FIG 7 − 4 − 2 Wake Encounter Counter Control 8/15/19 3/15/07 7/16/20 AIM  7110.65R CHG 2 AIM 2. The vortex circulation is outward, upward and around the wing tips when viewed from either ahead or behind the aircraft. Tests with larger aircraft have shown that the vortices remain spaced a bit less than a wingspan apart, drifting with the wind, at altitudes greater than a wingspan from the ground. In view of this, if persistent vortex turbulence is encountered, a slight change of altitude (upward) and lateral position (upwind) should provide a flight path clear of the turbulence. 3. Flight tests have shown that the vortices from larger aircraft sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft. Pilots should fly at or above the preceding aircraft’s flight path, altering course as necessary to avoid the area directly behind and below the generating aircraft. (See FIG 7 − 4 − 4.) Pilots, in all phases of flight, must remain vigilant of possible wake effects created by other aircraft. Studies have shown that atmospheric turbulence hastens wake breakup, while other atmospheric conditions can transport wake horizontally and vertically. 4. When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the ground at a speed of 2 or 3 knots. (See .FIG 7 − 4 − 5) a. Trailing vortices have certain behavioral characteristics which can help a pilot visualize the wake location and thereby take avoidance precau- tions. 1. An aircraft generates vortices from the moment it rotates on takeoff to touchdown, since trailing vortices are a by − product of wing lift. Prior to takeoff or touchdown pilots should note the rotation or touchdown point of the preceding aircraft. (See FIG 7 − 4 − 3.) FIG 7 − 4 − 3 Wake Ends/Wake Begins Wake Turbulence 7 − 4 − 2 Touchdown Rotation Wake Ends Wake Begins COUNTER CONTROL 7 − 4 − 4. Vortex Behavior