America's newest stealth fighters have a major problem: their pilots can't breathe, due to some sort of malfunction in the planes' oxygen-generation systems. For months, the Air Force has been studying the problem, which temporarily grounded the entire fleet of F-22 Raptors and may have contributed to a pilot's death. Today, the Air Force admitted they still don't know exactly what's causing the issue.

"We have looked at everything on that system at the nth degree, and the bottom line is that there’s no smoking gun," said Lt. Gen Herbert "Hawk" Carlisle, the Air Force's deputy chief of staff for Operations, Plans and Requirements. But Carlisle later told the Air Force Times that investigators from the Air Force Scientific Advisory Board have pinpointed a leak in the Raptor's cooling system. Somehow, the cooling fluid was getting into oxygen system, blocking air from getting to pilots.

Problems with the F-22's air supply date back years, resulting in two fleet-wide groundings in 2011 and the possible death of a pilot the previous year. The cause was likely a defect somewhere in the F-22's On-Board Oxygen-Generation System, or "OBOGS." With a failure in the OBOGS, pilots would be denied their oxygen. Then, it's only a matter of time before developing "hypoxia and decompression sickness" – which can lead to blackouts. But no one knew exactly what the problem was, perhaps until now.

Along with the cooling leak, another problem may be "an interaction between contaminants and the materials in OBOGS that the service has yet to uncover." Also, a problem has been found with the a valve connecting the oxygen system to the pilot's mask. So when there's a problem, the pilot may not have adequate warning or enough time to respond. Investigators have also previously pointed toward toxic nitrogen as a possible culprit, but this seems unlikely now.

But some combination of the above may have been what happened in the Nov. 10, 2010 crash in Alaska that killed Capt. Jeffrey Haney. According to investigators, a system responsible for channeling air away from the engines on board Haney's F-22 began leaking. To prevent the air-dependent OBOGS from being contaminated, the system shut down – no more air. The Air Force blamed the crash on pilot error, but an examination from the plane's black box recording found that Haney had attempted to switch on his emergency oxygen system before plummeting into the ground.

Meanwhile, the Air Force says it's adding backup oxygen systems to the Raptor and is continuing to investigate. The stealth fighters have also been flying since October's brief grounding. As a safety precaution the planes are flying above 50,000 feet, below the F-22's normal operating altitude of 60,000 feet. Nonetheless, the F-22 is still flying with a lemon oxygen system, and without a fix in sight, that puts pilots at risk.

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