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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