4/20/23 

AIM 

(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. 

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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 

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