By Andrea Shalal-Esa
WASHINGTON (Reuters) - The U.S. Air Force on Monday said it has not found a "smoking gun" to explain oxygen issues that grounded Lockheed Martin Corp's F-22 fighter jet for four months last year but has implemented steps to minimize problems.
Air Force Chief of Staff General Norton Schwartz told Reuters that a subpanel was due to brief the Air Force's Science Advisory Board this week about recurring problems with the system that supplies oxygen to pilots who fly the radar-evading warplane, but no single mechanical cause had been found.
He said he expected the advisory board to finalize its report on the issue by the end of January or early February.
The Air Force grounded its fleet of F-22 Raptor fighter jets in May 2011 but allowed flights to resume in September after concluding the planes were safe to fly.
Schwartz said the service installed new equipment to monitor the output from the oxygen producing system on board, as well as the level of oxygen in the blood of the pilots, but would continue to collect data.
"We haven't found a single mechanical deficiency that addresses some of the symptoms that we've seen," he said in an interview at his Pentagon office. "We've taken a range of both engineering and physiological actions to minimize the consequences of what we've seen, and continue to collect data so we can nail this down once and for all."
"The stand-down provides Air Force officials the opportunity to investigate the reports and ensure crews are able to safely accomplish their missions," the Air Force said in a statement.
The Raptor is the premier U.S. fighter and features cutting-edge shapes, materials and propulsion systems designed to make it appear as small as a swallow on enemy radar screens.
Lockheed rolled the last F-22 fighter out of its Marietta, Georgia facility last month, but the Air Force is preserving the hardware used to build the jet, which would allow it to restart production for about $200 million.
Schwartz said he considered it unlikely that the plane's production would ever be restarted. "I wouldn't say never, but I think it very unlikely," he said.
(Reporting By Andrea Shalal-Esa in Washington, D.C.; Editing by Steve Orlofsky)