HUMAN LIMITS IN FLIGHT—MATTHEWS 275 
heart but as its virtual weight increases the heart has difficulty in 
maintaining the blood supply to the head. The brain and the eyes can 
only function for a few seconds without their normal blood supply 
and loss of vision in blacking-out is due to failure of the circulation in 
the retina of the eye. If the acceleration is still greater, the whole 
blood supply of the brain fails and the pilot becomes unconscious. 
Blacking-out is a warning that the blood pressure in the cerebral 
arteries is getting low. If the control column is eased forward, the 
airplane straightens out, the centrifugal force ceases and within a few 
ACCELERATION 
= 
eT oom Et La 
HUMAN LIMIT OF TOLERANCE OF ACCELERATION 
ACTING VERTICALLY ON A MAN SEATED 
Figure 1.—Approximate relationship between duration of vary- 
ing degrees of acceleration and occurrence of loss of vision 
(lower curve) and loss of consciousness (upper curve). 
(Data from various sources.) 
seconds the circulation returns to normal. While this happens in the 
head the deficit of blood tends to gravitate to the legs and the phenom- 
enon can be regarded as the head losing blood to the feet. 
This draining of the blood from the head takes time. The graphs 
in figure 1 illustrate the limits of tolerance of acceleration—the greater 
the acceleration the less the time that the pilot can retain his sight. 
Many measures have been taken to reduce the effect of centrifugal 
force on the pilot; much may be done by posture and seating; if the 
pilot’s attitude is crouched with his legs raised, the distance through 
