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SYMPTOMS OF Gz EFFECTS
a. Positive Gz Effects. The +Gz induced effects may be described as follows:
(1) Grayout. There is graying of vision caused by diminished flow of blood to the eyes. Although there is no associated physical impairment, this condition should serve as a warning of a significant impairment of blood flow to the head.
(2) Blackout. Vision is completely lost. This condition results when the oxygen supply to the light sensitive retinal cells is severely reduced. Contrary to other common usages of the term, consciousness is maintained. In blackout, some mental activity and muscle function remains, thus the occurrence of blackout warns of seriously reduced blood flow to the head and of a high risk of loss of consciousness. Note: In some centrifuge studies, 50 percent of the pilots had simultaneous blackout and loss of consciousness. Therefore, a pilot cannot rely on blackout to precede loss of consciousness.
(3) Loss of Consciousness. When the blood flow through the brain is reduced to a certain level, the pilot will lose consciousness. He or she may have jerking, convulsive movements; these have been seen in many subjects of centrifuge studies and in some pilots during actual flight. The pilot will slump in his or her seat. Possibly, the pilot will fall against the controls, causing the aircraft to enter flight configurations from which it cannot recover even if consciousness is regained. In centrifuge studies, many pilots lost (and regained) consciousness without realizing they had done so.
(4) How long does induced unconsciousness last? In a series of studies of pilots in centrifuges, the pilots were unconscious for an average of 15 seconds. Following this, there was an additional 5 to 15 second interval of disorientation. Thus, if there is loss of consciousness due to +Gz forces, there will be a 20 to 30 second (or longer) period during which the pilot is not in control of his or her aircraft.
b. Negative Gz Effects. Negative Gz is encountered when acceleration is in a foot to head direction, such as might be obtained during inverted flight, or during an outside loop or pushover maneuver (see
Figure 2). Blood is then pushed toward the head, and the amount of blood returning from the head is diminished, so the blood tends to stagnate, particularly in the head. Under mild conditions of -Gz forces, the pilot will feel congestion, as when standing on his or her head. Engorgement of blood vessels causes a reddening or flushing of the facial skin. Blood vessels in the eyes will become dilated. Some persons may experience a headache. A condition termed "redout" may occur. This may be due in part to congestion but may also occur when the lower eyelid, reacting to -Gz, rises to cover the pupil, so that one sees light through the eyelid.
(1) Little is known about the effects of high -Gz on humans because -Gz accelerations have caused considerable discomfort in those studied. Aerobatic pilots have reported small hemorrhages in the eyes and skin. Harold Krier, an accomplished aerobatic pilot, wrote that if he got a pain in the top of his head while pulling -G's, he eased off on the controls to reduce the G loading.
(2) The blood vessels in the brain tolerate mild -Gz stresses well, but the increased blood pressure in the chest and neck causes a slowing of the heart in virtually all subjects. In a few individuals, there is such a marked slowing of the heart that there are intervals of several seconds between beats. In some people, the heart may beat irregularly after -Gz exposure. The slowing of the heart and irregularities of beats can add to the stagnation of blood in the brain. Thus, it appears that the greatest threat from -Gz is the loss of consciousness from the slowing of the heart, irregularities of the heartbeats, and stagnation of blood in the head.
