December 8, 2017: The U.S. Air Force temporarily grounded a hundred of its T-6 trainers after at least five incidents where personnel in the T-6 (four instructors and one student) reported problems with their air supply. This happened during a two week period and in all five cases the backup oxygen supply was activated and all aircraft were able to land safely. Air force pilot trainees spend about 90 flight hours in the two seat T-6A and problems with the air supply does not inspire confidence. The air force is checking the primary pilot oxygen system, which uses an OBOB (OnBoard Oxygen Generating) unit.

Back in the 1990s, the T-6A was selected by the U.S. Air Force as the replacement for the half century old T-37Bs (which were basically worn out). This was because the T-6A design is based on the very popular Pilatus PC-9, which already had an excellent reputation as a trainer aircraft. The U.S. Air Force began using the T-6A in 2008.

A single engine prop driven aircraft, the T-6 reduces fuel costs by over 60 percent (compared to the T-37B). The three ton T-6As cost about $8 million each while the AT-6Cs cost a bit more. The basic AT-6 can carry about half a ton of weapons (bombs, missiles, machine-gun pods). The “C” version includes hard points on the wings for carrying bombs and missiles, or pods for recon and intelligence collecting. Since it entered service in 2001 nearly 900 T-6s have been delivered or are on order.

Meanwhile pilot air supply has been a frequent problem with American warplanes since 2000. Earlier this year (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 17th. But the aircraft resumed flying only after some changes were made to the pilot oxygen system to bypass the OBOG. 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 prevented 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 is a nine ton, single engine two seat aircraft used to train pilots who will eventually fly jet fighters. The T-45can 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 have persisted but are part of a larger issues 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.