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.

FIG 7-4-2

Wake Encounter Counter Control

COUNTER
CONTROL

7-4-4. Vortex Behavior
a. Trailing vortices have certain behavioral
characteristics which can help a pilot visualize the
wake location and thereby take avoidance precautions.
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.)

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)

FIG 7-4-3

Wake Ends/Wake Begins

Touchdown

Wake Ends

7-4-2

Rotation

Wake Begins

Wake Turbulence