The U.S. Air Force’s most advanced stealth fighters have been grounded for so long that pilots of the F-22 Raptors are starting to run the risk of being disqualified from flying their assigned planes.

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. And no amount of time in a flight simulator can fix that; fighter jocks need to fly in real cockpits to stay qualified.

“Today’s simulator visuals are quite good, but nothing can truly replicate the physiological difficulties of long range visual pick-up of tactical aircraft or ground targets in the ‘real’ world,” Air Combat Command spokeswoman Capt. Jennifer Ferrau tells Danger Room.

The entire fleet of F-22 Raptors — the world’s most advanced dogfighters — has been grounded since May 3, after problems were discovered with the planes’ oxygen systems. So, too, is the F-35 Joint Strike Fighter fleet, which means that every stealth fighter in the U.S. inventory is currently out of commission.

The F-35s, at least, are expected to resume testing shortly. No one’s quite sure when the 165 Raptors (and their pilots) are coming back.

Instead of pumping in pure air, the F-22 was feeding its pilots lungloads of dangerous chemicals.

“Toxins found in pilots’ blood include oil fumes, residue from burned polyalphaolefin (PAO) antifreeze, and, in one case, propane,” reports Air Force Times’ Dave Majumdar. 14 pilots suffered “hypoxia-like symptoms.”

Some are even blaming the oxygen system in the fatal crash of an F-22 in November.

All of which means that Raptor pilots are stuck on the ground until further notice.

“The guys are getting antsy,” Lt. Col. Jason Hinds, director of operations for the 27th Fighter Squadron, told the Daily Press of Newport News, Virginia, last month.

Time in the simulator doesn’t relieve the stir-craziness. For one thing, it doesn’t resolve the pilots’ currency issues, Ferrau notes in an e-mail. Unlike commercial pilots, Air Force fighter jocks can’t use simulated takeoffs and landings to keep them current.

“The simulator is a controlled environment, while live fly includes the stress of heat, sweat, vibration, G force, blinding sun, motion of three-dimensional flight, uncertainties while flying in a crowded airspace and maybe most important — mortality,” she says. “You can run out of fuel and put the sim on ‘freeze’ but you cannot stop live flight to avoid a dangerous situation.”

Also, the waiting list to get into an F-22 simulator is getting rather long. There are only two Raptor sim complexes — one at Langley Air Force Base in Virginia, the other in Tyndall Air Force Base in Florida. Each complex has four ersatz cockpits. Pilots from as far away from Alaska and Hawaii are coming to the East Coast to use the facilities.

“For the next three months, we’ve got people booked to come here to operate in our simulators,” Capt. Travis Passey, training flight commander in the Operational Support Squadron, told the Daily Press.

Restoring currency to the Raptor pilot cadre could take months, once it’s lost. First, instructor pilots will have to re-qualify, through a combination of ground trial, test flights and time in an Air Force classroom. Then, they’d train a small number of pilots, who would get the others up to speed.

It’s not the first time the Air Force has faced a currency issue; in 2007 and 2008, previously undiscovered manufacturing errors kept 182 early model F-15s on the shelves.

But the F-22’s woes are part of a larger issue facing the military, one House Republican aide said. “The real problem is the effect all these defense cuts have on technical military training,” he notes.

“The Navy uses simulators in place of machines. It’s turned into a boondoggle. The Air Force is now having to rely more on simulators, too,” he said. “If you have only one simulator in say, a National Guard unit, how can you practice formation flying? Precision strike? This is where Obama’s defense cuts are killing us: military training and readiness.”

Perhaps. But, at the moment, losing the world’s most advanced and expensive fighter jets — priced at $411 million each, including R&D costs — isn’t exactly inhibiting the U.S. military as it fights wars around the globe.

The Air Force decided long ago to keep the F-22 out of Iraq and Afghanistan. The jet has also been missing from the air assault of Libya: there are no Raptors based in Europe, and the F-22 doesn’t have the ability to transmit data to other aircraft — only to other Raptors.

Even when it’s in the sky, this most evolved of jets is notable for its limitations.

Photo: USAF