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ground features, as when landing over water,
darkened areas, and terrain made featureless by snow,
can create the illusion that the aircraft is at a higher
altitude than it actually is. The pilot who does not
recognize this illusion will fly a lower approach.
5.3.5Atmospheric Illusions.Rain on the windscreen
can create the illusion of greater height, and
atmospheric haze can create the illusion of being at
greater distance from the runway. The pilot who does
not recognize these illusions will fly a lower
approach. Penetration of fog can create the illusion of
pitching up. The pilot who does not recognize this
illusion will steepen the approach, often quite
abruptly.
5.3.6Ground Lighting Illusions.Lights along a
straight path, such as a road, and even lights on
moving trains can be mistaken for runway and
approach lights. Bright runway and approach lighting
systems, especially where few lights illuminate the
surrounding terrain, may create the illusion of less
distance to the runway. The pilot who does not
recognize this illusion will fly a higher approach.
Conversely, the pilot overflying terrain which has few
lights to provide height cues may make lower than
normal approach.
6. Vision in Flight
6.1Introduction.Of the body senses, vision is the
most important for safe flight. Major factors that
determine how effectively vision can be used are the
level of illumination and the technique of scanning
the sky for other aircraft.
6.2Vision Under Dim and Bright Illumination
6.2.1Under conditions of dim illumination, small
print and colors on aeronautical charts and aircraft
instruments become unreadable unless adequate
cockpit lighting is available. Moreover, another
aircraft must be much closer to be seen unless its
navigation lights are on.
6.2.2In darkness, vision becomes more sensitive to
light, a process called dark adaptation. Although
exposure to total darkness for at least 30 minutes is
required for complete dark adaptation, the pilot can
achieve a moderate degree of dark adaptation within
20 minutes under dim red cockpit lighting. Since red
light severely distorts colors, especially on aeronautical
charts, and can cause serious difficulty in focusing
the eyes on objects inside the aircraft, its use is
advisable only where optimum outside night vision
capability is necessary. Even so, white cockpit
lighting must be available when needed for map and
instrument reading, especially under IFR conditions.
Dark adaptation is impaired by exposure to cabin
pressure altitude above 5,000 feet, carbon monoxide
inhaled in smoking and from exhaust fumes,
deficiency of Vitamin A in the diet, and by prolonged
exposure to bright sunlight. Since any degree of dark
adaptation is lost within a few seconds of viewing a
AIP ENR 1.15−7
United States of America 15 MAR 07
Federal Aviation Administration Nineteenth Edition
bright light, the pilot should close one eye when using
a light to preserve some degree of night vision.
6.2.3Excessive illumination, especially from light
reflected off the canopy, surfaces inside the aircraft,
clouds, water, snow, and desert terrain, can produce
glare, with uncomfortable squinting, watering of the
eyes, and even temporary blindness. Sunglasses for
protection from glare should absorb at least
85percent of visible light (15 percent transmittance)
and all colors equally (neutral transmittance), with
negligible image distortion from refractive and
prismatic errors.
6.3Scanning for Other Aircraft
6.3.1Scanning the sky for other aircraft is a key
factor in collision avoidance. It should be used
continuously by the pilot and copilot (or right seat
passenger) to cover all areas of the sky visible from
the cockpit. Although pilots must meet specific visual
acuity requirements, the ability to read an eye chart
does not ensure that one will be able to efficiently spot
other aircraft. Pilots must develop an effective
scanning technique which maximizes one’s visual
capabilities. The probability of spotting a potential
collision threat obviously increases with the time
spent looking outside the cockpit. Thus, one must use
timesharing techniques to efficiently scan the
surrounding airspace while monitoring instruments
as well.
6.3.2While the eyes can observe an approximate
200 degree arc of the horizon at one glance, only a
very small center area called the fovea, in the rear of
the eye, has the ability to send clear, sharply focused
messages to the brain. All other visual information
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