The United States has 540 F-35 combat jets and around half of them are not mission-capable, almost 15% under the acceptable limit, Bloomberg reported on Thursday.

Around 53.1% of the F-35 fleet was considered ready to carry out missions as of February, the jets' program manager Air Force Lieutenant General Michael Schmidt warned, stressing that this is well below the 65% limit. The full mission capable F-35 aircraft was under 30%, he told the House Armed Service Committee’s aviation subcommittee in a Wednesday hearing.

“This is unacceptable and maximizing readiness is my top priority,” Schmidt said, stating that he intends to achieve a 10% increase on the current rate within 12 months.

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

The fleet's average full mission-capable rate was roughly 39% in 2020; the readiness rates were down from that year, as per GAO. The fiscal year 2020's partial capability rate was 69%.

According to the operational test office of the Defense Department, the availability rate for planes used for combat missions was 65% at the end of fiscal 2022.

It's unclear whether the preparedness rates from last month signify the start of a long-term trend or a brief decline. Schmidt's statement doesn't specify the causes of the decline, but in the past, failures of parts and engine components more frequently than expected have been the main culprits.

Long wait periods for repairs at the depot and unexpectedly frequent need for repair or replacement of Pratt & Whitney engine power modules are other reported problems.

To address the issue, Schmidt’s plan focuses on unspecified “top degraders” of readiness by gathering program personnel, international users, Lockheed Martin Corp, Pratt & Whitney, and their subcontractors biweekly.

The GAO also found in the preview of its annual report on the F-35 that Lockheed Martin reported its poorest performance in six years late last year when it delivered 50% of the aircraft ordered. The Pratt & Whitney 2022 performance of Raytheon Technologies Corp. "indicates that the contractor again delivered nearly all engines late," according to the preliminary study.

Among other findings of the annual report, the Military found that an upgrade was necessary because the "Power and Thermal Management System," created by a Lockheed Martin subcontractor, "is under-performing, resulting in lower engine life."

The F-35 program has run into various bumps, including sourcing shortages issues of certain parts of the staple aircraft since 2019.

The multi-billion project's future has already looked quite grim, with grounded fifth-generation fighters only being able to carry out missions for 12 days on average in 2021 and 11 days in the first half of 2022.

In 2022, the United States Air Force even went as far as grounding its F-35A jet fleet, citing a potentially faulty component in the ejection seat that could endanger pilots in an emergency, a concern that has also grounded other types of military planes used for training.

The F-35 is considered the most expensive military equipment ever built, with a total cost of $412 billion in 2021, up from $398 billion in 2022, according to a Department of Defense report in September 2022.

Read next: F-35 defects put Israeli air force in dilemma: Israeli media