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Aeronautical Information Manual (AIM), page 522

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AIM                                                                                                          12/10/15



     5. There is a small segment of the aviation               vortex encounters when conducting approach and
community that have become convinced that wake                 landing operations. The pilot has the ultimate
vortices may "bounce" up to twice their nominal                responsibility for ensuring appropriate separations
steady state height. With a 200-foot span aircraft, the        and positioning of the aircraft in the terminal area to
"bounce" height could reach approximately 200 feet             avoid the wake turbulence created by a preceding
AGL. This conviction is based on a single                      aircraft.
unsubstantiated report of an apparent coherent
vortical flow that was seen in the volume scan of a              b. A crosswind will decrease the lateral movement
research sensor. No one can say what conditions                of the upwind vortex and increase the movement of
cause vortex bouncing, how high they bounce, at                the downwind vortex. Thus a light wind with a cross
what angle they bounce, or how many times a vortex             runway component of 1 to 5 knots could result in the
may bounce. On the other hand, no one can say for              upwind vortex remaining in the touchdown zone for
certain that vortices never "bounce." Test data have           a period of time and hasten the drift of the downwind
shown that vortices can rise with the air mass in which        vortex toward another runway. (See FIG 7-3-6.)
they are embedded. Wind shear, particularly, can               Similarly, a tailwind condition can move the vortices
cause vortex flow field "tilting." Also, ambient               of the preceding aircraft forward into the touchdown
thermal lifting and orographic effects (rising terrain         zone. THE LIGHT QUARTERING TAILWIND
or tree lines) can cause a vortex flow field to rise.          REQUIRES MAXIMUM CAUTION. Pilots should
Notwithstanding the foregoing, pilots are reminded             be alert to large aircraft upwind from their approach
that they should be alert at all times for possible wake       and takeoff flight paths. (See FIG 7-3-7.)


                                                      FIG 7-3-7
                                  Vortex Movement in Ground Effect - Tailwind




      Tail Wind


                             Light Quartering
                                Tailwind                            Touchdown Point


                                                           x




7-3-4                                                                                               Wake Turbulence

Page 522 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   521 -- Page 522 -- 523