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AIM

10/12/17

8−1−7

Fitness for Flight

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

/

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