F-22 Raptor stealth fighters at Langley Air Force Base in Virginia have been grounded after a pilot experienced oxygen loss mid-flight. It's the second stand-down this year for the U.S. military's most sophisticated dogfighter, and a foreboding sign for the Pentagon as it struggles to modernize its aerial armada.

Problems with the on-board oxygen system have vexed the $150-million-a-copy F-22 for more than a year. On May 3, the Air Force locked down the entire Raptor fleet while it investigated reports of pilot blackouts and disorientation – problems that might have contributed to a fatal F-22 crash in Alaska in November.

Investigators suspected a design flaw in the Raptor's oxygen generator that was allowing high levels of unbreathable nitrogen to leak into the pilot's air supply. But they could never pin down the precise flaw, and last month the Air Force brass ordered the 170 F-22s – accounting for nearly half of the Pentagon's air-superiority force – back into the air. "We now have enough insight from recent studies and investigations that a return to flight is prudent and appropriate," said Gen. Norton Schwartz, the Air Force chief of staff.

As added insurance, Raptor squadrons installed an extra air filter on the radar-evading jets built by Lockheed Martin. But the filter appears not to have solved the problem, if the Virginia incident is any indication.

The Air Force is being cagey about the new grounding. "Part of our protocol is to allow units to pause operations whenever they need to analyze information collected from flight operations to ensure safety," the flying branch said in a statement. "That is what is happening at Langley at the moment, and we support that decision."

Considering the Raptor's ongoing safety woes and continuing delays, cost overruns, maintenance woes and production cuts in the F-35 stealth fighter program, the Pentagon's next-generation air arsenal is looking more and more like history's most expensive hangar decoration. With many of the latest fighters unflyable, old-school F-15s and F-16s dating from the 1980s could be forced to hold the line for years to come.

Photo: Air Force

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