USAF fails to determine cause of F-22 Raptor onboard oxygen generation systems. episodes

Martin said during a Pentagon briefing: "From April 2008 until May 2011, the Air Force experienced 14 physiological incidents with the fleet of F-22s. Each incident was investigated, and of those incidents, 10 did not reveal a root cause."

"We were unable to determine a root cause, but we were able to put in place the proper safety measures and risk mitigation techniques that would allow the F-22 fleet to return to fly ... to ensure the integrity of the life support system." 

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