One of the most expensive weapons produced by the U.S. military, the F-22 Raptor, was grounded in May and again temporarily in Oct. 2011 due to problems with the oxygen system. Even after a lengthy investigation, the Air Force is still not completely sure about what's going on.
ABC News reported in January that from 2008 through 2011 there were 12 cases where pilots experienced "hypoxia-like" symptoms, which can include inattentiveness, blackouts and even seizures. It was these symptoms that caused all the F-22s to be grounded in May 2011 for investigation:
After an intense, nearly five-month investigation, the Air Force said it could not figure out what could be making the pilots feel the effects of hypoxia and cautiously sent the birds back into the skies in October.
But the Air Force told ABC News the problem persists -- in the 6,000 sorties flown since the grounding, pilots have reported another eight instances of suffering "hypoxia-like symptoms." In each of the new cases, the pilot followed proper procedures, returned to base and landed "without incident," the Air Force said.
More recently, ABC News reported three more incidents within the last two weeks where pilots have not been getting enough oxygen to the brain while in flight.
Wired reported Lt. Gen Herbert “Hawk” Carlisle, the Air Force’s deputy chief of staff for Operations, Plans and Requirements, as saying the Air Force has "looked at everything on that system at the nth degree" and still can't find the root cause. Still, Air Force Times reports that the agency believes that it has at least pinpointed several contributing factors.
Last year, it was thought perhaps coolant was leaking into the oxygen system, preventing air transfer to the pilots. Wired reports Carlisle saying that a potential leak alone was not the problem but, perhaps it is “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.
As the investigation continues, Wired says that planes are still flying but at lower than recommended altitudes and with backup oxygen systems.
Issues with the oxygen system is believed to have lead to the death of Captain Jeffery Haney in Nov. 2010. Wired reports that examination of the black box on the plane revealed that Haney had tried to turn on the emergency oxygen system.