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6/17/21 

AIM 

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 aeronauti-
cal 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. 

2. 

While 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 
that is not processed directly through the fovea will be 
of less detail. An aircraft at a distance of 7 miles 
which appears in sharp focus within the foveal center 
of vision would have to be as close as 

7

/

10

 of a mile 

in order to be recognized if it were outside of foveal 
vision. Because the eyes can focus only on this 
narrow viewing area, effective scanning is accom-
plished with a series of short, regularly spaced eye 
movements that bring successive areas of the sky into 
the central visual field. Each movement should not 
exceed 10 degrees, and each area should be observed 
for at least 1 second to enable detection. Although 
horizontal back-and-forth eye movements seem 
preferred by most pilots, each pilot should develop a 
scanning pattern that is most comfortable and then 
adhere to it to assure optimum scanning. 

3. 

Studies show that the time a pilot spends on 

visual tasks inside the cabin should represent no more 
that 

1

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4

 to 

1

/

3

 of the scan time outside, or no more than 

4 to 5 seconds on the instrument panel for every 
16 seconds outside. Since the brain is already trained 
to process sight information that is presented from 
left to right, one may find it easier to start scanning 
over the left shoulder and proceed across the 
windshield to the right. 

4. 

Pilots should realize that their eyes may 

require several seconds to refocus when switching 
views between items in the cockpit and distant 
objects. The eyes will also tire more quickly when 
forced to adjust to distances immediately after 
close-up focus, as required for scanning the 
instrument panel. Eye fatigue can be reduced by 
looking from the instrument panel to the left wing 
past the wing tip to the center of the first scan quadrant 
when beginning the exterior scan. After having 
scanned from left to right, allow the eyes to return to 
the cabin along the right wing from its tip inward. 
Once back inside, one should automatically com-
mence the panel scan. 

5. 

Effective scanning also helps avoid “empty-

field myopia.” This condition usually occurs when 
flying above the clouds or in a haze layer that 
provides nothing specific to focus on outside the 
aircraft. This causes the eyes to relax and seek a 

Fitness for Flight 

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