AIM
4/3/14
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