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The eye, and consequently vision, is vulnerable to just about everything: dust;
fatigue; emotion; germs; fallen eyelashes; age; optical illusions; and the
alcoholic content of last night's party. In flight, vision is altered by atmospheric
conditions, windshield distortion, too much (or too little) oxygen, acceleration,
glare, heat, lighting, aircraft design and forth.
Most of all, the eye is vulnerable to the vagaries of the mind. We can "see"
and identify only what the mind lets us see. For example, a daydreaming pilot
staring out into space sees no approaching traffic and is probably the number one
candidate for an in-flight collision.
Accommodation
One function of the eye that is a source of constant problems to the pilot (though
he or she is probably never aware of it) is the time required for accommodation.
Our eyes automatically accommodate for (or refocus on) near and far objects.
But the change from something up close, like a dark panel two feet away, to a
well-lighted landmark or aircraft target a mile or so away, takes one to two seconds
or longer for eye accommodation. That can be a long time when you consider that you
need 10 seconds to avoid in-flight collisions.
Empty-Field Myopia
Another focusing problem usually occurs at very high altitudes, but it can happen
even at lower levels on vague, colorless days above a haze or cloud layer when no
distinct horizon is visible. If there is little or nothing to focus on at infinity,
we do not focus at all. We experience something known as "empty-field myopia: "
we stare, but we see nothing, even opposing traffic, if it should enter our visual
field.
Binocular Vision
The effects of what is called "binocular vision" have been studied seriously
by the National Transportation Safety Board (NTSB) during investigations of in-flight
collisions, with the conclusions that this too is a casual factor. To actually accept
what we see, we need to receive cues from both eyes. If an object is visible to one eye,
but hidden from the other by a windshield post or other obstruction, the total image
is blurred and not always acceptable to the mind.
Tunnel Vision
Another inherent eye problem is that of narrow field of vision. Although our eyes
accept light rays from an arc of nearly 200 degrees, they are limited to a relatively
narrow area (approximately 10-15 degrees) in which they can actually focus and
classify an object. Though we can perceive movement in the periphery, we cannot
identify what is happening out there, and we tend not to believe what we see out of
the corner of our eyes. This, aided by the brain, often leads to "tunnel
vision."
Blossom Effect
This limitation is compounded by the fact that at a distance, an aircraft on a
collision course with you will appear to be motionless. It will remain in a
seemingly stationary position, without appearing either to move or to grow in
size for a relatively long time, and then suddenly bloom into a huge mass filling
one of your windows. This is known as "blossom effect." Since we
need motion or contrast to attract our eyes' attention, this effect becomes a
frightening factor when you realize that a large bug smear or dirty spot on the
windshield can hide a converging plane until it is too close to be avoided.
Environmental Effects
In addition to the built-in problems, the eye is also severely limited by environment.
Optical properties of the atmosphere alter the appearance of traffic, particularly
on hazy days. "Limited visibility" actually means "limited vision."
You may be legally VFR when you have three miles, but at that distance on a hazy
day, opposing traffic is not easy to detect. At a range closer than three miles, opposing
traffic may be detectable, but no longer avoidable.
Lighting also affects our vision stimuli. Glare, usually worse on a sunny day over
a cloud deck or during flight directly into the sun, makes objects hard to see and makes
scanning uncomfortable. Also, an object that is well lighted will have a high degree
of contrast and will be easy to detect, while one with low contrast at the same
distance may be impossible to see. For instance, when the sun is behind you, an
opposing aircraft will stand out clearly, but when you are looking into the sun and
your traffic is "backlighted," it's a different story.
Another contrast problem area is trying to find an airplane over a cluttered
background. If it is between you and terrain that is Varicolored or heavily dotted
with buildings, it will blend into the background until it is quite close.
Human Factors
And, of course, there is the mind, which can distract us to the point of not seeing
anything at all, or lull us into cockpit myopia - staring at one instrument without
even "seeing" it. How often have you filed IFR on a VFR day, settled back
at your assigned altitude with autopilot on, and then never looked outside,
feeling secure that "Big Daddy Radar" will protect you from all harm? Don't
fall for this trap. Remember, the radar system has its limitations too! It is fine
to depend on instruments, but not to the exclusion of the see-and-avoid system,
especially on days when there are pilots not under radar surveillance or control
flying around in the same sky. Also remember that our Air Traffic Control (ATC) system
is not infallible, even when it comes to providing radar separation between aircraft
flying on IFR flight plans.
As you can see, visual perception is affected by many factors. It all boils down
to the fact that pilots, like anyone else, tend to overestimate their visual
abilities and to misunderstand the limitations of their eyes. Since the number one
cause of in-flight collisions is the failure to properly adhere to the see-and-avoid
concept, we can conclude that the best way to avoid them is to learn how to use
our eyes in an efficient external scan.
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