SCIENTIFIC BULLETINS 
of the 
OFFICE OF THE AIR SURGEON 
HEADQUARTERS ARMY AIR FORCES 
Washington, D. C. 
SUMMARY REPORT NO. 5. 
ACCELERATION AND BLACKOUT 
Summary reports reviewing the more significant findings 
contained in reports from service and civilian laboratories in 
the ynited States and Allied Countries are issued from time to 
time. It is the purpose of these summaries to make available to 
flight surgeons and research personnel a brief survey of the 
information relating to a certain subject contained in the numer- 
ous and often voluminous reports in the files of the Air Surgeon. 
There is no assurance that the files are complete but it may be 
assumed that the summaries will fairly indicate the trend of work 
and opinions in the field. The information here reported is a re- 
view of the available reports and does not necessarily reflect the 
opinion of the Air Surgeon or his staff. 
0629, AF CONFIDENTIAL 
ACCELERATION AND BLACKOUT 
In operational living, the human body and the plane are 
frequently subjected to forces many times greater than that due to the 
gravitational pull of the earth (1). Since the forces are proportional 
to and in the same direction as the accelerations, it has become a 
custom to use the observed acceleration as a measure of the magnitude 
and direction of the forces acting on the parts of the body. Accord- 
ingly, the degree of change in these forces is expressed as a multiple 
of one G, the acceleration due to the force of gravity. In operational 
flying, pilots may be subjected for a few seconds to more than 7 G (1). 
One survey indicated the time of maximum stress during various maneuvers 
varied from 3 to 10 seconds with an average time of about-5 seconds (45). 
Experience in flight and experiments in the laboratory both show that 
such forces acting for even these short periods may cause dangerous 
physiological effects in some individuals unless special precautions are 
taken. The danger arises primarily from the possibility of a temporary 
loss of vision or consciousness at critical times during combat. 
Dimming of vision (Grey-out) is an early sign of dangerously 
high acceleration, (2, 45). If the acceleration is not increased 
further or not too prolonged, visual dimming may be the only effect. 
Otherwise this period of greyout is followed by blackout, or loss of 
vision (2, 45). At this time the subject 'is conscious but shortly may 
become unconscious (2). Under constant’conditions, the greyout. usually 
occurs at 1 G less than the blackout threshold, and unconsciousness is 
produced by 1 G more than that producing blackout (45). When the 
duration and magnitude of acceleration is such that this, sequence of 
events progresses only to blackout, there is a rapid uneventful re- 
covery in a few seconds after cessation of acceleration (2). Recovery 
from unconsciousness, while a matter of .seconds, may be accompanied by 
convulsions and may be followed by a period of disorientation (2). 
Thus, greyout or blackout should be regarded as important warnings to 
the pilot that he is close to his safe G-tolerance. 
The G-tolerance•of an individual depends principally upon 
the magnitude of the acceleration and its direction with respect to 
the long axis of the body, but, in addition, the time required to reach 
peak acceleration as well as the duration of this maximum stress are 
important (45). 
Since a factor involved in determining the G-tolerance of 
a man is the direction of the forces with respect to the body axis, 
posture is important in minimizing the effects of high acceleration. 
0629, AF 
CONFIDENTIAL CONFIDENTIAL 
The physiological effects of acceleration are most marked when the 
forces act in the direction of the long axis of the body. Thus, about 
5 to 6 G directed from the head to seat can cause unconsciousness in 
most men if continued a few seconds,(1, 45). When the force is 
directed from seat to head in man, the field of vision becomes red, 
probably because of vascular congestion in the head regions. However, 
this "redding out" is not as common in operational flying as is black- 
out. On the other hand, much higher values directed transversely to 
the long axis have no important effect. In - centrifuge experiments on 
animals, the forces most effective in disturbing visual function and 
brain waves were also those directed from the heart to the tail (5). 
Because of this importance of the direction of the force, the G-tolerance 
can be increased by assuming certain postures to be described later. 
(See pages 6-7.) 
It is customary to employ blackout as the endpoint in estima- 
ting G~tolerance. In such tests the force is directed from head to 
seat. When the tests are done in a centrifuge, the time to reach maxi- 
mum acceleration can be held constant. To further standardize the test, 
the magnitude of acceleration required to produce blackout in 5 seconds 
has been the criterion employed in Canada, (45). 
