stood at about 39%, according to the GAO. The partial capability rate was 69% in fiscal year 2020. The availability rate for jets assigned to combat missions stood by the end of fiscal 2022 at 65%, according to the Defense Department’s operational test office.

It’s not clear if last month’s readiness rates represent temporary dip or the beginning of a long-term trend. Schmidt’s statement doesn’t spell out the reasons for the drop but past culprits have been a lack of spare parts as well as parts and engine components breaking more frequently than anticipated.

Other problems include long depot repair times as well as Pratt & Whitney engine power modules needing repair or replacement faster than expected.

Schmidt’s plan to fix the problem will focus on addressing unspecified “top degraders” of readiness by gathering program personnel, international users, Lockheed Martin Corp, Pratt & Whitney and their subcontractors every two weeks.

In a preview of its annual report on the F-35, the GAO also found:

* Lockheed Martin delivered 50% of aircraft late last year, in what it said was the worst result in six years. A preliminary analysis of Raytheon Technologies Corp’s Pratt and Whitney 2022 performance “indicate that the contractor again delivered nearly all engines late.”

* The “Power and Thermal Management System” designed by a Lockheed Martin subcontractor that provides cooling for the engine “is under-performing, resulting in reduced engine life” so the Pentagon determined it must be upgraded.

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