The F-22 Raptor, built by Lockheed Martin.

The F-22 Raptor, built by Lockheed Martin. (Lockheed Martin)

SEOUL — The U.S. Air Force has grounded its fleet of F-22 Raptors, citing concerns over the aircraft’s oxygen system.

The 137 fighter jets were grounded Tuesday on the orders of Gen. William Fraser, commander of Air Combat Command at Langley Air Force Base, Va., according to Pacific Air Forces Command spokesman Capt. Martin Gerst.

“The stand-down is a prudent measure following recent reports of potential oxygen system malfunctions,” Gerst said Friday. “The stand-down provides Air Force officials the opportunity to investigate the reports and ensure crews are able to safely accomplish their missions.”

It is not immediately clear how long the Raptors will be grounded, or what it will take to put them back in operation, he said.

“The safety of our airmen is paramount, and we will take the necessary time to ensure we perform a thorough investigation,” Gerst said.

An investigation into the Raptors’ performance was launched in the wake of a November crash in Alaska. Since January, the jets have been restricted to altitudes of 25,000 feet because of concerns pilots might black out from a lack of oxygen, officials said.

Asked how the grounding of the Raptors might affect Air Force operations, Gerst said, “The temporary stand-down will have a negligible effect on real-world missions, as the F-22 remains available for national security-directed missions. Additionally … commanders may allow one-time flights if warranted and prudent.

“Crews will maintain proficiency through simulator and ground training events,” he said.

The F-22 program has been under intense scrutiny for years. Developed as the U.S. military’s premier fighter jet, the F-22 with its $400 million price tag has been hotly debated in Congress. When the 2010 Defense Authorization Bill was passed, F-22 production was capped to 187 total aircraft.

The stealth aircraft has never been flown in combat.

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