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G-LOC, abbreviated from G-force induced Loss Of Consciousness, is a term generally used in aerospace physiology to describe a loss of consciousness arising from excessive and sustained g-forces draining blood away from the brain causing cerebral hypoxia. The condition is most likely to affect pilots of high performance fighter and aerobatic aircraft or astronauts but is possible on some extreme amusement park rides. G-LOC incidents have caused fatal accidents in high performance aircraft capable of sustaining high g for extended periods. High-G training for pilots of high performance aircraft or spacecraft often includes the simulation of G-LOC in special centrifuges. Such man-rated centrifuges are made by AMST Systemtechnik in Austria (Austria Metall SystemTechnik), Latacoere in France, the Environmental Tectonics Corporation (ETC) and Wyle Laboratories in the USA.

Effects of G-forces

As g-force increases, or the longer it is sustained, the victim may suffer progressively:

Recovery is usually prompt following removal of g-force but a period of several seconds of disorientation may occur. Brief but vivid dreams have been reported to follow G-LOC.

The human body is much more tolerant of g-force when it is applied laterally (across the body) than when applied longitudinally (along the length of the body). Unfortunately most sustained g-forces incurred by pilots is applied longitudinally. This has led to experimentation with prone pilot aircraft designs which lies the pilot face down or (more successfully) reclined positions for astronauts.


The g thresholds at which these effects occur depend on the training, age and fitness of the individual. An un-trained individual not used to the G-straining maneuver, can black out between 4 and 6 g, particularly if this is pulled suddenly. A trained, fit individual wearing a g suit and practicing the straining maneuver, can, with some difficulty, sustain up to 9g without loss of consciousness. Prone position designs in aircraft have not proved successful and the problem has been addressed largely by the development of the G-suit.

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