Flying and Health

Flying and Health

A career in aviation brings the individual into situations not usually met in other professions. These situations are discussed below.

Acceleration

Acceleration is the rate of change in velocity, either as regards to speed or direction or both. Man is exposed to the forces of acceleration in one form or the other almost constantly throughout flight. Acceleration is referred to in aviation medicine as:

•Linear.

•Radial (centripetal).

•Angular.

Difference between Radial and Angular Acceleration

Angular acceleration involves rotation about an axis which passes through the pilot’s body whereas radial (centripetal) is where the axis is external to the pilot. Thus, when a very gentle turn is initiated, the body will initially experience angular acceleration but, as the turn develops, both angular and radial accelerations are experienced.

The effects of acceleration on the human body may be classified into long or short duration accelerations. In long duration acceleration the force will act for longer than one second. In the case of short acceleration, which lasts for one second or less, we are mainly concerned with impact forces.

G-forces

The human body has adapted to live under the force of gravity on the earth (the pull of the earth’s gravity giving the body weight). Acceleration in an aircraft can subject the body to forces much greater than this. For convenience, the forces are measured as multiples of our 1g terrestrial environment.

Acceleration in the fore and aft (the horizontal) plane is referred to as Gx, whereas acceleration in the lateral plane (side to side) is known as Gy. However the usual g-force encountered in aviation is that in the vertical plane which is termed as Gz.

Effects of Positive G-force on the Human Body

In long-term positive acceleration, the changes in g-force are perceived as:

•An increase in body weight so that limbs become harder to move, the head becomes heavy (2g and above). Mobility is impaired, for example, if the head is lowered it may be impossible to raise it again. At 2.5g it is impossible to rise from the sitting position.

•Internal organs are displaced downwards from their normal positions and the lower facial area feels “pulled down” (3-4g and above).

•An increase in hydrostatic variation of the blood pressure. Normally the blood pressure in the legs and lower body is greater than that at the heart. As the positive g-forces increase so the hydrostatic variation increases. The result is a pooling of blood in the lower body with a reduced venous return to the heart. There is a consequent reduction of blood pressure in

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•Smoking.

•Obesity.

•Alcohol.

FactorsThat IncreaseTolerance to Long Duration G-forces

The normal relaxed individual will usually be able to withstand or tolerate about +3.5g without serious effects such as greying out, other visual disturbances, or unconsciousness. The factors that can increase tolerance are:

Body Position

It has been found that, while in the sitting position, by raising the knees and feet and bending the trunk forward from the hips has a small benefit in delaying the effect of blackout. Firstly it decreases the vertical distance between the lower limbs and the heart, and secondly, it causes the acceleration force to drive the blood in the thighs towards the heart instead of away from it. Certainly lying in the prone or supine positions go a long way to eliminate the hydrostatic variation and it has been known for the human being to tolerate up to 12G in this position before blacking out.

Voluntary Manoeuvres

It is possible to raise the g tolerance by about 1-1.5g by taking a number of physical actions. The principle underlying the first three of the following manoeuvres is to raise the pressure in the abdominal cavity, so as to maintain the level of the diaphragm and facilitate the return of the venous blood. This can be done by:

•Straining.

•Shouting.

•Tensing leg muscles

It is essential that these measures are kept short and intermittent since they cause fatigue and will have a negative effect if prolonged.

Anti-g Straining Manoeuvre

This is now widely accepted as a means of combining the beneficial effects of the manoeuvres listed above. It is a combination of muscle tensing and the valsalva manoeuvre performed rhythmically every 3 to 4 seconds.

Anti-g Suits

The object of the suit is to provide more counter-pressure to the lower limbs and help to maintain the level of the diaphragm. The operation is automatic and varying pressures are delivered to the suit according to the g level experienced.

Combined with the anti-g straining technique, the tolerance level can be increased by 1.5 to 2g, however these techniques are normally confined to military flying.

The tolerance for negative g-forces is much lower being only -3g. This level can only be tolerated for a few seconds.

Short Duration Positive (Impact) g-Forces

The body can tolerate at most 25g in the vertical axis and 45g in the fore and aft axis. Forces above these levels will cause serious injury and death.

The lap strap, as worn by airline passengers, is the simplest form of restraint. Although tolerance

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