Washington ( CNN ) -- The U.S. Air Force has grounded its entire F-22 Raptor fleet amidst continuing concern over how the fighter jet provides oxygen to the pilot , according Capt . Jennifer Faerrau , a spokesperson for the Air Force 's Air Combat Command . The stand-down order was made Tuesday but is just now becoming public . It 's unclear how long the stand down will last . There are about 165 Raptors in the fleet . Since January , the aircraft already had been kept at altitudes under 25,000 feet during an ongoing investigation into a November crash . Flying above that altitude could cause a pilot to black out from lack of oxygen and lose control . The stand-down affects all routine training and other missions , Faerrau said . The Air Force is looking at all the systems on this new-generation fighter jet , but one area of focus is the On-Board Oxygen Generating System ( OBOGS ) , according to Ferrau . The order came from Gen. William Fraser , commander of Air Combat Command , which is based at Langley Air Force Base in Virginia .
The stand-down order was made Tuesday but is just now becoming public . The 165 Raptor fighter jets have been kept at lower altitudes during ongoing probe . Flying above 25,000 feet could cause pilots to black out . Order came from Gen. William Fraser , head of Air Combat Command .