http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/ACNumber/23234BF17C35918E862569D900744D21?OpenDocument TOLERANCE TO G'S. Because of the number of factors involved, it is difficult to predict how much acceleration a certain individual can withstand. Tolerance is related to the rate of onset of acceleration and to the duration of exposure. Individual tolerance depends on factors such as the height of the person, age, elasticity of the blood vessels, training, the responses of the heart and blood vessels, and on health. Because of the many variables involved, the centrifuge data in the following table are useful only as an estimate of the average civilian pilot's tolerance to +Gz (little is known of tolerance to -Gz). These data were collected from 1,000 Naval aviation pilots and aviation personnel and apply to rates of onset of about +1G per second - a rate that well may be encountered in civil aerobatic maneuvers.
Table 1. - Thresholds in Relation to +Gz Tolerance
Range Symptom Average
Threshold Standard
Deviation
2.2 to 7.IG Grayout 4.IG + 0.7G
2.7 to 7.8G Blackout 4.7G + 0.8G
3.0 to 8.4G Unconsciousness 5.4G + 0.9G
a. Note that at an onset rate of 1G per second, the G values in Table 1 could just as well be expressed in seconds. Thus, the "average" pilot accelerating at 1G per second could expect to have grayout at 4.1 seconds, blackout at 4.7 seconds, and unconsciousness at 5.4 seconds. More sensitive pilots, however, might have grayout, blackout, and unconsciousness at 2.2, 2.7, and 3.0 seconds. If the rate of onset were greater, then symptoms of grayout, blackout, and loss of consciousness would occur sooner.
b. A major outcome of these centrifuge studies was the demonstration of significant variations among subjects. The act of piloting an aircraft can raise the acceleration tolerance; thus, the results of some centrifuge studies during which the subject was passive may not apply directly to flight. This increased tolerance is not so great, however, that the aerobatic pilot should consider himself or herself immune to
G-incapacitation.
c. Tolerance of -Gz (foot-to-head) has not been studied intensively. About -lGz produces an unpleasant congestion of blood in the face and head; -2 to -3Gz causes severe congestion of the face, throbbing headache, progressively blurring, graying, or occasionally reddening of vision. After exposure to -Gz, there may be tiny hemorrhages in the skin and eyes and the eyelids may be swollen. Minus 5Gz for 5 seconds is probably the upper limit of tolerance; this level has seldom been achieved by volunteer subjects. Unlike military flying in which -Gz does not present much of a problem, aerobatic flying may demand that a pilot spend over half of his or her air time in inverted flight, pulling -Gz.
d. One little-known, but important, aspect of tolerance to G's is the effect of rapid changes from +Gz to -Gz, or vice versa. Because aerobatics induce such rapid changes, tolerance to changes could be highly significant. It is known, for example, that when one is subjected to -Gz, blood-pressure receptors in the head and chest respond to the increased pressure and cause a reflex slowing of the heart. A rapid change to +Gz (for example, when the pilot executes a half-roll during a maneuver) would suddenly drop blood pressure in these receptors and there would be a rapid speeding up of the heart to maintain pressure; but because the reflex system requires some time to sense the need, the heart could be delayed in responding to this demand and blood flow to the brain might suddenly decrease. Because of the sudden transition, and possible delay in response of the heart, a vertical 8 with an outside loop on top (-Gz) and an inside loop on the bottom (+Gz) may be one of the most threatening of aerobatic maneuvers. An inability of the cardiovascular system to react to the rapid change from -Gz to +Gz was probably the basis for the loss of consciousness reported in Case IV, Paragraph 6.d.
e. Obviously, some persons can withstand greater G's than others. Even experienced military pilots in a simulated aerial combat maneuver using anti-G procedures (M-1, L-1 maneuvers) could remain conscious only about 6 to 7 seconds. Everyone has a limit. Seasoned aerobatic pilots may represent a selected group because persons with less physiological resistance probably drop out of aerobatics before reaching the highest levels of performance. The aerobatic pilot will realize the potential hazards of G's and will seek to find the level of acceleration he or she can safely endure.
8. G'S IN AEROBATICS. The ordinary accelerometer in an aerobatic aircraft "pegs out" at a maximum and a minimum value and gives no indication of the duration of the G's, so that the accelerometer record has little significance as a record of physiological stress. National Aeronautics and Space Administration (NASA) data show that a range of +8Gz to -6Gz (the limit of the accelerometer) can occur during aerobatic competition, but such values lack the time element. In one study both G's and time were recorded during four competitive sequences or airshows. The pilot experienced negative G's about half of the total time spent in the performance.
a. Approximately 10 percent of the time he was pulling +2Gz or more with spikes to +5.4Gz, and about 10 percent of the time he pulled -3Gz or more with spikes to -5.2Gz. In an "obligatory" outside 360-degree turn the pilot experienced -2Gz or more for 32 seconds.
b. The horizontal rolling 360-degree turn produced rapid and repeated G oscillations. in 28 seconds the pilot experienced 6 major G excursions: -3.4, +2.3, -3.5, +2.0, -4.0, and +2.3. The first transition from negative to positive G's was 5.7G's in 2 seconds, or approximately 2.9c's/second.
c. The outside-inside vertical 8 was probably the most physiologically demanding; the pilot experienced a maximum of -5.2G's in the upper outside loop and 5 seconds later pulled +5.0 G's in the lower inside loop. This amounted to 10.2 G's in 5 seconds, or over 2G's per second for 5 seconds. Even G-tolerant pilots may have changes in vision or possibly loss of consciousness in this maneuver (Case IV, paragraph 6.d.)
d. During several maneuvers the rate of onset equaled or exceeded IG per second, and in some maneuvers, the rate of onset and duration of the G's were at levels that have been found to cause unconsciousness in some
subjects in centrifuge studies.
