Physiological Effects of Positive G Forces

This article is an attempt to relate some of the known effects of positive G loading to competition aerobatics and/or airshow type flying. Much of what is contained herein is information given to me by employees of the Johnson Space Centre, and most of the tables used are by Drew Lundgren - a Pitts jockey and competitor.

Some terms used in this article will have the following definitions:

Grey-out — Loss of peripheral vision (tunnel vision) with loss of colour perception, and no loss of consciousness, The pilot can still hear, feel, and think. Recovery time two to three seconds after release of positive G force.

Blackout — Complete loss of eyesight, no loss of consciousness. Pilot can hear, feel, and think. Recovery time two to three seconds after re- lease of positive G force.

L.O.C. — Loss of consciousness. The subject cannot hear, feel, think, or function. Frequently accompanied by seizure activity and/or loss of bladder and bowel control. Recovery does not occur on the average for 15 to 20 seconds after the G force is terminated. The time required to return to consciousness may vary from nine to 20 seconds, and the subject does not re- turn to normal function for several minutes.

+ G — Positive G forces exerted through the vertical axis of the body. The effect of positive G loading (± G is a function of the G load, the time exposed to the G load, and the rate at which the G load is produced.

Figure 1 is a graph of the time plotted against G loads and showing grey- out, blackout and L.O.C. The information on this graph has been known to aerospace medicine since published by White in 1961. The graph would indicate that we can function in the first five seconds with heavy to even astronomical G loads without even a grey-out or blackout, provided how- ever, that we return to less than four G’s within five seconds. This explains the 12 G corners that the Russians pull with the Sukhoi Su-26M.

Referring to Table 2, a 12 G pull-up at 300 MPH would last 1.8 seconds. In reality, the exposure time would be slightly longer because the speed of the aircraft would be decreasing throughout the pull, but not nearly long enough to produce visual changes or L.O.C. (See Figure 1). Again, using Table 3, considering an inverted humpty where you pull six G’s at 240 MPH, the pull will last 5.7 seconds. Plot this on Figure 1 and you will be right on the edge of L.O.C. You would, however, pass through the areas of grey-out and blackout before reaching L.O.C. and, therefore, would have been forewarned of the imminent L.O.C. This warning ordinarily causes us to slack off the G load and avoid disaster.

It has been demonstrated many times in the centrifuge and in F-16’s, etc., that if you enter the L.O.C. area (Figure 1), you will not regain consciousness for approximately 15 to 20 seconds. Fifteen seconds is time to carry you 3,960 feet at 180 MPH, during which time the pilot might well have convulsive jerking movements of all extremities with or without loss of control of bowels and bladder. It is certainly this 15 to 20 second period after the ± G~ load has been relieved that has killed so many military pilots and probably several competition and airshow pilots as well,

The very high performance aircraft which are now being flown by the Russians and are presently being developed in this country are quite capable of carrying us directly into L.O.C. without any visual disturbance to warn us. Consider an inverted humpty at 300 MPH pulling eight + G, the duration of the pull would be 5.4 seconds (Table 3). If this were to happen in the aerobatic box, the result would almost certainly be fatal.

Consider now the same pull up from a vertical line down to a vertical line up at ± 12G. On Table 3, we see that the duration of the pull is 3.6 seconds, which is 1.4 seconds short of the time required to reach the L.O.C. area, and there would be no grey-out or black- out. The 12G pull is infinitely safer and more comfortable if your equipment is stressed for it, and the pilot is positioned in such a way that the load is not too uncomfortable.

A study has been done to determine time to loss of consciousness with complete interruption of circulation to the brain — that is, circulation being cut off with a cuff apparatus around the neck which could be inflated so rapidly that all circulation was cut of between heartbeats. Un- consciousness occurred between five and six seconds. Some of the subjects in this study denied having lost consciousness though they were unable to respond to stimuli and even had convulsive seizures during their unconscious episode. Some failed to respond to a flashing light signal for several seconds after circulation was restored though they stated that they could see the light flashing.

