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Aviation Learning Center Document How to Avoid a Mid Air Collision - P-8740-51
Author: Federal Aviation Administration Date: Unknown
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Limitations of the Eye
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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.


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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