CRS INSIGHT
Out of Breath: Military Aircraft Oxygen Issues
June 21, 2017 (IN10723)
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Jeremiah Gertler
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Jeremiah Gertler, Specialist in Military Aviation (jgertler@crs.loc.gov, 7-5107)
The Air Force recently grounded some of its newest aircraft, F-35A strike fighters, due to incidents in which pilots
became physiologically impaired with symptoms of oxygen deficiency while flying. Although the root cause of the F-35
incidents has not yet been established, the grounding has renewed attention on hypoxia, a physical condition caused by
oxygen deficiency that may result in temporary cognitive and physiological impairment and possible loss of
consciousness. Hypoxia has affected pilots of F-22, F/A-18, and T-45 aircraft in recent years.
Military aircraft (and jet aircraft generally) operate at altitudes where there is not enough oxygen for humans to retain
consciousness without supplemental oxygen supplies. For several decades, combat aircraft relied on canisters of
compressed liquid or gaseous oxygen to provide pilots sufficient oxygen. However, the finite oxygen supply limited
mission duration, and its handling complicated aircraft maintenance. This led to the development of On-Board Oxygen
Generation Systems, or OBOGS. Developed in the early 1980s, OBOGS was conceived as a source of limitless oxygen.
The system works by purifying air drawn from the plane's engine compressor (called "bleed air") before it reaches
combustion. That air is run through a series of scrubbers, or "sieve beds," that remove nitrogen. The resulting gas going
to the pilot is approximately 95% oxygen and 5% argon.
OBOGS systems are made by a number of contractors, including Honeywell
, Cobham, Air Liquide, and others.
How is this different from commercial aircraft?
Most commercial aircraft also use engine bleed air to provide air for cabin climate control and pressurization. That air is
not filtered to remove nitrogen, because commercial aircraft cabins are pressurized to a constant altitude, usually at or
around 8,000 feet. At this pressure altitude, the cabin air climate retains sufficient oxygen for human needs.
In contrast, combat aircraft pressurization varies as a function of altitude, with a fixed difference between cabin and
outside pressure. Military aircraft can change altitude (and thus cabin pressure) quite rapidly, which could cause
physical complications (such as "bends," or the formation of nitrogen bubbles in the blood) were a higher ratio of
nitrogen present.
F-22 incidents
