US Air Force leaders are considering a redesign of the Lockheed Martin F-22 but still have no answers for the oxygen system breakdowns responsible for another operational disruption.
F-22s based in Alaska were grounded for one day in mid-February after three separate pilots reported hypoxia symptoms, the Air Combat Command confirmed to Flightglobal.
It was at least the third temporary stand-down for the F-22 since the USAF deactivated the entire fleet for four months until last September.
But air force officials are no closer to identifying the cause of the string of incidents, including one fatal crash in November 2010 that was preceded by a failure of the pilot's oxygen supply.
USAF officials hoped an expert panel led by retired Gen Gregory Martin might yield the answer. The team has now reported its findings, but found no "smoking gun", said Lt Gen Herbert Carlisle, deputy chief of staff for operations, plans and requirements.
The USAF is considering a broad range of options, including redesigning the F-22 to include a back-up oxygen supply, Carlisle said. This would automatically detect an oxygen system malfunction and activate, he added.
The F-22 already is equipped with an emergency oxygen system, but it must be manually activated by the pilot after the onboard oxygen generation system (OBOGS) stops working.
Capt Jeff Haney was killed on 16 November 2010 when his F-22 crashed in Alaska. Haney inadvertently steered the aircraft into the ground while trying to reach a handle to activate his emergency oxygen system.
The OBOGS had already stopped functioning during the incident. An unexplained oxygen leak in the engine compartment prompted an automatic fire protection system to shut down the supply of bleed air to the OBOGS.
While Haney's supply of breathing air was cut off, other F-22 pilots have reported symptoms suggesting their air supply was not filtered properly.