Positive and Negative “G”
Many pilots think that unless they are performing aerobatics, knowledge about acceleration (G) is unnecessary. However, this force affects pilots in all aircraft – from the smallest ultralight to the biggest jet. G is the symbol for the rate of change of velocity and so represents both a force and a direction.
What is a G?
The most common example is the force of gravity (g), which is 32 feet/sec2. This means a body in a vacuum would fall at a speed that increases by 32 feet/sec in each second of the fall. By international convention, G is described in three planes relative to the body. These are transverse (Gx), lateral (Gy), and longitudinal (Gz).
Convention also requires an indication of whether the force is positive (+) or negative (-). For example, acceleration from the feet to the head is positive Gz and from the head to the feet is negative Gz. The effect of acceleration on the body is due to the displacement of blood and tissues. It is important to realize that the displacement is caused by the inertia of the tissues and this will be opposite in direction to the acceleration force. If you were fired into the air from a cannon, the acceleration would be upward, but inertia would result in a relative downward displacement of your organs and blood.
Only Gx and Gz are of practical significance to civilian pilots and the most significant result of Gx is disorientation. Thus, when we speak of positive or negative G, we are referring to Gz unless otherwise noted.
The Effects of G
G tolerance varies greatly with the individual. Because the symptoms are caused by the displacement of blood and tissues, we would expect that a pilot with good muscle tone would have a better tolerance. This is correct. Tolerance is lowered by obesity, ill health, low blood pressure, pregnancy and many medications. It may vary from day to day in relation to fatigue, smoking, hypoxia or hangovers.
In absolute figures, G tolerance is affected by the peak value, the duration of the G force and the rate of onset. If the rate of onset is very high, positive G can result in unconsciousness, known as G-loss of consciousness (G-LOC), without any other symptoms.
The increased weight of limbs and organs interferes with movement, and forces greater than +3G make it almost impossible to escape from an aircraft in uncontrolled flight. Fine movements are less affected. Heavy equipment such as a protective helmet can cause problems with increasing G. At about +6G a pilot’s head would be flexed on the chest by the increased weight of a crash helmet.
The most serious effect of positive G is the draining of blood away from the head toward the feet, causing (stagnant) hypoxia of the brain, the first symptom is vision deterioration. As G forces are experienced, the blood pressure to the retina decreases because the weight of the column of blood between the heart and the eye (and therefore the work of the heart) increases. Therefore, the retinal blood supply decreases. Vision, beginning in the periphery, starts to become dim and colourless; this is called “grey-out.” As the G forces increase further, the blood flow in the back of the eye will be completely interrupted and “black-out” (temporary loss of vision) will occur, although the pilot remains conscious. There is a delay of 5–7 s between the onset of G and the visual changes because of the oxygen dissolved in the fluids of the eyeball. If G forces stabilize, there may be an improvement in the visual symptoms after l0–12 s because the body’s reflexes automatically increase blood pressure.
Grey-out begins at about +2G and black-out is usually complete at +4G in the relaxed, unprotected pilot. As the G force increases, hypoxia of the brain develops and consciousness is usually lost in the unprotected pilot at over +6G (G-LOC). When the G forces decline, consciousness is quickly recovered, but there is always a brief period of confusion on awakening.
Negative G is poorly tolerated. Here, because the acceleration is from feet to head, blood pressure in the eyes and the brain is increased so “red-out” (a red haze in the vision) is experienced. Negative G in excess of -5G may cause rupture of small blood vessels in the eyes and prolonged negative G may cause brain damage. Negative G is experienced in a push-over or “bunt” and in an outside loop.
Transverse G is well tolerated; this is why astronauts recline on blastoff. Levels of up to +50 Gx can be tolerated for short intervals without tissue damage, although the acceleration interferes with breathing. In current aircraft, Gy is not a significant problem.