A seven-month investigation by the U.S. Air Force Scientific Advisory Board into unexplained hypoxia, or oxygen deprivation, incidents experienced by F-22 pilots has not determined the root cause of the problem, the service said March 29. The investigation did produce a number of safety recommendations, and the Air Force continues to study the problem.

The scientific advisory board’s investigation into the suspected cause of the incidents—the aircraft’s onboard oxygen generation system (Obogs)—drew on the results of ground tests and a series of flight tests with sensors installed in the aircraft to monitor the end-to-end process of the oxygen system. “Despite those efforts, we do not have this day the root cause in hand,” retired Gen. Gregory Martin, chairman of the scientific board's F-22 study, told a Pentagon press briefing. “We have some pretty good ideas and we have a series of tests that the Scientific Advisory Board believes were necessary to continue to explore the envelopes of the system and to understand it completely.”

Martin said the F-22 fleet logged 14 “physiological” incidents from April 2008 through May 2011, 10 of undetermined cause. The fleet was restricted to flying below 25,000 feet in January 2011 during an Air Force investigation of a fatal F-22 crash in November 2010 in Alaska that was later attributed to loss of control by the pilot. The entire fleet was grounded for four months beginning last May.

Maj. Gen. Charles Lyon, Air Combat Command director of operations, said F-22s have flown 10,000 sorties since returning to service last September, with an incident rate of 0.1 percent. F-22 pilots now fly with pulse oximeters to measure blood oxygen level and HEPA filters to trap particulates. A new, larger handle is used to access the emergency oxygen system. “We're implementing the recommendations given to us by Gen. Martin's group and others, and we continue aggressively to pursue the root cause of these unexplained incidents,” Lyon said.