Senior US Air Force and NASA officials testified 13 May before Congress on the steps the service is taking to ensure that the Lockheed Martin F-22 Raptor is safe to fly.
The USAF is installing a redesigned upper pressure garment inflation/deflation valve on the Combat Edge upper pressure garment, a new back-up oxygen system and it is changing the oxygen schedule for the jet's onboard oxygen generation system (OBOGS).
"We've reduced the potential negative effects created by high oxygen concentration levels produced by the OBOGS through cockpit selectable oxygen settings," says Maj Gen Charles Lyon, director of operations at the USAF's Air Combat Command.
The USAF continues to fine tune the F-22's oxygen schedule, he adds.
USAF documents show that a modified oxygen schedule was proposed in 2005, but was rejected by the service. The current work on modifying the oxygen concentration entering the pilot's mask is far more comprehensive than the schedule proposed seven year ago, Lyon says.
The F-22 system programme office has also modified the algorithms that control the Raptor's air cycle machine to give the OBOGS a higher priority over other systems, says retired Gen Greg Martin, who led a USAF Scientific Advisory Board study on a series of hypoxia-like incidents that has plagued the powerful twin-engined stealth fighter.
The USAF expects to modify the first F-22 with the new back-up oxygen system in January 2013, Lyon says. Subsequently, the USAF hopes to have the first full squadron of Raptors equipped by that spring. Modifications to the entire 185 aircraft fleet should be complete by mid-2014, he says.
The F-22 was originally designed with a back-up oxygen system, but that apparatus was deleted as a weight saving measure. At the time, the device was not considered a safety critical item, Martin says.
"In retrospect, that was not an appropriate decision," he says.
The back-up oxygen system weighs only 15lbs and will have no impact upon the performance of the jet when it is installed.
The modified BRAG valve, which will prevent the Combat Edge upper pressure garment from improperly inflating when not under g-loading, will be fielded by the end of 2012.
The changes were needed to prevent acceleration atelectasis-the result of too much oxygen under g-loading--and a form of hyperventilation caused by an improperly inflating Combat Edge pressure vest.
The problems with the Raptor's oxygen supply led to a four-month grounding of the F-22 fleet in May last year with over a dozen pilots complaining of hypoxia-like symptoms.
Clinton Cragg, principle engineer at the NASA engineering safety center, says that his agency concurs with the USAF's findings that toxin substances are not responsible for the Raptor's problems. But, he notes, the F-22 cockpit has no effective means of filtering out "irritants." Some of those irritants could cause breathing problems, Cragg says.