Marine Corps pilot Maj. N.H. “Robo” Thayer conducts a conventional landing of a F-35B Lightning II at Naval Air Facility El Centro, Calif., in February 2021. (Drew Verbis/U.S. Navy)

WASHINGTON — F-35 fighter jets are available to fly only 55% of the time, and 73% of replacement parts must be sent back to suppliers because the Pentagon’s maintenance depots are inadequate, according to a new report from government auditors.

“The military services must take over management of F-35 sustainment by October 2027 and have an opportunity to make adjustments — specifically to the contractor-managed elements,” the Government Accountability Office wrote in its 96-page report released Thursday. “Reassessing its approach could help [the Defense Department] address its maintenance challenges and reduce costs.”

The report comes days after an F-35B Lightning II crashed in South Carolina after a Marine Corps pilot encountered a problem while in flight and ejected. Questions have been raised about what prompted the pilot to eject and how the $100 million warplane was able to keep flying pilotless for 60 miles before crashing.

The F-35 program is one of the Pentagon’s most expensive, costing $1.7 trillion in its lifespan. Of that amount, $1.3 trillion has been spent on the cost of operating and maintaining the fighter jets, according to the GAO report.

The 55% average availability rate as of March 2023 is below the goal of a mission-capable rate for different versions of the aircraft of 85% to 90%.

The report state the GAO spoke to military maintenance staff at three military bases who said they were not allowed to look up spare part numbers for the F-35 because the proprietary database was controlled by Lockheed Martin, the company that makes the jets.

“Not having ready access to part numbers hinders the repair of the aircraft because it delays the ordering and receipt of needed parts,” the auditors wrote. “Since then, DOD has added new F-35s to its fleet, increasing the demand for repairs, while continuing to face delays establishing military service depot repair capacity. As a result, the number of parts waiting for repair increased to over 10,000 as of March 2023.”

As of March, the Pentagon was sending 73% of F-35 components back to the original equipment manufacturer because of delays in standing up a full depot repair capability at military service depots. Maintenance on components of fighter jets is behind, leaving more than 10,000 components waiting to be repaired.

The F-35 is used by the Air Force, Marine Corps and the Navy. Previous maintenance concerns have led to a push for the Pentagon to take over management of F-35 maintenance, but that shift is not scheduled to occur until late 2027.

The GAO made seven recommendations to address maintenance delays, and the Pentagon concurred with all of them, according to the report.

“We will work with, and be part of, the Sustainment Implementation Working Group to drive improvements in sustainment across the F-35 enterprise. We are pursuing a more resilient sustainment structure,” Air Force Lt. Gen. Mike Schmidt said in a statement, the Pentagon’s F-35 program manager.

The GAO released a report in May that found F-35 engines are being overlooked and could lead to an extra $38 billion in overhauls in the next few decades to help cool radar and other components of the fighter jet.