This aticle first appeared in Aerospace Daily & Defense Report.
The U.S. Air Force will continue to investigate the oxygen concentration levels in F-22 Raptor cockpits in the wake of reports about fluctuations of those numbers noted during studies of recent pilot breathing problems.
"We also will study and as necessary revise the schedule by which the Obogs (On-Board Oxygen Generation System) adjusts the oxygen concentration delivered to the pilot to better mitigate against the identified effects of too low or too high a concentration," says service spokesman Lt. Col. Edward Sholtis.
Oxygen concentration level fluctuations were noted in the Air Force Scientific Advisory Board (SAB) Report on Aircraft Oxygen Generation, which delved into the recent Raptor pilot breathing problems and was released last month.
"As the aircraft descends and the pilot puts eight Gs on the aircraft, the percentage of oxygen produced by the Obogs is reduced," the report says. "As the pilot reduces the G load, the Obogs begins to recover and then the percentage of oxygen produced by the Obogs is reduced again when the pilot reapplies the Gs."
The report says, "The amount of oxygen being produced does decrease to between 60 percent and 70 percent."
A software "deep dive" is under way, the report says, as well as a further assessment of the reasons for the drop in oxygen concentration noted under G-loads.
By design, the Obogs oxygen level for the F-22 is lower by five or six percentage points than levels required for earlier-type fighters, and the SAB report says there is "limited understanding of the aviation physiology implications of accepting a maximum 93-94 percent oxygen level instead of the 99+ percent previously required."
The report says, "Given the F-22's unique operational envelope, there is insufficient feedback to the pilot about the partial pressure of oxygen in the breathing air," adding there is "no indication of pilot oxygen saturation throughout the F-22 flight envelope."
The report also cautions that "ECS (environmental control system) shutdowns are more frequent than expected and result in Obogs shutdown and cessation of breathing air to the pilot."
Noting "some anomalies in the performance of the F-22 oxygen and anti-G delivery systems when the ECS cuts back or shuts down in-flight, or during the onset of High-G forces," the report says the incidents "merit further analysis and testing."
Obogs efficiency is key for flight operations, the report says. "Unlike most other aircraft oxygen generation systems, the breathing air to the F-22 pilot is not diluted with cockpit air to obtain the appropriate oxygen partial pressure (PPO2) necessary to maintain physiological function at a particular altitude, but rather it is concentrated to the necessary PPO2 by controlling the cycling of the Obogs."
Credit: U.S. Air Force