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. Obstructions having poor reflective surfaces, such as wires  and small tree limbs, are difficult to detect. The best way to  Obstruction Detection Figure 12-5.  Scanning pattern. Figure 12-6.  Night vision. 4 2 3 1 10° 6 seconds 6 seconds 6 seconds 6 seconds 6 seconds 3 seconds 3 seconds 3 seconds 3 seconds 3 seconds 3 seconds 4 seconds 4 seconds 4 seconds 4 seconds 12-5