The body is able to withstand the effects of acceleration up to certain thresholds. These thresholds depend upon both the intensity and duration that the forces are applied. Normally, acceleration is divided into 2 areas:
Short Term Acceleration
Impact acceleration forces that last less than one second.
The forces the body can withstand are directly related to its own strength:
- In the vertical axis the body can withstand 25G.
- In the fore and aft axis 45G.
- In the lateral (side) axis 10-15G
Any force above these levels cause injury.
Long Term Acceleration
Forces that last more than one second. As a human being we are used to the effects of gravity. When we fly we are subject to the acceleration forces that can be applied when flying the aircraft. The value of long term acceleration is usually given as either "positive G" or "negative G".
Perceived as an increase in body weight, the more the G pulled the harder it becomes to move freely. If enough G is pulled then organs can be displaced from their normal position. As seen earlier we measure the blood pressure in the upper arm as this equates to the blood pressure in the heart. If we were standing it would be fair to say that the blood pressure in the head will be less than that in the heart and that the blood pressure in the feet will be greater than that in the heart.
If G is applied then the blood pressure in the head will be reduced because the force will drive the blood to the lower half of the body. The blood supply can be cut off meaning that the eyes and the brain are starved of blood. As G increases we notice the effect on the eyes firstly by greyout (gradual greying of the vision) and followed by unconsciousness.
Greyout will appear at approximately 3.5G if the pilot is totally relaxed. By using straining manoeuvres the delay of greyout and unconsciousness can be up to 7-8G. The military use G-suits to help the pilot in long term acceleration.
G tolerance is reduced by many factors such as:
- Low blood sugar
The effects of negative G are the opposite of those for positive G. Negative G manoeuvres in an aircraft are much more uncomfortable than positive G manoeuvres. Facial pain can be experienced and in extreme cases small blood vessels can burst. Negative G is associated with the term "redout", where the lower eyelid is pushed up under the eye. Maximum negative G is considered as -3G and then for short periods only.