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scan effectively, pilots look from side to side. They should begin scanning at the greatest distance at which an object can be perceived high on the horizon, thence moving inward toward the position of the aircraft. Figure 12-5  shows this scanning pattern. Because the light-sensitive elements of the retina are unable to perceive images that are in motion, a stop-turn-stop-turn motion should be used. For each stop, an area about 30 degrees wide should be scanned. This viewing angle includes an area about 250 meters wide at a distance of 500 meters. The duration of each stop is based on the degree 

of detail that is required, but no stop should last more than two or three seconds. When moving from one viewing point to the next, pilots should overlap the previous field of view by 10 degrees. This scanning technique allows greater clarity in observing the periphery. Other scanning techniques, as illustrated in Figure 12-6,  may be developed to fit the situation. 

## Obstruction Detection 

Obstructions having poor reflective surfaces, such as wires and small tree limbs, are difficult to detect. The best way to 

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