The new American F-22 fighter may be having a problem with its OBOG (OnBoard Oxygen Generating) system, causing pilots to get drowsy, or even black out, from lack of oxygen. As a result, on May 3rd, all F-22s were grounded because of suspected problems in the OBOG. But the U.S. Air Force is checking the OBOGs in F-16, F-15E, A-10, F-35 and T-6 aircraft as well. The problem may just be with the F-22 OBOG, or a general problem with all air force OBOGs.
OBOGs have been around for over half a century, but only in the last two decades have OBOGs 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, get an OBOG tweaked for space, weight or other conditions specific to that warplane design.
Because aircraft have been staying in the air longer (because of in-flight refueling), carrying enough compressed oxygen has become untenable, and OBOG solves 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.
Even helicopter pilots sometimes need additional oxygen. The U.S. Army has encountered this in Afghanistan. That's because helicopter pilots there often operate at high (over 3,200 meters/10,000 feet) enough altitudes that they need oxygen to maintain alertness. Currently, crew members get the oxygen via tubes from 45 kg (hundred pound) tanks. This restricts mobility inside the helicopter. So the army has developed a portable (2.3 kilograms/5 pound) device that you can wear on your chest. A sensor gives you additional oxygen when it detects a need. The PHODS (Portable Helicopter Oxygen Delivery System) provides enough oxygen for 2-3 hours.