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AIM

10/12/17

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

steady state height. With a 200−foot span aircraft, the

“bounce” height could reach approximately 200 feet

AGL. This conviction is based on a single

unsubstantiated report of an apparent coherent

vortical flow that was seen in the volume scan of a

research sensor. No one can say what conditions

cause vortex bouncing, how high they bounce, at

what angle they bounce, or how many times a vortex

may bounce. On the other hand, no one can say for

certain that vortices never “bounce.” Test data have

shown that vortices can rise with the air mass in which

they are embedded. Wind shear, particularly, can

cause vortex flow field “tilting.” Also, ambient

thermal lifting and orographic effects (rising terrain

or tree lines) can cause a vortex flow field to rise.

Notwithstanding the foregoing, pilots are reminded

that they should be alert at all times for possible wake

vortex encounters when conducting approach and

landing operations. The pilot has the ultimate

responsibility for ensuring appropriate separations

and positioning of the aircraft in the terminal area to

avoid the wake turbulence created by a preceding

aircraft.

b. A crosswind will decrease the lateral movement

of the upwind vortex and increase the movement of

the downwind vortex. Thus a light wind with a cross

runway component of 1 to 5 knots could result in the

upwind vortex remaining in the touchdown zone for

a period of time and hasten the drift of the downwind

vortex toward another runway. (See FIG 7−3−6.)

Similarly, a tailwind condition can move the vortices

of the preceding aircraft forward into the touchdown

zone. THE LIGHT QUARTERING TAILWIND

REQUIRES MAXIMUM CAUTION. Pilots should

be alert to large aircraft upwind from their approach

and takeoff flight paths. (See FIG 7−3−7.)

FIG 7−3−7

Vortex Movement in Ground Effect − Tailwind

Light Quartering

Tailwind

Light Quartering

Tailwind

x

Tail Wind

Tail Wind

Touchdown Point

Touchdown Point