. 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.
On April 5th the U.S. Navy grounded all its T-45C jet trainers for three days to interview pilots and instructors about oxygen problems. It turned out the problem was real and the trainers remained grounded until April 17
The T-45 is a nine ton, single engine two seat aircraft used to train pilots who will eventually fly jet fighters. The Hawk 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.
The OBOG problems are not new, nor are problems in general with pilot oxygen systems. The U.S. Navy had a similar problem with its F-18s. There were 64 incidents in 2002-9, resulting in two dead pilots. The navy found that the problem was carbon monoxide getting sucked into the aircraft air system. The navy modified the oxygen system, eliminating the problem. 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 is anything similar going on with the newly introduced F-22s. The air force has 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. Yes, 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 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, requiring OBOGs to solve 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 was 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.