Under these conditions the average threshold determined on 
88 cadets was 4 to 6 G for most of the men. The total range of thresh- 
olds was 3.5 to 9 G, the individual thresholds being determined from 
at least 10 runs. These data indicate that the G-tolerance of a man 
may vary j 1 G from day to day, but the variation on any one day is much 
less. The results are comparable to estimates based on flying experience 
where the test conditions are necessarily less well defined. Pilots in 
action, for instance, have reported tolerance of 3.5 G on one day to 
5.5 G on the next (3). When special care was taken to keep the time of 
maximum acceleration constant, the blackout threshold on one individual 
in flight varied only,0,5 G (1). 
Experience has shown that, within limits, the higher the ac- 
celeration the shorter the time to blackout. In tests or in flight, 
therefore, the.shorter the time of exposure the higher the acceleration 
that can be tolerated without effects such as blackout,(45). For 
example, the average cadet can tolerate about 6-G for two seconds and 
only about 4-G for ten seconds. In cats a similar relation between 
force and time holds when the reversible extinction of brain waves is 
taken as. endpoint. Thus at 3-G or below, there is no extinction and 
above 8-G the time to extinction reaches minimum values of 8 to 12 
seconds, depending upon the individual animal. This minimum time does 
not change with further increase in Hie force (5, 7). At 3 to 4 G 
the time to extinction of brain waves varies among individual animals 
C-629, AF 
CONFIDENTIAL 20NFXDKNTXAL 
from 80 to 120 seconds (5). In general, as the force was increased 
the time to abolish the electrical activity of the brain was decreased. 
Though extinction of brain waves may be the more severe test when com- 
pared with loss of consciousness, it is reasonable to suppose that the 
same factors are involved in causing these two degrees of cortical 
depression. Thus the findings in animal experiments, using this end- 
point, are of value in interpreting observations on man. 
The most complete information on the Mforce-time” relations 
has been obtained on animals tested in a centrifuge, using killing 
rate as an endpoint. Over a range of Z to 8 minutes and 6 to 25-G, the 
killing effect on rats is measured by a constant product of force and 
time, 50 G minutes (6). Since the endpoint used in these experiments 
was death of the animal, these results may have little bearing upon 
the study of the brief reversible effects of acceleration that are 
actually encountered in man. 
Greyout and blackout are probably caused by loss of function 
in the retinal sensory cells. In the first place, a similar amaurosis 
is produced by a pressure of 300 millimeters of Hg, on the eyeball 
(7, 8), which visibly obstructs the retinal blood flow. It is thought 
that the resulting tissue anoxia raises the threshold for initiation of 
nerve impulses in the light sensitive retinal cells. It is also possible 
that this increased pressure has a direct effect upon the retinal cells. 
However, this seems unlikely because loss of response to a strong light 
is produced by that value of pressure that just stops retinal flow (7). 
A similar reduction of retinal blood flow is supposed to occur when 
the forces due to acceleration during flying impede the action of the 
heart in pumping blood to the head regions. 
Since the degree of retinal ischemia and level of retinal 
anoxia can be graded, it is not surprising that there are graded 
changes in visual threshold up to complete blackout. Because of such 
gradations of threshold, the intensity of light is an important factor 
in testing for blackout or greyout (1). Thus, blackout in dim light 
may be only a greyout in bright light. As a consequence, the degree 
of acceleration tolerated by pilots before blackout depends upon the ex- 
isting light intensity and is much less at dusk than in the day time. 
How much less is not known since the effects of acceleration on the 
sensitivity of the dark adapted eye have not been reported. 
One explanation for the unconsciousness that occurs on ex- 
posure to high values of G for a few seconds is that the circulation 
to the brain is impeded or stopped just as is that to the retina. It 
0629, AF 
CONFIDENTIAL CONFIDENTIAL 
has been observed in cats and monkeys that under these conditions there 
is stasis in the pial arteries but no visible drainage of blood from 
the brain (7). In these animals the carotid blood pressure may fall 
to very low values in one second at 6 to 7 G. 
It is thus possible that greyout and blackout precede uncon- 
sciousness in man because the intra-ocular pressure stops blood flow 
to the retina at a time when the systemic pressure still maintains 
some flow to the brain. Thus greyout or blackout should be regarded 
by the pilot as a useful warning of impeding unconsciousness if the 
existing acceleration is either continued or increased in magnitude. 
