(c) Runway and terrain slopes illusion. An upsloping runway, upsloping terrain, or both, 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. A downsloping runway, downsloping approach terrain, or both, can have the opposite
effect.
(d) Featureless terrain illusion. An absence of 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.
(e) Atmospheric illusions. Rain on the windscreen can create the illusion of greater height, and
atmospheric haze the illusion of being at a 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.
(f) Ground 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 a lower than normal approach.
8-1-6. Vision in Flight
a. Introduction. 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.
b. Vision Under Dim and Bright Illumination.
1. Under 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.
2. In 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, a 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 altitudes 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 bright light, a pilot should
close one eye when using a light to preserve some degree of night vision.
3. Excessive 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 85 percent of visible light (15
percent transmittance) and all colors equally (neutral transmittance), with negligible image distortion from
refractive and prismatic errors.
c. Scanning for Other Aircraft.
1. Scanning 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.

Fitness for Flight

8-1-7