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AIM

10/12/17

7

−3−2

Wake Turbulence

2.

Counter control is usually effective and

induced roll minimal in cases where the wingspan
and ailerons of the encountering aircraft extend
beyond the rotational flow field of the vortex. It is
more difficult for aircraft with short wingspan
(relative to the generating aircraft) to counter the
imposed roll induced by vortex flow. Pilots of short
span aircraft, even of the high performance type, must
be especially alert to vortex encounters. 
(See FIG 7

−3−2.)

FIG 7

−3−2

Wake Encounter Counter Control

COUNTER
CONTROL

3.

The wake of larger aircraft requires the

respect of all pilots.

7

−3−4. Vortex Behavior

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

−3−3.)

2.

The vortex circulation is outward, upward

and around the wing tips when viewed from either
ahead or behind the aircraft. Tests with large 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 and lateral
position (preferably upwind) will provide a flight
path clear of the turbulence.

3.

Flight tests have shown that the vortices from

larger (transport category) 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. Atmospheric
turbulence hastens breakup. Pilots should fly at or
above the preceding aircraft’s flight path, altering
course as necessary to avoid the area behind and
below the generating aircraft. (See FIG 7

−3−4.)

However, vertical separation of 1,000 feet may be
considered safe.

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

−3−5.)

FIG 7

−3−3

Wake Ends/Wake Begins

Touchdown

Rotation

Wake Ends

Wake Begins