3.
SPATIAL DISORIENTATION AND VISUAL ILLUSIONS
Although the visual system is not the most reliable of the senses,
there are illusions which can result from misinterpreting what is seen.
Even with references outside the cockpit and the instrumentation inside,
pilots must be careful to interpret visual information correctly and
understand that what is being seen is not always what is actually happening.
a. Relative Motion. This illusion is often encountered in formation
flights where a pilot sees the motion of another aircraft and interprets it
as motion of his own. Another area where this can occur is hovering a
helicopter over tall grass and interpreting the wave action of the grass
(due to the rotor wash) as aircraft movement.
b. Confusion of Ground Lights. Many pilots have put their aircraft
into very unusual attitudes to keep ground lights above them, having
mistaken them for stars. Less frequent, but just as dangerous, are the
illusions caused by certain patterns of ground lights imagined to be things
they are not. Some pilots have interpreted lights along a seashore to be
the horizon and come dangerously close to the ocean while thinking they were
flying straight and level. Aviators have also confused certain geometric
patterns of ground lights (such as a moving train) with runway and approach
lights, again causing a dangerous situation.
c. False Vertical and Horizontal Cues. Cloud formations may be
confused with the horizon or ground (Figure 17). Momentary confusion may
result when an individual looks for outside reference after prolonged
attention inside the cockpit.
d. Structural Illusions. Structural illusions are caused by heat
waves, rain, snow, sleet or other obscurants to vision. For example, a
straight line may appear curved when seen through the heat wave from a
desert, or a wing tip light may appear as a double light or in a different
location when viewed through a rain shower.
e. Autokinetic Illusions. Autokinetic illusion (autokinesis) is the
illusory phenomenon of movement exhibited by a static light when stared at
for a long enough period of time. The cause is not known but appears to be
due to the uncontrolled movement of the eye in attempting to find another
reference point in the field of vision. Autokinesis is not exclusively
limited to periods of darkness. It can occur whenever a small, bright,
still object is stared at against a dull, dark, still object in a light,
structureless environment.
f. Flicker Vertigo. A great deal of time and research have been
devoted to the study of flicker vertigo. Light flickering at a rate of
between 4 and 20 cycles per second can produce unpleasant and dangerous
reactions. Fatigue, frustration and boredom tend to increase the severity
of these reactions and, although not a serious problem in Army aviation,
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