The U.S. Air Force is admitting that it is having a problem with the pilot air supply on the F-22, a problem they have not been able to find the cause of. Despite this, the air force is going to continue flying its F-22s. The decision to keep flying was made because the air supply problems have not killed anyone yet and they are rare (once every 10,000 sorties).
The 14 incidents so far were all cases of F-22 pilots apparently experiencing problems. The term "apparently" is appropriate because the pilots did not black out and a thorough check of the air supply system and the aircraft found nothing wrong. There have been nearly 30 of these "dizziness or disorientation" incidents in the last four years, with 14 of them serious enough to be called real incidents. Only one F-22 has been lost to an accident so far and, while that did involve an air supply issue, it was caused by pilot error, not equipment failure.
Meanwhile the air force is spending $7 million to install commercial oxygen status sensors in the air supply systems of its F-22 fighters. This is part of a year-long effort to find out what's causing the air supply on F-22s to get contaminated and cause pilots to become disoriented or pass out. Twice in the past year the entire F-22 fleet was grounded because of the air supply problems. The first grounding lasted 140 days and ended last September. The second grounding lasted a week and ended five months ago. The 180 F-22s comprise the most powerful component of the air force's air combat capability and the brass are eager to find out what is wrong.
The air force has already found some problems with the air supply system (too much nitrogen and other contaminants). The main problem was always about something bad in the air supply. But the air does not go bad in any predictable fashion nor does it become bad enough to cause problems for the pilot. So the air force is still looking for causes. Thus F-22 pilots, for example, give blood samples after most flights and maintainers pay extra attention to the oxygen system. And now there will be all the data from all the new oxygen sensors.
The U.S. Navy had a similar problem with its F-18s. There were 64 incidents from 2002 to 2009, resulting in two deaths. The navy found that the problem was carbon monoxide getting sucked into the aircraft air system (which the navy modified, eliminating the problem). The air force looked into the navy experience to see if there is anything similar going on with the F-22s. No luck. The air force has looked into a lot of potential causes, without a lot of success.
The air force woes began when it appeared that the F-22 might be having a problem with its OBOG (OnBoard Oxygen Generating) system, causing pilots to get drowsy or even black out from lack of oxygen. The U.S. Air Force also checked the OBOGs in F-16, F-15E, A-10, F-35, B-1, B-2, CV-22, and T-6 aircraft as well. Apparently there were no problems there. The air force believed, at one point, that the F-22 problem might not just involve the OBOG.
The chief suspect in all this, OBOGs, have been around for over half a century. It's only in the last two decades that OBOGs have become compact, cheap, and reliable enough to replace the older compressed gases or LOX (liquid oxygen) as a source of breathable air for high flying aircrew. Each aircraft, especially the F-22 and F-35, gets an OBOG tweaked for space, weight, or other conditions specific to that warplane design. It's this custom design that was also closely studied, to find out how the toxins got in.
One problem is that aircraft have been staying in the air longer (because of in-flight refueling) and carrying enough compressed oxygen has become untenable. Thus the need for OBOGs to solve the problem. Since the 1990s, most American military aircraft have replaced older oxygen systems with OBOG. Most Western nations, and Russia, have followed, at least with their latest model aircraft. Most OBOG systems work by using a chemical reaction to remove carbon dioxide from the air taken in to the OBOG and then sending out air with the proper amount of oxygen to the aircrew.