A report by the US Office of Government ability (GAO) points out that at least eight different aircraft models from the Navy (USN) and Air Force (USAF) have shown low levels of mission availability.
And rates have only gotten worse since 2015, especially for the Air Force's F-22 and Navy's KC-130T. The F-22 fleet has 50,3% availability, while the Navy and Marine Corps KC-130s have 36,5%.
“Mission rates – a metric used to assess the health and readiness of an aircraft fleet – and other related maintenance metrics trends have worsened since FY 2015 for eight selected aircraft,” says the GAO report, published last Wednesday (15).

“While the Air Force and Navy have initiatives to address unit-level maintenance challenges, neither service has mitigated the lingering sustaining risks of fixed-wing aircraft.”

Aircraft mission capable is characterized by GAO as the percentage of time an aircraft spends when it is capable of “fly and accomplish at least one mission”. An unavailable aircraft cannot fly either because it is waiting for necessary maintenance or if the unit does not have a spare part needed to repair the aircraft.
In this way, the GAO found that the Availability rates for all eight fleets have decreased since 2015With the F-22 and KC-130T showing the worst ratings: Both experienced a 16,7% drop in availability. The F/A-18E/F (3,9%) and KC-135 (4,1%) fleets, while still declining, saw the smallest changes in mission capability rates.

While the Office acknowledges that organizations have taken some steps to stop declining numbers (such as the Navy's implementation of its Naval Sustainment System), it still finds that the USN and USAF have not concluded “maintenance reviews” for specific fleets, required in the Fiscal Year 2017 National Defense Authorization Act.
USAF officials told the GAO that the Air Force had not yet conducted the reviews because of ambiguity in how they initially interpreted the law, but began reviews after the statute was clarified in 2021. and P-8 will be completed in fiscal 2022 and 2023, respectively.

“Air Force officials have reported that they plan to work through the backlog of the remaining systems through the end of fiscal 2025,” according to the GAO report. “Navy officials reported that they expect to complete maintenance reviews for 54 systems over the next 13 years, between now and FY 2035.”
As is customary in all ing Office reports, the watchdog has made a number of recommendations to the Pentagon, which primarily focus on both services completing their necessary sustainment reviews, as well as developing mitigation plans to contain the declining availability rates.

Answering for the Department of Defense, Vic Ramdass, a senior Pentagon standby official, broadly agreed with the GAO's recommendations.
The only recommendation disputed by Ramdass was that the Navy accelerate its planned assessments of each fleet, arguing that the service has crafted a schedule that balances a variety of programmatic needs. "without creating a wave of sustaining revisions in any year."