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scratched and/or
dirty instrumentation, use
of cockpit red lighting,
inadequate cockpit environmental
control (temperature
and humidity),
inappropriate sunglasses
and/or prescription glasses/
contact lenses, and
sustained visual workload
during flight.
Focusing
The natural ability to focus your eyes is critical to flight safety. It is important to
know that normal eyes may require several seconds to refocus when switching views
between near (reading charts), intermediate (monitoring instruments), and distant
objects (looking for traffic or external visual references).
Fatigue can lead to impaired visual focusing, which causes the eyes to overshoot
or undershoot the target, and can also affect a pilot’s ability to quickly change focus
between near, intermediate, and distant vision. The most common symptoms of visual
fatigue include blurred vision, excessive tearing, “heavy” eyelid sensation, frontal
or orbital headaches, and burning, scratchy, or dry eye sensations.
Distance focus, without a specific object to look at, tends to diminish rather
quickly. If you fly over water or under hazy conditions with the horizon obscured or
between cloud layers at night, your distance focus relaxes after about 60-80 seconds.
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If there is nothing specific on which to focus, your eyes revert to a relaxed intermediate
focal distance (10 to 30 ft). This means that you are looking without actually
seeing anything, which is dangerous. The answer to this phenomenon is to condition
your eyes for distant vision. Focus on the most distant object that you can see, even
if it’s just a wing tip. Do this before you begin scanning the sky in front of you. As
you scan, make sure you repeat this re-focusing exercise often.
Dark Adaptation or Night Vision Adaptation
Dark adaptation is the process by which the eyes adapt
for optimal night visual acuity under conditions of low
ambient illumination. The eyes require about 30 to 45
minutes to fully adapt to minimal lighting conditions.
The lower the starting level of illumination, the more rapidly
complete dark adaptation is achieved. To minimize
the time necessary to achieve complete dark adaptation
and to maintain it, you should:
avoid inhaling carbon monoxide from smoking or exhaust
fumes
get enough Vitamin A in your diet
adjust instrument and cockpit lighting to the lowest
level possible
avoid prolonged exposure to bright lights
use supplemental oxygen when flying at night above 5,000 ft (MSL)
If dark-adapted eyes are exposed to a bright light source (searchlights, landing
lights, flares, etc.) for a period in excess of 1 second, night vision is temporarily
impaired. Exposure to aircraft anti-collision lights does not impair night vision adaptation
because the intermittent flashes have a very short duration (less than 1
second).
Visual Scanning
Scanning the sky for
other aircraft is a very important
factor in avoiding
midair collisions, and it
should cover all areas of
the sky visible from the
cockpit. Most of us are
instinctively alert for potential
head-on encounters
with another aircraft. Actually,
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Pilot Safety Brochures飞行员安全手册(40)
