The US Air Force has stopped flying all Lockheed Martin F-22s for an indefinite period over concerns about a possible glitch in the onboard oxygen generation system.
The stand-down order issued on 3 May by Air Combat Command (ACC) chief Gen Will Fraser comes about six weeks after the F-22s were restricted to flying below 25,000ft due to the same problem.
The order is in effect until "further notice", the ACC says.
"The standown is a prudent measure following recent reports of oxygen system malfunction," Fraser said in a statement.
While the F-22s remain parked, investigation teams will continue to determine the cause of the malfunction, the statement added.
The ACC emphasized that the stand-down order is technically not considered a "grounding", which occurs only in response to a catastrophic technical issue. A stand-down is voluntary and precautionary.
Despite keeping the F-22s on the ground, ACC believes the stand-down order will not change the air force's ability to perform missions such as air superiority alert. The F-22 is still available for "national security directed missions", the ACC said. One-time flights can also be approve by the heads of major commands, inclduing repositioning flights.
The air force has been flying the F-22s operationally for more than five years, with more than 150 already in service. All 186 F-22s on order are expected to be delivered by early next year.