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Pilot's Handbook of Aeronautical Knowledge
Aeromedical Factors

Vision in Flight

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Night blind spot.
Figure 16-14. Night blind spot.

Hypoxia also affects vision. Sharp clear vision, (with the
best being equal to 20-20 vision) requires significant oxygen
especially at night. As altitude increases, the available oxygen
decreases, degrading night vision. Compounding the problem
is fatigue, which minimizes physiological well being. Adding
fatigue to high altitude exposure is a recipe for disaster.
In fact, if flying at night at an altitude of 12,000 feet, the
pilot may actually see elements of his or her normal vision
missing or not in focus. Missing visual elements resemble
the missing pixels in a digital image while unfocused vision
is dim and washed out.

For the pilot suffering the effects of hypoxic hypoxia, a simple
descent to a lower altitude may not be sufficient to reestablish
vision. For example, a climb from 8,000 feet to 12,000 feet for
30 minutes does not mean a descent to 8,000 feet will rectify
the problem. Visual acuity may not be regained for over an
hour. Thus, it is important to remember, altitude and fatigue
have a profound effect on a pilot's ability to see.

Several things can be done to keep the eyes adapted to
darkness. The first is obvious: avoid bright lights before and
during flight For 30 minutes before a night flight, avoid any
bright light sources, such as headlights, landing lights, strobe
lights, or flashlights. If a bright light is encountered, close
one eye to keep it light sensitive. This allows the use of that
eye to see again when the light is gone.

Red flight deck lighting also helps preserve night vision, but
red light severely distorts some colors and completely washes
out the color red. This makes reading an aeronautical chart
difficult. A dim white light or a carefully directed flashlight
can enhance night reading ability. While flying at night, keep
the instrument panel and interior lights turned up no higher
than necessary. This helps to see outside references more
easily. If the eyes become blurry, blinking more frequently
often helps.

Diet and general physical health have an impact on how well
a pilot can see in the dark. Deficiencies in vitamins A and C
have been shown to reduce night acuity. Other factors, such
as CO poisoning, smoking, alcohol, certain drugs, and a lack
of oxygen also can greatly decrease night vision.

Night Vision Illusions
There are many different types of visual illusions that
commonly occur at night. Anticipating and staying aware
of them is usually the best way to avoid them.

Autokinesis is caused by staring at a single point of light
against a dark background for more than a few seconds.
After a few moments, the light appears to move on its own.
To prevent this illusion, focus the eyes on objects at varying
distances and avoid fixating on one target. Be sure to maintain
a normal scan pattern.

False Horizon
A false horizon can occur when the natural horizon is obscured
or not readily apparent. It can be generated by confusing
bright stars and city lights. It can also occur while flying
toward the shore of an ocean or a large lake. Because of the
relative darkness of the water, the lights along the shoreline
can be mistaken for stars in the sky. [Figure 16-15]

Night Landing Illusions
Landing illusions occur in many forms. Above featureless
terrain at night, there is a natural tendency to fly a lower than-
normal approach. Elements that cause any type of
visual obscurities, such as rain, haze, or a dark runway
environment can also cause low approaches. Bright lights,
steep surrounding terrain, and a wide runway can produce the
illusion of being too low, with a tendency to fly a higher-than normal approach. A set of regularly spaced lights along a road
or highway can appear to be runway lights. Pilots have even
mistaken the lights on moving trains as runway or approach
lights. Bright runway or approach lighting systems can create
the illusion that the airplane is closer to the runway, especially
where few lights illuminate the surrounding terrain.