Aviation physiology is the study of physiological responses of the body in aviation environment. Flying exerts great effects on the body through accelerative forces and gravitational forces, which are developed during the flight maneuvering. Pilots and other crew members of aircraft are trained to overcome the effects of these forces.


Acceleration means change in velocity. Flying straight in horizontal plane with constant velocity has minimum effects on the body. However, changes in velocity produce severe physiological effects. Accelerative forces are developed in the flight during linear, radial or centripetal and angular acceleration.


Gravitational force (G force) is the major factor that develops accelerative force. G force and the direction

in which body receives the force are responsible for physiological changes in the body during acceleration. Force or pull of gravity upon the body is expressed in G unit. On the earth, this pull is responsible for body weight. Force of gravity while sitting, standing or lying position is considered to be equal to body weight and it is referred as 1 G. G unit increases in acceleration. If we say that G unit increases to 5 G during acceleration, it means that the force of gravity on body at that moment

is equal to five times the body weight. While traveling in an airplane, elevator or a car, if there is a sudden change in speed or direction, passengers are thrown or centrifuged in the opposite direction. It is because of change in the G unit. G unit may increase or decrease. Increase in G unit is called positive G and decrease in G unit is called negative G. Positive G occurs while increasing the speed (acceleration).

Negative G occurs while decreasing the speed (slowing down; deceleration). G unit is altered during the change in direction also. While flying, both positive G and negative G cause physiological changes in the body.


Effects of Positive G

Major effects of positive G during acceleration are on the blood circulation. When G unit increases to

about 4 to 5 G, blood is pushed toward the lower parts of the body including abdomen. So the cardiac

output decreases, resulting in reduced blood supply to the brain and eyes. Decreased blood flow in turn,

decreases oxygen supply (hypoxia) to the head and leads to following disturbances:

1. Grayout

Grayout is the graying of vision that occurs when blood flow to eyes starts diminishing. It occurs because

the retina is more sensitive to hypoxia than brain. Though physical impairment does not occur, grayout

is considered as a warning for decreased blood flow to head.

2. Blackout

Blackout is the complete loss of vision that occurs when retinal function is affected by hypoxia. Consciousness and muscular activities are still retained. But it indicates the risk of loss of consciousness.

3. Loss of consciousness

When force increases beyond 5 G, hypoxia reaches the critical level and causes loss of consciousness.

It may be associated with convulsions. Unconscious state may last for about 15 seconds. After recovery

from unconsciousness, the person needs another 10 to 15 minutes for orientation. If the affected person

happens to be a lone pilot, then he will loose control over his aircraft.

4. Fracture of bones

When force increases to about 20 G, bones, particularly the spine, becomes susceptible for fracture even during sitting posture.

Effects of Negative G

Negative G develops while flying downwards (inverted flying). It causes the following disturbances:

1. Hyperemia

When the force decreases to –4 to –6 G, hyperemia (abnormal increase in blood flow) occurs in head

because the blood is pushed towards head. Sometimes the blood accumulates in head, resulting in brain

edema. There is congestion, flushing of face and mild headache. Negative G at this level is tolerable and the effects are only momentary. Brain also can withstand hyperemia in such conditions.

2. Redout and headache

Redout is the blurring of vision and sudden reddening of visual field, caused by engorgement of blood vessels in head. When the negative G reaches to about –15 G to –20 G, there is dilatation and congestion of blood vessels in head and eyes, resulting in redout and headache. Blood vessels in brain may not be affected much because of CSF. When blood accumulates in brain, there is simultaneous pooling of CSF in cranium. The high pressure exerted by CSF acts as a cushion (buffer) and protects the blood vessels of brain.

3. Loss of consciousness

High negative G affects the body by other means. It increases the pressure in the blood vessels of chest

and neck. It causes bradycardia or irregular heartbeat, which adds to stagnation of blood in head. All these factors ultimately lead to unconsciousness.


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