G-LOC (Gravity induced loss of consciousness)


During manoevres, big or small, the pilot effectively alters the G-loading (the force which acts upon the pilot when attempting a manoevre) being imposed on his or her body. During the early years of flight, physiological problems related to G-loading were unreported because aircraft speeds and power during those times were limited. The first reported instance of G-LOC could have been in 1908, when a pilot of a Sopwith Triplane reported that the sky turned to gray (grayout) before he fainted in a tight turn which he induced 4.5G on himself. The pilot also reported that in a sustained high G condition such as a 4.5G powered spiral, he lost his sight for a short time (blackout). However he retained all body controls such as his limbs, and had no difficulty controlling the aircraft, except that his vision was blacked out (Beaudette, 1984)1.

As a pilot, or manager, we understand that when flying an aircraft, Situational Awareness is paramount to the safety of the aircraft. This awareness could be in regards to the actions inside the aircraft (monitoring of instruments) or outside the aircraft (maintaining Visual Awareness). When we put our minds under stress our minds intially get aroused and our performance increases. However as stress increases even more, our performance would start to decrease, as seen from the Yerkes & Dodson (1908)2 Curve.

This topic covers the essentials of G-Force induced Loss of Consciousness, or put simply, how our mind fades and potentially becomes unconscious with the increased application of gravity over a period of time such as in an aerobatic manoevre.



G-Induced Loss of Consciousness is a condition caused by a reduction in bloodflow to the brain at which point the neurons fail to function in the absense of oxygen replenishment by normal blood flow (Bureau of Air Safety Investigation, 1988)3.

A video example of G-LOC

What Gs are.

When we talk about G Forces, we need to identify the direction of these accelerations (G-Force is a misnomer, it is actually an acceleration), they can come in the form of three diferent axis. We have Lateral G which go from left to right and vice versa. We have Longitudinal G which go from back to front (felt when we depress the brakes or accelerate in a car). And finally we have Vertical G which go from top to bottom.

An F1 driver can pull up to 5 or 6 Gs in the lateral (sideways) and longitudinal (back to front) axis as seen in the short video below.

However, flying is very different as compared to driving because one more axis is introduced, the Vertical axis. When there are accelerations in the vertical axis, we either feel that there is a very heavy object on top of us, or we feel that we could be floating (negative G).


The force acting in the vertical axis can be represented by "Gz" and can be read as 'G in the vertical axis' or 'G in the Z axis'. When a pilot pitches up, positive Gz is produced, and we annotate that as "+Gz". When the pilot pitches down, the force acts from down to up, and we feel we are being lifted from our seat, "-Gz" can be used to annotate negative Gz.

How G-LOC Occurs

We first need to understand how our brain and eyes function and what 'fuel' they need. Our brain and eyes function with the help of Oxygen and Glucose. They both have a very small store of sugar and virtually no stored Oxygen. A constant supply of these two nutrients are crucial for normal function these nutrients are transport via our bloodstream and the heart helps to pump it.

When we pull +Gz, what happens is that our body is being acclerated downwards, as a result blood will be drained from our brains and eyes.
When this +Gz is sustained, the bloodflow will act against the direction being pumped by the heart, as a result our brain and eyes will be starved of oxygen and cease to function. (Watson, 1990)4

The Symptoms - Greyout, Blackout

When the pilot pulls light amounts of G, maybe 2G, his body will feel cumbersome, if your arm weighs 15 pounds, in a 3G manoevre, the arm weighs 45 pounds. Moderate amount of G, such as any manoevre between 3 and 5 G, the pilot will start to experience Greyout, which is a result of the eye gradually losing bloodflow, but not completely. The pilot's peripheral vision actually starts to deteriorate as soon as the stick starts to come back towards their lap and by the time they notices any 'tunnelling', 75% of the visual field is already gone.

As the Gs continue to increase, the pilot will blackout, many people perceive blackout as the pilot losing consciousness, this is not so. Blackout is the result of the eye losing all of its oxygen and glucose supply. In fact "RAF pilots training for the Schneider trophy in the mid 1920s became quite adept at pulling just enough G to blackout but not lose consciousness, maintaining control of their aircraft by 'feel'" (Watson 1990).

Because the eyes typically lose its function before the brain does, when a person pulls Gs to a point of blackout, it is a symptom that a G-LOC will shortly occur if such high G is maintained for an extended period of time. The induction of G-LOC is often very sudden, almost without warning, the sufferer will instantly lose consciousness. Once a person enters a G-LOC state and no effort is made to reduce the amount of G-Loading, the person can suffer from brain tissue damage or even more seriously, brain death.

Prevention of G-LOC

The most obvious way of preventing G-LOC would be to pull no G, however this is unrealistic because there will definitely be situations when pulling significant G is required, such as an avoidance manoevre.

Frequent exposure to G-stress may "tune" the human system, making it less sensitive to higher G-loads. If you have not flown aerobatics for some time, begin with the simpler, less stressful maneuvers when you take it up again.

Physical conditioning does not seem to increase tolerance to G-loads; marathon running lowers tolerance. On the other hand, a well-tuned cardiovascular system seems to recover more rapidly from many different kinds of stress. Keep in shape.

Aircraft seat angle can also have a profound effect on the pilot's G-tolerance. The USAF F-16 has it's seat reclined 30 degrees and the Soviet Su-25M 35 degrees each giving about 1G added protection while still allowing good visibility. A seat reclined to about 80 degrees allows a pilot to easily sustain 15G but this is of little practical use as such a position impairs forward, and downwards, visibility.

Anaerobic fitness, especially involving abdominal and biceps muscles, improves G-tolerance. The duration of High G Tolerance can be extended 53% by an aggressive anaerobic weight program . There is considerable debate, and a lot of ongoing research, into the relative merits of aerobic and anaerobic fitness in the protection against G-LOC.

Mastering the limits of G-LOC

This is what the best aerobatic pilots can do, pushing their bodies to the limit and still fly safely, enjoy!

Want to know more?

  1. *Effects of G forces on aerobatic pilots.*
  2. Wikipedia G-LOC
  3. G-LOC - Could it happen to you?

Contributors to this page

Authors / Editors

Noel Chua

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