PENTAGON (CNN) -- A few pilots have told the Air Force they won't fly their expensive F-22 Raptor stealth jets because no cause has been found for oxygen deprivation incidents in the cockpit, the head of Air Combat Command for the U.S. Air Force told reporters.
The Air Force has been looking for the cause of about a dozen unexplained incidents related to hypoxia, or oxygen deficiency, with pilots, but so far has been unable to pinpoint it, Gen. Mike Hostage with Air Combat Command said in a media briefing.
Hostage noted it was a very small group of pilots who opposed flying the Raptors. Pilots began experiencing problems starting four years ago.
“For some reason, the on-board oxygen generating system and the environmental control system that feeds it may be inputting some contaminant,” Gen. Gregory Martin, a retired Air Force veteran, told CNN affiliate WAVY in Virginia.
Hostage said if a contaminant is not the problem, there may be something else hindering pilots from getting enough oxygen.
Hostage spoke at length with reporters about the issue, which has plagued the fleet since problems with the F-22’s oxygen supply system were first reported in 2008. The jets have previously been grounded to examine the issue , but one year ago the Raptors were again cleared and allowed to fly. In January 2011, the jets were limited to altitudes under 25,000 feet during an ongoing investigation into a November 2010 crash. Flying above that altitude could cause a pilot to black out from lack of oxygen and lose control.
"We are diligently pursuing a variety of hypotheses to try and understand and characterize the exact circumstances we've been experiencing," he said.
The Air Combat Command said it still has not identified the "root cause" of the oxygen issue, but is making progress with its investigation and hopes to soon determine the exact cause of the problem.
“The smoking gun is disassembled in a mosaic in front of us. ... At some point we’re going to have the smoking gun assembled,” Maj. Gen. Charles Lyon, the director of operations for ACC, told the Air Force Times.
While Hostage said that there was certainly a concern about the group of incidents, he didn't think it was necessary to pull the entire group of jets, which have had 12,000 deployments and a total of 15,000 flight hours since September 11 and only a handful of problems. The Air Force has also made sure to add new emergency oxygen deployment handles, should a pilot encounter any issues.
And the F-22s are still being used when needed, including a recent deployment by the Air Force of a squadron of the Raptors to southwest Asia.
"I fully expect we'll get to a solution," Hostage said. "I won't give you a timetable, but we have made great progress to that effect and am confident we'll put this behind, we'll be able to explain it, and we'll retool the airplane to make this problem go away."