The most expensive fighter jet ever built, the U.S.’s F-22, has not only sat on the sidelines in Iraq, Afghanistan and Libya—the entire fleet has been grounded indefinitely since early May due to a problem that’s caused some mysterious symptoms for pilots.

Wired notes that if this doesn’t change in the next few months, the Air Force’s F-22 pilots will need to be retrained on how to fly the jets:

Air Force requires pilots to fly a certain number of sorties in their aircraft every month, in order to stay fresh. If they don’t fly for 210 days, the pilots lose their “currency,” as it’s known in military jargon. Then, they have to be retrained on their jets, nearly from scratch.

Ordinarily, that’s a problem for an individual pilot when he gets sick, goes on leave, or takes a desk job somewhere. But now, the Air Force is facing the possibility of it happening to hundreds of their very best fighter pilots.

As we noted in March, there’d been some buzz in defense circles about whether the F-22 would get its first taste of combat in Libya—but that never happened. Air Force Chief of Staff Norton Schwartz told Bloomberg that the planes were up to the task but simply based too far away to join the fight.

But concern about problems with the planes has been growing for months. Last November, an F-22 crashed over Alaska, killing the pilot. After other pilots reported symptoms of hypoxia, or low oxygen levels, the Air Force initially placed restrictions on the altitudes at which the plane could be operated, until it finally grounded the entire fleet in early May. It’s still investigating what caused the symptoms but has been looking at possible carbon monoxide poisoning or flaws with the oxygen system. No root cause has yet been identified, AOL Defense reported last week.

The problem is just the latest in a series of snafus for the F-22, which has faced a number of hurdles in its three-decade-long development. In 2009, the Washington Post noted early structural deficiencies and computer flaws with the F-22s, as well as problems with the jet’s radar-absorbent coating, which required costly and time-consuming maintenance.

The United States has spent more than $65 billion on developing the F-22s, which have never been used in combat. Air Force officials told the Los Angeles Times recently that the F-22 hasn’t been used in conflicts yet because it’s “designed for high-threat environments, not what we’ve seen in Iraq, Afghanistan and Libya.”

In 2009, then-Defense Secretary Robert Gates successfully pushed to Congress to stop buying more of the planes.

The Pentagon—which has warned that steep budget cuts written into the recent debt deal could “have devastating effects on our national defense”—may soon have to find other programs to cut back as well. (Another Lockheed Martin stealth fighter jet, the F-35, has also been plagued by cost overruns and could be eyed for cuts. The U.S. fleet of F-35s was also grounded earlier this month for an electrical problem.)

The new debt deal requires about $350 billion in cuts over the next 10 years from a broad category of defense and security funding that includes funding for Defense, Homeland Security and even foreign aid. Defense Secretary Leon Panetta has said that the Defense Department will be able to make those cuts.

But if the super-committee of 12 members doesn’t reach another agreement, a second round of so-called “trigger cuts” would require an additional $500 billion in cuts to the Pentagon budget. Panetta has warned those would have “devastating effects on our national defense.”