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.

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 of them 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 CBS News’s “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 narrow oxygen hoses were leaking and 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.