1. In rare instances, a wake encounter could cause catastrophic inflight structural damage to an aircraft.
However, the usual hazard is associated with induced rolling moments that can exceed the roll-control authority
of the encountering aircraft. During inflight testing, aircraft intentionally flew directly up trailing vortex cores
of larger aircraft. These tests demonstrated that the ability of aircraft to counteract the roll imposed by wake
vortex depends primarily on the wingspan and counter-control responsiveness of the encountering aircraft.
These tests also demonstrated the difficulty of an aircraft to remain within a wake vortex. The natural tendency
is for the circulation to eject aircraft from the vortex.
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 7-4-2.)
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.)
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)

7-4-2

Wake Turbulence