Blackout is a temporary loss of vision and/or consciousness when a person is subjected to high accelerations and reaches maximum G.

Another term for blacking out due to acceleration is ‘g-force induced loss of consciousness’ (abbreviated as G-LOC, pronounced “JEE-lock”) This is a term used in aerospace physiology to describe a loss of consciousness occurring from excessive and sustained g-forces draining blood away from the brain.

So first of all what is a ‘G’ short for? – ‘Gravitational Force’ –

(1G is the force of Earth’s gravity — it is this force that determines how much we weigh. At 5 G’s, a pilot or astronaut experiences a force equal to five times his or her weight).

High speed does not produce harmful symptoms. It’s high accelerations which produce multiples of gravity, or g’s. When catching a ride to space in a rocket, an astronaut may be subjected to an acceleration as high as 3g. Specially designed outfits, called anti-g suits, apply pressure to the abdomen and legs, counteracting the tendency for blood to accumulate in those areas.

At high g’s the pilots blood pressure changes and the flow of oxygen to the brain rapidly decreases. This happens because the pressure outside of the pilot’s body is so much greater than the pressure a human is normally used to.

“If a force of 4 to 6 G is sustained for more than a few seconds, the resulting symptoms range from visual impairment to total blackout.”

The standard G-suit provides passive protection to about 5.5 +Gz.”

As the g forces climb up toward 7 g’s, you can no longer see colour. Everything appears in black and white. An instant later, your field of vision is shrinking. It now looks like you are seeing things through a pipe. In a flash you see black.

You have just “blacked out.” You are unconscious until the number of g’s are reduced and the blood returns to your brain. This could be fatal if you are flying an aircraft, race car or rocket and you black out in the process.

Check out these two videos on g-force training – what g force do you think you could take before passing out?

G-force, jerk, and a giant centrifuge

Be prepared to see some funny faces!  Nasa G-Force Training