• Only one in four US F-35 fighters was fully mission-capable las year, according to a new report.
• The GAO said readiness rates have been decreasing in recent years due to maintenance and spare parts issues.
• Efforts to improve readiness rates haven't yielded results.

The US fleet of F-35 stealth fighters has seen falling readiness, as maintenance issues drag down mission-capable rates, according to a new watchdog report.

The F-35 Lightning II Joint Strike Fighter, made by defense giant Lockheed Martin, is the Pentagon's most expensive weapons program, with total acquisition and sustainment costs now projected to exceed $2 trillion. But as the price tag has climbed, persistent problems have limited how much the aircraft can fly. The military has spent billions of dollars on improving readiness, but those efforts often haven't delivered the desired results.

Last week, the Government Accountability Office, a congressional watchdog agency, reported that the F-35's readiness has been trending down in recent years.

Per the GAO report, the mission-capable rate for the advanced aircraft, or the amount of time it is available to perform one mission, fell from 67% to 44% between fiscal years 2021 and 2025. The full mission-capable rate, or the percentage of time the F-35 can perform all its missions, declined from 38% to 25%.

The GAO has identified several problems behind the decline, including spare-parts shortages and software issues. Those maintenance challenges have left too many F-35s grounded for extended periods. The Pentagon is trying to reverse that trend with a $13.7 billion readiness plan launched last year, aimed at improving the fleet's availability by the end of the decade.

But the plan may require more money than previously anticipated, the GAO said. And there are severe risks that could prevent its success, including heavy reliance on contractors for support, capacity constraints for parts, and cost gaps in keeping the F-35 running over its life-cycle.

The US has been paying fees to contractors to help incentivize fixing these issues, but, the GAO said, until DoD's plan can ensure incentives result in better F-35 performance goals, "it risks rewarding contractor performance that does not help meet program goals."

The US military operates three versions of the F-35: the Air Force's F-35A, built for conventional runways; the Marine Corps' F-35B, which can take off from shorter runways and land vertically; and the Navy's F-35C, designed for aircraft carriers.

Each variant is tailored to different service needs. The aircraft is built for a broad set of missions, including air-to-air combat, ground attack, surveillance, and electronic warfare.

US defense officials have pointed to the aircraft's growing importance across the fleet, including as a stealth system to penetrate hostile and denied airspace, as well as a quarterback for combat assets, leveraging its advanced sensor suite to direct action.

The eventual retirement of the A-10 Thunderbolt II attack aircraft could demand even more from the F-35, requiring the fighter to take over close-air support missions, including the "Sandy" role of protecting combat search-and-rescue missions.

The GAO issued three recommendations, suggesting DoD ensure risk mitigation plans are developed, incentives for contractors better align with goals for the aircraft, and the Pentagon tracks these incentives better.

The F-35 Joint Program Office's said it agreed with the GAO's findings. "We fully support all three recommendations to enhance fleet readiness, improve contract incentive structures, and implement rigorous financial quality controls," the office told Business Insider.