The US Air Force is working to modify the Lockheed Martin F-22 Raptor's on-board oxygen generation system (OBOGS) with a new oxygen concentration schedule.
"The programme office is in the process of implementing a change to the OBOGS concentration control schedule," the USAF says.
The modified schedule is designed to reduce the concentration of oxygen reaching the pilot's lungs at lower cockpit altitudes. The high concentration of oxygen has been identified (along with high g-forces) as the cause of acceleration atelectasis, a condition where air sacs in the pilot's lungs collapse. It is the formal medical term for the so-called "Raptor cough" that has been afflicting F-22 pilots.
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Acceleration atelectasis has been identified as a contributing factor to a series of physiological incidents that have plagued the USAF's F-22 fleet from as far back as 2008. The main culprit, however, according to the service, is a faulty valve in the Combat Edge upper-pressure garment.
At present, the USAF is trying to figure out the best way to implement the modification to the digital OBOGS found on most of its F-22s.
"It's a firmware change. Therefore, the hardware needs to be opened up in order to load the software," it says. "The change is not minor. In addition to the concentration schedule change, the warning band needs to be modified to accommodate the new schedule. In order to change the warning band, other features need to be incorporated, such as an automatic back-up oxygen system."
The USAF is implementing the change now, after having rejected a similar modification proposed in 2005 due to cost reasons.
In the meantime, the service is taking steps to mitigate the effects of the high oxygen concentration levels. It has rescinded guidance to use the OBOGS's maximum oxygen concentration mode - which was implemented when the air force believed the problem was too little oxygen - to the more gradual automatic mode. "Procedural guidance to use the OBOGS' automatic mode below certain altitudes is already in place that will reduce the concern for high oxygen concentration," the USAF says.
Source: Flight International
