RFT 251: Visual Illusions
Visual illusions are familiar to most of us. As children, we
learned that railroad tracks—contrary to what our eyes
showed us—don’t come to a point at the horizon. Even
under conditions of good visibility, you can experience
visual illusions including:
Aerial Perspective Illusions may make you change
(increase or decrease) the slope of your final approach. They
are caused by runways with different widths, upsloping or
downsloping runways, and upsloping or downsloping final
approach terrain.
Pilots learn to recognize a normal final approach by
developing and recalling a mental image of the expected
relationship between the length and the width of an average
runway.
A final approach over a flat terrain with an upsloping
runway may produce the visual illusion of a high-altitude
final approach. If you believe this illusion, you may respond
by pitching the aircraft nose down to decrease the altitude,
which, if performed too close to the ground, may result in
an accident.
A final approach over a flat terrain with a downsloping
runway may produce the visual illusion of a low-altitude
final approach. If you believe this illusion, you may respond
by pitching the aircraft nose up to increase the altitude,
which may result in a low-altitude stall or missed approach.
A final approach over an upsloping terrain with a flat runway
may produce the visual illusion that the aircraft is higher than
it actually is. If you believe this illusion, you may respond by
pitching the aircraft nose-down to decrease the altitude,
resulting in a lower approach. This may result in landing short
or flaring short of the runway and risking a low-altitude stall.
Pitching the aircraft nose-down will result in a low, dragged-in
approach. If power settings are not adjusted, you may find
yourself short of the runway, needing to add power to extend
your flare. If you do not compensate with power, you will land
short or stall short of the runway.
A final approach over a downsloping terrain with a flat
runway may produce the visual illusion that the aircraft is
lower than it actually is. If you believe this illusion, you
may respond by pitching the aircraft’s nose up to gain
altitude. If this happens, you will land further down the
runway than you intended.
A final approach to an unusually narrow runway or an
unusually long runway may produce the visual illusion of
being too high. If you believe this illusion, you may pitch
the aircraft’s nose down to lose altitude. If this happens too
close to the ground, you may land short of the runway and
cause an accident.
A final approach to an unusually wide runway may
produce the visual illusion of being lower than you actually
are. If you believe this illusion, you may respond by
pitching the aircraft’s nose up to gain altitude, which may
result in a low-altitude stall or missed approach.
A Black-Hole Approach Illusion can happen during a final
approach at night (no stars or moonlight) over water or
unlighted terrain to a lighted runway beyond which the
horizon is not visible. When peripheral visual cues are not available to help you orient yourself relative to the earth, you may have the
illusion of being upright and may perceive the runway to
be tilted left and upsloping. However, with the horizon
visible you can easily orient yourself correctly
using your central vision.
A particularly hazardous black-hole illusion involves
approaching a runway under conditions with no lights
before the runway and with city lights or rising terrain
beyond the runway. Those conditions may produce the
visual illusion of a high-altitude final approach. If you
believe this illusion you may respond by lowering your
approach slope.
The Autokinetic Illusion gives you the impression that a
stationary object is moving in front of the airplane’s path; it
is caused by staring at a fixed single point of light (ground
light or a star) in a totally dark and featureless background.
This illusion can cause a misperception that such a light is
on a collision course with your aircraft .
False Visual Reference Illusions may cause you to orient
your aircraft in relation to a false horizon; these illusions are
caused by flying over a banked cloud, night flying over
featureless terrain with ground lights that are
indistinguishable from a dark sky with stars, or night flying
over a featureless terrain with a clearly defined pattern of
ground lights and a dark, starless sky.
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