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Pilot's Handbook of Aeronautical Knowledge
Airport Operations

Wake Turbulence

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Vortex generation.
Figure 13-20. Vortex generation.

Traffic Advisories
Figure 13-19 Traffic Advisories.

Wake Turbulence

All aircraft generate wake turbulence while in flight. This
disturbance is caused by a pair of counter-rotating vortices
trailing from the wingtips. The vortices from larger aircraft
pose problems to encountering aircraft. The wake of these
aircraft can impose rolling moments exceeding the roll
control authority of the encountering aircraft. Also, the
turbulence generated within the vortices can damage aircraft
components and equipment if encountered at close range. For
this reason, a pilot must envision the location of the vortex
wake and adjust the flightpath accordingly.

During ground operations and during takeoff, jet engine
blast (thrust stream turbulence) can cause damage and upset
smaller aircraft at close range. For this reason, pilots of small
aircraft should consider the effects of jet-engine blast and
maintain adequate separation. Also, pilots of larger aircraft
should consider the effects of their aircraft's jet-engine blast
on other aircraft and equipment on the ground.

Vortex Generation
Lift is generated by the creation of a pressure differential over
the wing surface. The lowest pressure occurs over the upper
wing surface, and the highest pressure under the wing. This
pressure differential triggers the rollup of the airflow aft of
the wing resulting in swirling air masses trailing downstream
of the wingtips. After the rollup is completed, the wake
consists of two counter rotating cylindrical vortices. Most of
the energy is within a few feet of the center of each vortex,
but pilots should avoid a region within about 100 feet of the
vortex core. [Figure 13-20]

Vortex Strength
The strength of the vortex is governed by the weight, speed,
and shape of the wing of the generating aircraft. The vortex
characteristics of any given aircraft can also be changed by the
extension of flaps or other wing configuration devices as well
as by a change in speed. The greatest vortex strength occurs
when the generating aircraft is heavy, clean, and slow.

Vortex Behavior
Trailing vortices have certain behavioral characteristics
that can help a pilot visualize the wake location and take
avoidance precautions.

Vortices are generated from the moment an aircraft leaves the
ground (until it touches down), since trailing vortices are the
by product of wing lift. [Figure 13-21] The vortex circulation
is outward, upward, and around the wingtips when viewed
from either ahead or behind the aircraft. Tests have shown
that vortices remain spaced a bit less than a wingspan apart,
drifting with the wind, at altitudes greater than a wingspan from
the ground. Tests have also shown that the vortices 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.