upward relative to the pilot. This gives the pilot the sensation that he is
tilting over backward until he is inverted. To try to correct this illusory
attitude, he is likely to push the nose of the aircraft abruptly downward,
thus intensifying the illusion. This illusion is especially dangerous
because some aircraft cannot be safely recovered once a nose-down, negative
angle-of-attack attitude is entered.
(2) Elevator illusion: The otolith organs can monitor changes in the
length of the gravity vector as well as changes in its direction. The
elevator illusion results when the utricle and saccule respond to changes in
the length of the gravity vector. An increase in the length of the applied
gravity vector (as during an acceleration in an upward direction) results in
the compensatory downward tracking eye movement. This tracking eye movement
results as the body tries, through the vestibular ocular reflex, to maintain
visual fixation on the environment during upward acceleration. Since the
instrument panel, situated directly in front of the pilot, does not move
relative to him while his eyes are tracking downward, the pilot will see the
instrument panel (hence the nose of the aircraft) rise.
g. Oculogyral Illusion. The term oculogyral illusion has been used to
describe the apparent relative motion of an object in front of a person when
both the person and the object are subject to angular acceleration. For
example, in the dark, if a pilot fixes his eyes on a light which rotates as
the pilot is turning, the light will appear to move in the direction the
pilot is turning. When the pilot stops his turn, the light will move
rapidly in the direction opposite his rotation in a series of jerks and may
appear to be displaced as much as 60 degrees from its actual position.
SPATIAL DISORIENTATION AND PROPRIOCEPTIVE ILLUSIONS
Proprioceptive illusions as a pure entity probably do not occur. They
are intimately associated with the vestibular system and to a lesser degree
with the visual system. The proprioceptive information input to the brain
may also lead to a false perception of the true vertical (Figure 20).
During turns, banks, climbs and descending maneuvers, proprioceptive
information is fed into the central nervous system. A properly executed
turn vectors gravity and centrifugal force through the vertical axis of the
aircraft. In the absence of visual reference the only sensation experienced
by the body is an awareness of being pressed firmly into the seat. This
sensation is normally associated with a climb and may be falsely interpreted
by the pilot as such. Recovering from turns lightens the pressure on the
seat creating the illusion of descending and may cause the pilot to pull
back on the stick and reduce his airspeed.