Rather than receiving too little oxygen, the pilots of one of the world’s most advanced fighter aircraft could be taking in too much, according to a NASA assessment of the F-22 ’s life support system.

Pilots of the Raptor have for years experienced breathing difficulties and something now called “ Raptor cough.” The Air Force launched serious investigations into the problem in 2008, after pilots reported an increasing number of “hypoxia-like symptoms.”

Since that time, the Air Force has pointed to a “mosaic” of interrelated problems in the cockpit.

Independently, the NASA Engineering Safety Center conducted its own assessment, and principle engineer Clinton Cragg presented the findings during a House Armed Services Committee hearing Sept. 13. The assessment found that a high concentration of oxygen at low altitudes can lead to “absorption atelectasis,” in which too much oxygen can wash away necessary nitrogen within the lungs and cause lung tissue to collapse. The effects of high oxygen levels are compounded by the aircraft’s “inevitable acceleration,” Cragg said.

“Inappropriate inflation” of the pilots’ upper pressure garment further restricted breathing and reduced overall cardiac output, Cragg’s testimony said. In addition, “uncharacterized F-22 life support system vulnerabilities, such as pressure drops across components in the cockpit,” contributed to the trouble.

Cragg’s testimony also took issue with the F-22 pilot community’s reaction to the aircraft’s problems. “Differences in pilot breathing in the F-22 from other platforms was widely known and accepted as a normal part of flying the advanced aircraft,” Cragg ’s testimony said. “The acceptance of these phenomena as ‘normal’ could be seen as a ‘normalization of deviance.’” That is a NASA term of art to describe the lower standard of excellence accepted on components of the Challenger before the shuttle’s devastating explosion.

Air Force Maj. Gen. Charles Lyon says the service has put in place a system of fixes that will take care of the problem. That starts with replacing a valve on the upper pressure garmet that underwent a critical design review this week. “We’ll start getting it out to the fleet in November,” Lyon told reporters after the hearing. He added that a backup oxygen system will begin to be put into the fleet in January.

Lyon will also recommend to Defense Secretary Leon Panetta that he allow the F-22 fleet to resume operations at normal altitudes. “The fact that we’ve gone six months without an incident is a confidence-building measure,” the general said.

Lyon said he is still working on the oxygen concentration issue. The problem , Lyon said, is that the Air Force provides the pilots with more oxygen than they need to help them retain consciousness should a worst-case scenario disaster result. But that is complicated by the fact that unlike any other aircraft , the F-22 flies at higher altitudes and higher levels of acceleration – or “High-G.” “We’re working the balance between catastrophic possibility and day-to-day flying,” Lyon said.

But Rep. Jackie Speier (D-Calif.) is not convinced that they have discovered what’s wrong. “I’m just not convinced that we have the answer,” she said. “I think NASA needs to be privy to all the information. And all the reports should be made public.”