On August 30th all restrictions on F-35A operations at an Arizona airbase were lifted. This comes after flights were resumed, with altitude restrictions (stay under 11,300 meters) in late June. The problems began in early May and by June 8th five F-35 pilots had reported signs of hypoxia (air supply problems) and F-35As at the base were grounded for two weeks as the aircraft were checked out. Nothing definitive could be found and training flights were resumed but with altitude restrictions. Still nothing conclusive but a new set of safety checks was developed that could catch the problem before it occurred again.
There has been a lot of this sort of thing in the last fifteen years with several different types of aircraft developing mysterious problems with hypoxia. For example earlier in 2107 there were hypoxia problems with U.S. Navy aircraft and on April 5th the navy grounded all its T-45C jet trainers for three days to interview pilots and instructors about air supply problems. It turned out the problem was real and the trainers remained grounded until April 17th. But the aircraft resumed flying only after some changes were made to the pilot oxygen system to bypass the OBOG (OnBoard Oxygen Generating). This limited how high the aircraft could fly. The T-45s will fly at lower altitudes until the navy can determine what was wrong with the OBOG or something else in the pilot air supply system. This restriction will prevent the T-45s from performing about a quarter of the training they normally provide but it averted a threatened pilot strike over the issue. The T-45 problems have yet to be diagnosed, understood and fixed.
While the F-35 is the most modern jet fighting in American service the T-45 is a nine ton, single engine two seat aircraft that has been used since early 1990s to train pilots who will eventually fly jet fighters. The T-45 can also be armed and used for ground attack. In this mode they can carry up to three tons of weapons, including a pod with a 30mm autocannon. In comparison the F-35A is a 31 ton single engine stealth fighter-bomber that began entering service in August 2016.
OBOG problems are not new, nor are problems in general with pilot air/oxygen systems. The U.S. Navy had a similar (to the T-45) problem with its F-18s. There were 64 incidents in 2002-9, resulting in two dead pilots. The navy found that the main problem was carbon monoxide getting sucked into the aircraft air system. The navy modified the oxygen system, eliminating most of the problems. Similar failures continued with the major difficulty finding out exactly what was going on so it could be fixed.
After 2009 the air force looked into the navy experience, to see if there was anything similar going on with the newly introduced F-22 stealth fighters. The air force looked into a lot of potential causes, without a lot of success. In 2012 the air force admitted that its rate of "pilot air supply" incidents for its 187 F-22 fighters has reached the rate of 26 per 100,000 flight hours. For most other aircraft, the rate is closer to 2-3 incidents per 100,000 hours.
All aircraft have occasional problems with their air supply but nothing like what the F-22 was going through. The air force woes began when it appeared that the F-22 might be having a problem with its OBOG system. OBOGs have been around since the 1950s but it’s only since the 1980s 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 recent solution is to constantly monitor the air quality and enable the OBOG to adjust its operation to cope or, failing that, alert the pilot. Another common feature of modern aircraft (and many other vehicles) is a performance monitor that notes and records certain performance data. Sort of a black box, but with a longer memory. These devices have been useful in diagnosing and fixing the hypoxia problems.
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, often making OBOGs mandatory to deal with the problem. Since the 1990s, most American military aircraft have replaced older oxygen systems with OBOGs. 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. The F-22, F-35. F-15E and many other modern aircraft were designed to fly long distances and spend long periods of time in the air. Thus an OBOG is mandatory. But OBOGs proved to be the culprit only part of the time and it has become quite a puzzle to unscramble these air supply problems that keep showing up.