It is obvious' that G-tolerance is a quantitative property and the mag- 
nitude of G acting parallel to his body axis should be' known by. the 
pilot at all times during training. For this purpose accelerometers 
should be in all training planes capable of achieving sufficient ac- 
celeration to produce deleterious physiological effects on the pilot. 
A recent report from Canada (46) shows that in cats and 
monkeys subjected abruptly (in 1 to 2 seconds) to values of 7 to 10 G 
which are well above their threshold, the spontaneous cortical activity 
disappears first, then the cortical response to light disappears and 
finally the retinal cells and those of the optic pathway fail to respond. 
In such an experiment, the time course of events is similar to that' 
following complete arrest of cerebral circulation by clamping the aorta. 
Under these conditions the delay in onset of failure of these several 
parts must be due to other than circulatory properties and is probably 
related to the relative oxygen requirements needed to maintain the 
various degrees of cellular action measured. In contrast, when the 
value of G is near threshold then the peripheral mechanisms fail first. 
As previously mentioned, this is probably due to the intraocular pressure 
stopping the blood flow to the retina,at a systemic pressure that is 
still able to supply some blood to the brain. Thus, for threshold 
values of G, the peripheral vasomotor factors are important in deter- 
mining time to blackout but when the acceleration is sufficient, essentially 
to stop blood flow to the head, then the susceptibility of the cells to 
anoxia seems to determine the time to'loss of vision or consciousness. 
The possibility that direct mechanical forces on the brain 
cells may play a part cannot be neglected. The intra-cranial pressure 
drops well below atmospheric pressure during positive G and rises 
above it during recovery (46). However, the effect of these forces 
is to prevent marked distortion of the brain and is thus probably 
beneficial. 
C-629, AF 
confidential CONFIDENTIAL 
The previously mentioned relation between f orce and time in 
producing a given effect in cats is presumably an expression of the 
relation between the degree of circulatory arrest and the rate at which 
the subsequent cellular anoxia develops. On this basis, one would expect 
that the minimum time for extinction of brain waves would occur when the 
acceleration was sufficient to arrest blood flow to the brain. In sup- 
port of this idea, is the observation in animals that such a minimum 
time, 7 to 10 seconds, does exist (5, 7) for extinction of brain 'waves 
and is approximately equal to the extinction time following clamping 
of the aorta. 
Perhaps a similar interpretation of the minimum times to 
produce unconsciousness or blackout by, acceleration applies to man. 
In that event, the time to blackout and unconsciousness produced by oc- 
cluding the carotid and vertebral arteries in man should, be approxi- 
mately equal to the minimum time for high acceleration to produce similar 
effects. The former measurements have been made on normal subjects 
(10). The time required to produce unconsciousness in the average sub- 
ject was about 6 seconds. According to this hypothesis and these data, the 
minimum time to blackout during very high acceleration would be about 6 
seconds in the average young man and would be produced by forces suffi- 
cient to arrest cerebral arterial flow. These predictions apply to the 
average healthy young man. One of the most interesting results revealed 
by this study (10).is the wide difference in individual susceptibility to 
the effects of cerebral ischemia. Thus the minimum time to unconscious- 
ness was about 3 seconds and the maximum 12 to 14 seconds. Individual 
differences in human susceptibility to effects of acceleration have 
also been noted. This, however, is undoubtedly due to differences in 
vasomotor properties as well as to differences in resistance to the 
effects of cerebral anoxia. Which of these two factors is predomin- 
antly responsible for an individual’s G-tolerance under operational 
conditions remains to be determined. 
The fact that individuals differ in tolerance has lead to 
the suggestion that personnel be tested and classified according to 
their ability to withstand forces of acceleration (3, 11). To. this end, 
the human centrifuge has been developed for research, indoctrination, 
and classification purposes. 
In addition to the individual differences in the acceleration 
required to cause blackoutf there are daily variations in the toler- 
ance of the individuali.. It has been considered, however, that pilots 
differ sufficiently so that personnel with high G-tolerance can be 
selected by tests in a centrifuge (11). The data in report 45 on 88 
cadets, show that classification is possible. About 2.5 percent of 
tested cadets can withstand 8 G for 5 seconds and 4.5 percent have a 
G threshold below 4 G. Host of the tested cadets, (21,5$) had a 
threshold at 5 G for 5 seconds. 
