For F-22, Oxygen Problems Elude Air Force's Fixes JOINT BASE LANGLEY-EUSTIS, Va. - Capt. Jeff Haney was at 51,000 feet on a night flight above Alaska in November 2010 when the oxygen system in his F-22 Raptor fighter jet shut down, restricting his ability to breathe as he plummeted faster than the speed of sound into the tundra below. His plane burned a crater into the ice, froze 40 feet beneath the surface and was not fully recovered until the spring thaw. Ian C. Bates for The New York Times Lt. Colonel Pete Fesler explains how an F-22 jet works at Langley Air Force Base in Virginia. The Air Force is still investigating repeated problems with pilots experiencing low oxygen while flying the jets. Captain Haney's death unnerved the elite community of F-22 pilots, as did a series of episodes over the next 18 months in which an alarming number experienced symptoms of hypoxia, or oxygen deprivation. The Air Force grounded the Raptor, the jewel of its fleet, but could not find anything wrong, so it put the jet back in the air - only to have the episodes increase. In May, two seasoned pilots took the extraordinary step of telling "60 Minutes" that they refused to fly the plane. Last month, a breakthrough seemed to come at last. Investigators believed that a malfunctioning pressure vest was restricting pilots" breathing and that leaking narrow oxygen hoses were not delivering enough air. Pilots began flying without the vest, and, buoyed by three months without an episode, Air Force officials told the news media that they might be close to a solution. But last week, as Air Force officials escorted a reporter and a photographer to the Langley flight line to watch F-22s roaring on and off the runway for an ostensible good-news story, it happened again. A pilot pulled his emergency oxygen handle sometime after landing because of what the Air Force characterized as "discomfort" from intermittent air flow into the pilot's mask during flight. The Air Force is investigating but so far has said little. Senator Mark R. Warner, for one, is outraged by the episode. "I've been pressing them about the explanation for this, and we still don't have an answer," he said in an interview on Friday. We don't even have the full details yet. Mr. Warner, a Virginia Democrat who has taken up the cause of the two pilots who spoke to "60 Minutes" because they are constituents who fly out of Langley, said he was equally frustrated that the Air Force was only now coming to the conclusion that there might be a problem with the jet's oxygen flow. "Wouldn't this have been the first question to be asked?" he said. The F-22, at $400 million the world's most expensive jet, was conceived during the cold war when the Air Force wanted a plane to counter improvements in Russian MiGs. But the Soviet Union disappeared long before Lockheed Martin built the first F-22 prototype in 1997. By then critics had branded it a relic. It was not until 2009 that Congress, pushed by President Obama and the defense secretary at the time, Robert M. Gates, agreed to limit the number of planes it would pay for to 187, the number now in service. Although the stealthy jet is a technological wonder that can fly higher, faster and with more maneuvers than any other, it has never been used in combat. A squadron of F-22s is deployed to a base in the Persian Gulf as a deterrent to Iran. The plane sat out the wars in Iraq and Afghanistan and the conflict in Libya because it was not needed. "Last I checked the Taliban air force was pretty small," said Richard L. Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va. But Mr. Aboulafia, who compares the F-22 to a Maserati and the newer and relatively less expensive F-35 Joint Strike Fighter to a BMW sedan, said he supported the F-22 as a hedge against future developments in Chinese aircraft. The Air Force says that since the plane was put into operation in 2005, pilots have experienced 21 unexplained episodes of hypoxia-like symptoms. At least three episodes occurred before Captain Haney's death. His crash is not included in the 21 episodes because the Air Force counts it as one of 15 additional "explained" hypoxia-like events - anything from a loose air hose to a total failure of the life support system. It was not until 10 unexplained episodes had occurred that the Air Force took the drastic step in May 2011 of grounding the entire F-22 fleet. Investigators combed through the planes, focusing on whether there were contaminants in the oxygen system that might be making pilots disoriented. They found nothing conclusive. But as a precaution - and for further testing - the Air Force gave pilots devices to monitor their oxygen levels during flight and installed charcoal filters in the air system to block potential poisons.