Aeronautical Information Manual (AIM), page 520

Index   519 -- Page 520 -- 521

AIM                                                                                                        12/10/15

     2. Counter control is usually effective and           wake location and thereby take avoidance precau-
induced roll minimal in cases where the wingspan           tions.
and ailerons of the encountering aircraft extend
beyond the rotational flow field of the vortex. It is            1. An aircraft generates vortices from the
more difficult for aircraft with short wingspan            moment it rotates on takeoff to touchdown, since
(relative to the generating aircraft) to counter the       trailing vortices are a by-product of wing lift. Prior to
imposed roll induced by vortex flow. Pilots of short       takeoff or touchdown pilots should note the rotation
span aircraft, even of the high performance type, must     or touchdown point of the preceding aircraft. (See
be especially alert to vortex encounters.                  FIG 7-3-3.)
(See FIG 7-3-2.)                                                 2. The vortex circulation is outward, upward
                                                           and around the wing tips when viewed from either
                      FIG 7-3-2                            ahead or behind the aircraft. Tests with large aircraft
          Wake Encounter Counter Control                   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
    3. The wake of larger aircraft requires the            below the generating aircraft. (See FIG 7-3-4.)
respect of all pilots.                                     However, vertical separation of 1,000 feet may be
                                                           considered safe.

7-3-4. Vortex Behavior                                          4. When the vortices of larger aircraft sink close
                                                           to the ground (within 100 to 200 feet), they tend to
  a. Trailing vortices have certain behavioral             move laterally over the ground at a speed of 2 or
characteristics which can help a pilot visualize the       3 knots. (See FIG 7-3-5.)

                                                    FIG 7-3-3
                                            Wake Ends/Wake Begins

                                  Touchdown                                Rotation

                                        Wake Ends                              Wake Begins

7-3-2                                                                                            Wake Turbulence

Page 520 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   519 -- Page 520 -- 521