C-629, AF 
CONFIDENTIAL CONFIDENT I A L 
In early tests the endpoint for G-tolerance was usually black- 
out though many subjects have become unconscious for a few seconds. (2). 
Recently, it has been shown (12, 13) that changes in peripheral circula- 
tion to the head can be employed as an index of the effect of acceler- 
ation. Thus, the blood content of the ear, measured by a photocell 
method, is reduced when a subject is accelerated in a centrifuge or in 
a plane (12, 13). The degree of change is related to the force acting 
from head to seat and the effect appears at lower values of G than are 
required to produce blackout. This procedure makes such tests much more 
acceptable to the subject, but it has not been extensively employed ex- 
cept for research purposes. In tilt-table tests, this objective sign 
appeared before greyout (44). 
: -The most apparent disadvantage of the temporary blackout that 
may occur at. high accelerations is that the pilot is handicapped for 
a few seconds during each such combat maneuver. Furthermore, there is 
evidence that repeated blackout may lower a man’s G tolerance (4-5), thus 
making him more and more unfit for effective combat flying. There have . 
also been reports of accumulative fatigue resulting from-such stresses 
(l) and it is possible that this may contribute to the snydrome of 
chronic pilot fatigue. It is well to remember that acceleration, less 
than' that required to blackout a man, may nevertheless cause temporary 
decrease of the cerebral oxygen supply. Such minor degrees of anoxia 
repeated a sufficient number of times may add up to an undesirable total 
duration of mild cerebral anoxia. This problem is closely related to 
the similar problem of accumulative effects of mild anoxia experienced 
for long times, at, for example, 12,000 feet with normal blood flow. 
The effects of moderate unsaturation of the blood undoubtedly aggravate 
the effects of temporary reduction in blood flow to the brain produced 
by acceleration. These relations are urgently in need of Quantitative 
determination if we are effectively to prescribe the limits of reversible 
cellular strain resulting from unsaturation of the blood and decreased 
flow during acceleration at altitude. Tests made in a plane have demon- 
strated that anoxia does lower a subject’s blackout threshold (4-0^42). 
At 10,000 feet the blackout threshold, 6.9 G when breathing oxygen, was 
lowe.red 0.5 G when breathing air. At 15,000 feet, breathing air, the 
blackout threshold dropped to 5.9 G in about one-half hour. This 
observer also reports no change in greying threshold up to 32,000 feet 
when breathing oxygen. 
METHODS TO INCREASE G-TOLERANCE 
It has been found that a crouching position increases G- 
tolerance by 1 to 2 G (14, 15? 16). This benefit can easily be obtained 
by raising the rudder bar so that the usual heart to ankle vertical 
distance is reduced by 6 inches or more (16). The protection afforded 
by the crouch position is probably obtained because of the prevention 
C 0 N F I D E N T I A L 
C-62 9, AF CONFIDENTIAL 
of venous congestion in the abdominal veins which improves venous re- 
turn of blood to the heart. Another factor is that the component of 
force acting from head to heart is reduced. 
For this latter reason, the reclining position also affords 
some protection against effects of high acceleration. Reclining back- 
ward at 45° was sufficient to protect one subject against blackout at 
5 G for 10 to 20 seconds. In the upright position, these same stres- 
ses uniformly produced blackout in this subject (17). It was estimat- 
ed that the average fighter pilot could withstand action of 6 to 6.5 G 
in this position. 
The most practical and effective method yet devised for pro- 
tection against the forces developed during high acceleration seems to 
be the pressure suit. The principle is sound because it prevents the 
undesirable forces from acting upon the body by automatically produc- 
ing opposite and equal forces during the acceleration. In effect the 
body is suspended in water so that the external hydrostatic force de- 
veloped during acceleration everywhere compensates the action of the 
force within the body. In practice the water is contained in suitably 
placed pockets in a suit that is readily fitted to pilots. A rather 
surprising degree of protection is obtained from the 4 well-distributed 
pints of water (18, 19, 20, 21, 22, 23, 24, 25, 26)* The suit has been 
considered favorably by pilots and greatly increases G-tolerance when 
tested in flight (21, 22, 25, 26). One subject whose blackout thresh- 
old in flight was 4,5 G for 10 seconds could withstand 6 G for 12 
seconds when wearing the suit (26). One pilot reported MIt is the 
opinion of the writer that the Franks suit in its present form (July, 
1941) is a practical solution for the blacking out problem. It has 
been conclusively demonstrated that the wearing of the suit delays, 
if not eliminates blackout tendencies to beyond the practical limits 
of acceleration for which fighter aircraft are designed" (18). Such 
a sweeping endorsement of a "gadget" is seldom forthcoming after field 
testing. 
