Another focusing problem occurs on drab, colorless days above a haze or cloud layer when no
distinct horizon is visible. If there is little or nothing to focus on, we do not focus at all. We
experience something known as "empty-field myopia;" we stare but see nothing, not even
opposing traffic. In general, the threat of a midair collision can be avoided by scanning both 60º
to the left and right horizontally and 10º up and down. Figure 4-1 illustrates the minimum scan
range for the T-34C.
Figure 4-1 Outside Scan
The effects of "binocular vision" have been studied by the National Transportation Safety Board
(NTSB) during investigations of midair collisions. The board concluded that this is also a causal
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 windscreen post or other obstruction, the total
image is blurred and not always accepted by the mind.
Another inherent eye problem is that of narrow field of vision. Although our eyes accept light
rays from an arc of nearly 200º, they are limited to a relatively narrow area (approximately
10-15 degrees) in which they can actually focus on and classify an object. Though we can
perceive movement in the periphery, we cannot identify what is there. We tend not to believe
what we see out of the corner of our eyes. This often leads to "tunnel vision."
That limitation is compounded by the fact that at a distance, an aircraft on a collision course 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 the canopy. This is known as "blossom effect." We need motion or contrast to
attract our eyes' attention. A large bug smear or dirty spot on the windscreen can hide a
converging plane until it is too close to be avoided.
In addition to 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 Visual Flight Rules (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-even though detectable-opposing traffic may not be avoidable.
FUNDAMENTAL FLIGHT CONCEPTS