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highest pressure under the wing. This pressure
differential triggers the roll up of the airflow aft of the
wing resulting in swirling air masses trailing
downstream of the wing tips. After the roll up is
completed, the wake consists of two counter-rotating
cylindrical vortices. (See FIG 7-3-1.) Most of the
energy is within a few feet of the center of each
vortex, but pilots should avoid a region within about
100 feet of the vortex core.
FIG 7-3-1
Wake Vortex Generation
7-3-3. Vortex Strength
a. The strength of the vortex is governed by the
weight, speed, and shape of the wing of the generating
aircraft. The vortex characteristics of any given
aircraft can also be changed by extension of flaps or
other wing configuring devices as well as by change
in speed. However, as the basic factor is weight, the
vortex strength increases proportionately. Peak
vortex tangential speeds exceeding 300 feet per
second have been recorded. The greatest vortex
strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW.
b. Induced Roll
1. In rare instances a wake encounter could
cause inflight structural damage of catastrophic
proportions. However, the usual hazard is associated
with induced rolling moments which can exceed the
roll‐control authority of the encountering aircraft. In
flight experiments, aircraft have been intentionally
flown directly up trailing vortex cores of larger
aircraft. It was shown that the capability of an aircraft
to counteract the roll imposed by the wake vortex
primarily depends on the wingspan and countercontrol
responsiveness of the encountering aircraft.
AIM 2/14/08
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 precautions.
1. Vortices are generated from the moment
aircraft leave the ground, 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 7-3-4.)
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 7-3-3.)
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
3knots. (See FIG 7-3-5.)
FIG 7-3-3
Wake Ends/Wake Begins
Touchdown Rotation
Wake Ends Wake Begins
2/14/08 AIM
Wake Turbulence 7-3-3
FIG 7-3-4
Vortex Flow Field
AVOID Nominally 500-1000 Ft.
Sink Rate
Several Hundred Ft.,/Min.
FIG 7-3-5
Vortex Movement Near Ground - No Wind
No Wind
3K 3K
FIG 7-3-6
Vortex Movement Near Ground - with Cross Winds
6K
(3K + 3K)
3K Wind
0 (3K - 3K)
AIM 2/14/08
7-3-4 Wake Turbulence
5. There is a small segment of the aviation
community that have become convinced that wake
vortices may “bounce” up to twice their nominal
　
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