The US Air Force expects to start fielding life support systems modifications for its fleet of Lockheed Martin F-22 Raptor air superiority fighters this January.

"UPG [upper pressure garment] valves are being shipped now, and the vests will be modified locally," says the USAF's Air Combat Command (ACC). "We expect units will start flying with the modified vests later this month, which alleviates the current restriction that has them flying without the vests at altitudes below 44,000 feet unless otherwise directed."

The Raptor, which is regarded as the USAF's premier air-to-air fighter, had been operating under a number of operational limitations because of a suspected problem with the aircraft's onboard oxygen generation system (OBOGS). The problem was severe enough that the USAF was forced to ground the Raptor for four months in 2011 after more than a dozen "physiological incidents" with symptoms resembling hypoxia. After the USAF concluded that the pilot's Combat Edge upper pressure garment was the source of the problem, the service has slowly been lifting restrictions on the Raptor's operations. Other than the altitude limitation, the only other remaining restriction on F-22 is that the aircraft is currently precluded from executing the Alaska air sovereignty mission.