G-LOC
Encyclopedia
G-LOC, pronounced 'GEE-lock', is the abbreviation of G-force induced Loss Of Consciousness, a term generally used in aerospace physiology
to describe a loss of consciousness occurring from excessive and sustained g-force
s 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 astronaut
s 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 ground training for G-LOC in special centrifuges, with some profiles exposing pilots to 9 gs for a sustained period. Such 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.
(Under negative g, blood pressure will increase in the head, running the risk of the dangerous condition known as redout
, with too much blood pressure in the head and eyes.)
The reason why vision is affected before other brain functions is because intraocular pressure
within the eyes works against blood pressure in the head. Skilled pilots can use this loss of vision as their indicator that they are at maximum turn performance without losing consciousness. 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. If G-LOC occurs at low altitude, this momentary lapse can prove fatal
and even highly experienced pilots can pull straight to a G-LOC condition without first perceiving the visual onset warnings that would normally be used as the sign to back off from pulling any more g's.
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.
Another physiological effect that can happen from high levels of sustained gs is a temporary condition that pilots call the 'g-measles', where the skin around the buttocks or other extremities becomes mottled because blood capillaries have been broken from too much pressure.
.
Aerospace physiology
Aerospace physiology refers to effects on the human body caused by characteristics of the aerospace environment.This subject includes, but is not limited to, the following topics:* Spatial disorientation and airsickness* Vestibular system...
to describe a loss of consciousness occurring from excessive and sustained g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
s draining blood away from the brain causing cerebral hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
. The condition is most likely to affect pilots
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
of high performance fighter
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
and aerobatic
Aerobatics
Aerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in normal flight. Aerobatics are performed in airplanes and gliders for training, recreation, entertainment and sport...
aircraft or astronaut
Astronaut
An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....
s 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
High-G training
High-G training is done by aviators and astronauts who are subject to high levels of acceleration . It is designed to prevent a g-induced Loss Of Consciousness , a situation when g-forces move the blood away from the brain to the extent that consciousness is lost.Incidents of acceleration-induced...
for pilots of high performance aircraft or spacecraft often includes ground training for G-LOC in special centrifuges, with some profiles exposing pilots to 9 gs for a sustained period. Such 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
Under increasing positive g-force, blood in the body will tend to move from the head toward the feet. For higher intensity or longer duration, this can manifest progressively as:- Greyout - a loss of color vision
- Tunnel visionTunnel visionTunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision.- Medical / biological causes :Tunnel vision can be caused by:...
- loss of peripheral vision, retaining only the center vision - Blackout - a complete loss of vision but retaining consciousness.
- G-LOC - where consciousness is lost.
(Under negative g, blood pressure will increase in the head, running the risk of the dangerous condition known as redout
Redout
A redout occurs when the body experiences a negative g-force sufficient to cause a blood flow from the lower parts of the body to the head. It is the inverse effect of a greyout or brownout, where blood flows away from the head to the lower parts of the body. Redouts are potentially dangerous and...
, with too much blood pressure in the head and eyes.)
The reason why vision is affected before other brain functions is because intraocular pressure
Intraocular pressure
Intraocular pressure is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk from glaucoma...
within the eyes works against blood pressure in the head. Skilled pilots can use this loss of vision as their indicator that they are at maximum turn performance without losing consciousness. 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. If G-LOC occurs at low altitude, this momentary lapse can prove fatal
Controlled flight into terrain
Controlled flight into terrain describes an accident in which an airworthy aircraft, under pilot control, is unintentionally flown into the ground, a mountain, water, or an obstacle. The term was coined by engineers at Boeing in the late 1970s...
and even highly experienced pilots can pull straight to a G-LOC condition without first perceiving the visual onset warnings that would normally be used as the sign to back off from pulling any more g's.
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.
Another physiological effect that can happen from high levels of sustained gs is a temporary condition that pilots call the 'g-measles', where the skin around the buttocks or other extremities becomes mottled because blood capillaries have been broken from too much pressure.
Thresholds
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-suitG-suit
A G-suit, or the more accurately named anti-G suit, is worn by aviators and astronauts who are subject to high levels of acceleration force . It is designed to prevent a black-out and G-LOC caused by the blood pooling in the lower part of the body when under acceleration, thus depriving the...
.