After an extensive investigation and grounding, the Air Force doesn't know why some F-22 pilots have suffered symptoms similar to oxygen deprivation while flying the fighter (including one last week) and has returned the full fleet to service. During the more than four months that the aircraft was grounded the Air Force failed to find a common thread that linked at least 12 reported incidents in which pilots reported hypoxia-like symptoms while flying the jet. According to Air Force Chief of Staff General Norton Schwartz, the oxygen system was not the cause of a fatal crash during a November 2010 nighttime training mission. Prior reports published by the Air Force Times have stated that tests performed on Raptor pilots have found toxins in the pilots' blood. And reports previously published by the Air Force Times, and an Air Force accident report, suggest that not everyone is convinced the jet's oxygen system is trouble-free.
A September article published by the Air Force Times states that multiple sources had said a bleed-air issue led to last November's fatal F-22 Raptor crash. According to the Air Force Times, the Air Force accident report for that crash says the aircraft suffered a bleed air malfunction that caused both the jet's Environmental Control System and its On-Board Oxygen Generating System to shut down. That report is not publicly available. If those systems shut down, a pilot flying at altitude would need to switch on an emergency oxygen supply and dive to a lower altitude. According to a source cited by the Air Force Times, it is not clear if the pilot of the crash aircraft had switched on the emergency oxygen supply. The source says the aircraft's descent rate suggests that, if the pilot had suffered oxygen deprivation, he should not have fallen unconscious. In that specific case, says the source, the pilot should have been only suffering from symptoms of hypoxia before the aircraft reached safe altitudes.