October 25, 2011: For the second time this year, the U.S. Air Force has grounded all its F-22 fighters. Same reason, problems with the pilot's oxygen supply. This time, a pilot experienced loss of oxygen during flight. He was able to land safely, but this reoccurrence of the oxygen led to the prompt grounding of all 170 F-22s until the problem could be fixed. At the moment, F-22s comprise the most powerful component of the air force's air combat capability.

It was only on September 21st that the air force allowed its F-22 fighters to fly again. The aircraft had been grounded for 140 days because of problems with the oxygen system. The air force is not giving out many details on exactly what the problems is, although they say a report on the F-22 oxygen system will be out by the end of the year. It has been mentioned that there appeared to be a problem with two much nitrogen getting into the pilot's air, and that an additional filter was added to the oxygen system to help keep potential contaminants out.

During the last month, each F-22 underwent a detailed inspection before it was cleared to fly. This may have something to do with earlier remarks about toxins somehow getting into the pilot's air supply. The problem was always about something bad in the air supply. This has kept all F-22s grounded since May. The only exception was a squadron based on the Virginia coast that was given permission to fly out of the way of hurricane Karina. Those F-22s encountered no problems with their air supply.

The U.S. Navy had a similar problem with its F-18s (there were 64 incidents between 2002-9, resulting in two dead pilots). 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. The air force has looked into a lot of potential causes, without a lot of success.

The air force problem began when it appeared that the F-22 fighter 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. There were 14 reported incidents of pilots feeling drowsy, or even passing out, because of "bad air." Because of that, all F-22s were grounded last May. But 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.

During the grounding, pilots and ground crews used simulators and (for the ground crews) maintenance exercises on the grounded aircraft to keep their skills sharp. In addition, everyone helped checking out a growing list of aircraft components in support of the search for the breathing problem. The air force was also under time pressure to fix the problem. That’s because after 210 days on the ground, aircrew have to undergo extensive retraining to regain combat flying status. This added a little more urgency to fixing the problem quickly. The grounding also left a dozen F-22s stranded at a training base, where they were for live weapons exercises. At least six new F-22s could not be delivered because of the grounding.

The chief culprit 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.

Because aircraft have been staying in the air longer (because of in-flight refueling), carrying enough compressed oxygen has become untenable, and OBOG solved 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 nitrogen from the air taken in to the OBOG, and then sending out air with the proper amount of oxygen to the aircrew.