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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
x
Tail Wind
Touchdown Point
2/14/08 AIM
Wake Turbulence 7-3-5
7-3-5. Operations Problem Areas
a. A wake encounter can be catastrophic. In 1972
at Fort Worth a DC-9 got too close to a DC-10
(twomiles back), rolled, caught a wingtip, and
cartwheeled coming to rest in an inverted position on
the runway. All aboard were killed. Serious and even
fatal GA accidents induced by wake vortices are not
uncommon. However, a wake encounter is not
necessarily hazardous. It can be one or more jolts with
varying severity depending upon the direction of the
encounter, weight of the generating aircraft, size of
the encountering aircraft, distance from the generating
aircraft, and point of vortex encounter. The
probability of induced roll increases when the
encountering aircraft's heading is generally aligned
with the flight path of the generating aircraft.
b. AVOID THE AREA BELOW AND BEHIND
THE GENERATING AIRCRAFT, ESPECIALLY
AT LOW ALTITUDE WHERE EVEN A
MOMENTARY WAKE ENCOUNTER COULD BE
HAZARDOUS. This is not easy to do. Some
accidents have occurred even though the pilot of the
trailing aircraft had carefully noted that the aircraft in
front was at a considerably lower altitude. Unfortunately,
this does not ensure that the flight path of the
lead aircraft will be below that of the trailing aircraft.
c. Pilots should be particularly alert in calm wind
conditions and situations where the vortices could:
1. Remain in the touchdown area.
2. Drift from aircraft operating on a nearby
runway.
3. Sink into the takeoff or landing path from a
crossing runway.
4. Sink into the traffic pattern from other airport
operations.
5. Sink into the flight path of VFR aircraft
operating on the hemispheric altitude 500 feet below.
d. Pilots of all aircraft should visualize the
location of the vortex trail behind larger aircraft and
use proper vortex avoidance procedures to achieve
safe operation. It is equally important that pilots of
larger aircraft plan or adjust their flight paths to
minimize vortex exposure to other aircraft.
7-3-6. Vortex Avoidance Procedures
a. Under certain conditions, airport traffic controllers
apply procedures for separating IFR aircraft. If a
pilot accepts a clearance to visually follow a
preceding aircraft, the pilot accepts responsibility for
separation and wake turbulence avoidance. The
controllers will also provide to VFR aircraft, with
whom they are in communication and which in the
tower's opinion may be adversely affected by wake
turbulence from a larger aircraft, the position, altitude
and direction of flight of larger aircraft followed by
　
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