By Carlo Munoz - 03/30/12 05:56 PM EDT
The majority of the Lockheed Martin-built fighters are stationed at Langley Air Force Base in Virginia and Joint Base Elmendorf-Richardson in Alaska. F-22 flight operations have since resumed at both bases.
But after numerous tests and evaluations on both the aircraft and pilots involved in the incidents, Air Force officials "do not have this day the root cause in hand," Martin told reporters.
"We have some pretty good ideas and we have a series of tests that the [Air Force] believes were necessary to continue to explore the envelopes of the system and to understand it completely," he said during the briefing at the Pentagon.
For example, Air Force analysts could not explain why pilots suffered from oxygen deprivation at altitudes where the issue would normally not be a concern.
Usually instances of hypoxia take place at altitudes of 25,000 feet and above, according to Martin.
"We have seen those incidents in an area where you would not expect them," he told reporters. "We don't have the root cause for why that is. We have some pretty good assessment ... but we don't have the answer."
Martin added that no Air Force pilots have died as a result of oxygen deprivation while flying the F-22.
Maj. Gen. Charles Lyon, director of operations at Air Combat Command, declined to comment on whether the Air Force has limited the altitude at which the Raptor fleet can fly.
The F-22 is "operating in an envelope that expands and goes beyond that of any other fighter aircraft that we have today," Lyon said.
The ACC operations chief did point out that the handful of hypoxia incidents among F-22 pilots represented "0.1 percent" of all Raptor flights flown by the command.
"We've had a 99.9 percent effective flying rate," Lyon said at the same Thursday briefing. "But that's not good enough. We will not rest, we will not stop ... until we carry that 99 to the farthest right decimal point we can."