In this study, circulation to the brain was stopped for 100 seconds in some of the subjects. Consciousness and normal function returned within 30 to 40 seconds and the subjects were able to walk out of the room within two minutes after the procedure. Some subjects were exposed to this procedure several times and no after- effects were observed. Remember that this study was not done with heavy positive G loading but simply by interrupting circulation to the brain. It is when unconsciousness is produced by heavy + G loads that the subject remains unconscious for 15 to 20 seconds after the G load has been reduced to one + G.

The numbers used in Figure 1 are average numbers, the time to unconsciousness varying from time to time with the subject’s physiological state and training. We are referring to the state of hydration, rest, and nutrition, and the ability to perform the M-1 maneuver which is used by many people to help withstand positive G loads. It has also been demonstrated that repeated exposure to ± G forces conditions the pilot to withstand more and longer G forces.

Our primary protection against blackout and loss of consciousness have consisted of, number one, the M-1 maneuver which requires some training and practice. It is essentially breathing out against a closed glottis much in the way that you would strain to lift or pass stool. This is a rather dangerous maneuver to use under normal G loads because it produces considerably higher pressures within the venous system of the brain; however, under heavy G loads it is perfectly safe.

The second system of protection against G loads would be the G suit and valve, which improves G tolerance by less than one G, even if the best equipment is used. Breathing positive pressure oxygen or air under pressures up to approximately 60 mm mercury is very uncomfortable; how- ever, it does improve our ability to withstand G loads to a small degree.

The last method has not effectively been used in military aircraft to any extent to date but which is probably the system of the future. It is reclining the pilot in the cockpit.

Consider a subject sitting erect in the seat of an aircraft. The average distance from his heart to the base of the brain is 30 cm and it takes approximately 24 mm of mercury pressure to raise a column of blood 30 cm in a normal standing environment of one G. It would, of course, take the same pressure to raise a column of blood from the heart to the wrist if the wrist is held at eye level as it takes to de- liver blood to the brain.

In a study in which 250 centrifuge runs were made on human volunteers, it was demonstrated that systolic blood pressure in the radial artery held at eye level was reduced by 32 mm of mercury for every G added to the ± G2 force. Visual disturbances occurred when the systolic blood pres- sure at the base of the brain was reduced to 50 mm of mercury and complete loss of vision occurred when the pressure was reduced to 20 mm of mercury. Loss of consciousness occurred when the systolic pressure at brain level was reduced to zero. This would be equivalent to a five ± G pull, i.e., 5 x 32 = 160, where the blood pressure at the base of the brain would be reduced to zero if the systolic blood pressure in the subject was 160 at the heart level.

These forces were sustained in the study for a period of 15 seconds and the time to recovery unfortunately was not reported. It was noted, how- ever, that under a load of five G’s, the systolic blood pressure was reduced by five mm of mercury and the diastolic pressure was unaffected if the head is lowered to the level of the heart. To extrapolate from that information a positive G force in itself would not produce unconsciousness or blackout if the head is lowered to heart level.

Tables 2 and 3 show the time of exposure to ± G loads when pulling from horizontal flight to vertical (Table 2) and from vertical down to vertical up (Table 3) as in an inverted humpty at various speeds from 100 to 300 MPH and at varying G loads from two G’s to 12 G’s. Table 1 shows blood pressure at the base of the brain with varying seat tilt angles and 0 loads up to 12 G’s. Remember that L.O.C. occurs when the brain blood pressure has been zero for five seconds or more.

Permanent injury from excessive positive G loads have been studied in several ways. Very high G loads experienced in the ejection seat for a very short span of time has produced no permanent injury or after-effect. Positive G loads up to 15 have been used experimentally in human volunteers with no long term effects. Complete stoppage of circulation to the brain with an inflatable cuff around the neck for 20 seconds after unconsciousness occurred with no lasting effects.

In summary, positive G’s in the area that we are concerned with are not damaging long term, but the period of L.O.C. may be very disastrous if reached. Another forthcoming article will be devoted entirely to the effect of negative G forces.