The Marine Corps' F-35B fighters are among the oldest operating aircraft to come out of the Joint Strike Fighter program, and they're beginning to run into some maintenance issues. While many of the repairs being made to the aircraft are upgrades to bring them in line with the latest software releases and accompanying hardware upgrades, a fair share is for typical aircraft maintenance issues—and those issues are leading to an increasing number of aircraft left grounded waiting for parts.

As Defense News' Valerie Insinna reports, that's often because the wait time for delivery on some parts is a month or longer. And according to a GAO report, the lead time for some F-35 replacement parts could be two years or longer. Between waiting for parts and time spent offline for upgrades, F-35s worldwide were unable to fly 22 percent of the time between January and August of 2017. The GAO report, published last October, warned of "sustainment challenges" faced by the F-35 program—many of them because of poor planning and delays in bringing repair parts suppliers and depots onboard.

If anything major goes wrong with an F-35, it could spend a lot more time waiting to be repaired. "DOD does not have enough capacity to repair F-35 aircraft parts because the establishment of repair capabilities at the military depots is six years behind schedule," the GAO report stated. "Repair capabilities at the military depots were originally planned to be completed by 2016, but program officials told us that some capabilities have now been delayed until 2022."

As a result, some maintenance crews have turned to cannibalizing some F-35s for parts to keep others operational. It's not an uncommon practice to borrow a part off one broken aircraft to fix another, especially among fighter aircraft. But that approach, without a logistics channel behind it to deliver parts in a reasonable amount of time, can snowball—as it has with the German Luftwaffe's Eurofighter Typhoon aircraft, which are now down to a handful of flyable aircraft because of parts shortages.

It doesn't help, for example, that the average life of the short take-off, vertical landing (STOVL) F-35B's landing gear tire is less than 10 landings—a problem resulting from the aircraft's heavier weight than its conventional take-off cousins. It's simply difficult to engineer a tire capable of handling both vertical low-speed and conventional high-speed landings. The F-35B has also failed some structural testing, with the first aircraft used in tests literally falling apart. And the Autonomic Logistics Information System (ALIS)—software that combines diagnostics and repair functions with part inventory and verifies that the correct parts have been installed properly—has continued to get poor reviews.

A maintenance sergeant at VMFAT-50, the Marine Corps' F-35B training squadron at Marine Corps Air Station Beaufort, South Carolina, told Insinna that ALIS was slow for him, though Marines at MCAS Air Station Yuma, Arizona, reported better speeds. “We have a bunch of aircraft… And we have only so many servers,” the sergeant told Insinna. “They have less aircraft, and their servers aren’t booting up as much information.”

But things might be looking up. A Lockheed representative told Defense News that the company is exploring 3D-printing some parts to speed up repair cycles. And some of the lead times for parts provided to repair crews end up being very conservative estimates, with parts arriving in a month instead of the year lead time given to the services.

So, hopefully, things will get better. But the long-term cost of maintaining the F-35 fleet as it ages is still something of a wild guess at the moment. Based on some estimates, that price may be so high that the Air Force will have to eliminate a third of its planned F-35 purchase to stay on budget.