The F-35, the military’s most expensive weapon, keeps breaking down and takes too long to repair, the Pentagon’s chief weapons tester told Congress on Wednesday.

"None of the F-35 variants is meeting either the reliability or maintainability metrics” set by the Pentagon, said Robert Behler, the Department of Defense’s director of operational testing, in testimony before the House Armed Services Committee. “The operational suitability of the F-35 fleet remains at a level below Service expectations.”

Fleetwide, monthly availability of the aircraft between September 2018 and September 2019 was below the 65% target, and none of the planes met the 80% “mission capable” goal set by former Secretary of Defense Jim Mattis, Behler said. The Air Force’s F-35A variant had the best performance, followed by the Marine Corps’ F-35B and the Navy’s F-35C.

"In short, for all variants, aircraft are breaking more often than planned and taking longer to fix," Behler said.

The F-35 has deployed at least twice. Its first combat mission was in September 2018, when a Marine Corps F-35B attacked a Taliban target in Afghanistan. Another was in April, when two Air Force F-35As participated in strikes against ISIS tunnels and weapons caches in Iraq.

“Despite the confirmation that the program achieved initial operational capability, the F-35 basic design still remains a prototype,” Dan Grazier, a military fellow at the Project for Government Oversight, told the Washington Examiner. “It’s very concerning to me that the services have prematurely pressed these into a combat role when the basic design hasn’t been proven yet.”

Aircraft performance has improved recently, Behler testified, thanks to progress with maintenance and a greater availability of spare parts.

Additionally, aircraft readiness has gone up, testified Gregory Ulmer, vice president of Lockheed Martin’s F-35 program.

“Readiness rates continue to rise across the fleet, and today we see on average a mission capable rate of more than 70% on combat-coded aircraft,” Ulmer said in his opening remarks. A “combat-coded” aircraft is an aircraft assigned to a wartime mission.