A US Air Force report has said the regular oxygen system stopped working before a fatal Lockheed Martin F-22 crash in Alaska last November.
The accident investigation board still blames the accident on the pilot, Captain Jeffrey Haney, who failed to activate an emergency oxygen supply (EOS) that could have saved his life and the aircraft.
But the failure of an engine bleed air system that feeds the Honeywell onboard oxygen generation system (OBOGS) in the moments before the crash is a new twist in the evolving story.
The regular oxygen system stopped working before a fatal Lockheed Martin F-22 crash in Alaska last November.
Only two months after the accident, the USAF internally acknowledged serious concerns about the F-22's oxygen system. In January, the USAF limited F-22s to flying below 25,000 feet (7,620 metres). Starting on 3 May, the USAF grounded the entire fleet for more than four months, citing concerns about the pilot's oxygen system.
Although the F-22s resumed flying in September, the cause of dozens of reported oxygen system failures over several years was never pinpointed.
The investigators said Haney failed to activate the EOS during the 31s period after his normal oxygen supply became restricted.
The F-22's oxygen supply was automatically cut off after onboard computers detected bleed air leaking out of the engine bay, which could cause a fire, the report said. USAF investigators were unable to determine the cause of the bleed air leak.
Shutting down the bleed air system caused the OBOGS to fail, the report said.
The report concluded that Haney inadvertently pointed the aircraft at the ground while trying to activate the EOS, a procedure that calls on the pilot to pull up on a small ring tucked into the side of his ejection seat.
Simulator tests later concluded that this manoeuvre may have led to inadvertent stick or rudder movements.
The investigators ruled out loss of consciousness as a possible cause, despite possible oxygen deprivation.