In the suit developed by Dr. Cotton of Sydney University, 
the forces produced in the body by acceleration are opposed by an air 
pressure gradient distributed over the body by means of proper valves 
(27, 28, 29, 30,. 31, 32). The device must be connected to an air com- 
pressor and thus ties the wearer to-the plane. The advantage of the 
method over the water-suit is that the air pressure need only be turned 
on when the acceleration is anticipated. Wearing this suit, Dr. Cotton 
was able to withstand the effects of 11 G for 13 seconds in a centri- 
fuge (27). Recent tests of pneumatic suits during mock combat have 
shown them to be practical and effective in increasing G-tolerance. 
C-629,AF 
CONFIDENTIAL C 0 N F ID E N T I k L - 
Tests have been made. to. determine whether or not abdominal 
pressure belts would significantly increase G-tolerance by prevent- 
ing pooling of blood in the abdominal veins (33). A prolonged constant 
pressure was found to, lower blackout threshold while inflation of the 
belt just preceding the acceleration gave some protection (33). The 
belt is considered, a dangerous device because even if adjusted right, 
long duration of G can cause it to block venous return. The method 
was considered cumbersome and not fool-proof. Tests in the air of 
Spencer acceleration belts and leggings showed an increase of 1 G in 
the blackout threshold of the wearer. The devices were inflated 10 to 
30 seconds before the acceleration (34). Other tests of similar appar- 
atus' led to the conclusion that 1.5 pounds pressure in leggings and 
belt protected against persistent blackout as long as the acceleration 
was less than 5.5. G (35). In these tests an initial blackout at 5 G 
always occurred for 1 to 4 seconds regardless of the pressures employ- 
ed, but the vision then cleared for 12 to 34 seconds. Early tests in- 
dicated that abdominal belts alone are of no use (36). 
A thorough study of the effect of abdominal belts on the flow 
and distribution of blood has been made (37). The calf volume in- 
creased when the pressure in the abdominal belt was 100 ram Hg. Venous 
return of blood to the heart was impeded to a degree that was greater 
when the subject was vertical then when supine. The total blood flow 
remained essentially constant but more blood circulated below the 
heart than above it when the subject was erect and wearing the belt. 
The relation of these basic observations to the use of pressure belts 
and leggings is discussed. 
In another series of tests (43), benzedrine, glucose, adren- 
alin, and eschatin were found to have no effect on the blackout thresh- 
old. The possible action of desoxycorticosterone has been discussed 
(41),. 
There is one aspect of the acceleration problem that has re- 
ceived too little attention: The investigation of the effects of de- 
grees of .cerebral anemia insufficient to cause blackout or other im- 
mediate symptoms. Such moderate cerebral anemia must be present even 
at Z or 3 G. Though not subjectively annoying, 'it may be a cause of 
pilot fatigue. In principle, the gradbd action on man of all tolerable 
ranges of G should be examined experimentally. If moderate values of 
acceleration, insufficient to produce blackout or unconsciousness, have 
important physiological effects on pilots then protective devices may 
be even more widely useful than is now apparent. The partial-protection 
devices such as inflatable belts and leggings may here find their cor- 
rect application. Such applications of G-suits are to be regarded as 
protective in nature, but a G-suit or other means devised to enable 
CONFIDENTIAL 
C-629.AF C C N F IDENTIAL 
men to increase their top limits of G-tolerance is to be regarded as 
a method to gain tactical advantage at the expense of some physiological 
strain associated with marked cerebral anemia. 
CONFIDENTIAL 
C-6£9,AF CONFIDE K T I A L 
BIBLIOGRAPHY 
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C-629,AF 
CONFI D E N T I d L CONFIDE'NTI » L 
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CONFIDENT!;, L CONFIDENTIAL 
4-4-. "Objective determination of Circulatory Changes Preceding, Dur- 
ing, and Following Greying, Blackout and Syncope on the Tilt-* 
, Table", G, C, Ham and J. G. Hcrtsnstine, C, A, M. #54> May 28, 
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pendix R, October 22, 1942. 
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