background image

6/17/21 

AIM 

Section 4.  Wake Turbulence 

7

4

1.  General 

a. 

Every aircraft generates wake turbulence while 

in flight. Wake turbulence is a function of an aircraft 
producing lift, resulting in the formation of two 
counter

rotating vortices trailing behind the aircraft. 

b. 

Wake turbulence from the generating aircraft 

can affect encountering aircraft due to the strength, 
duration, and direction of the vortices. Wake 
turbulence can impose rolling moments exceeding 
the roll

control authority of encountering aircraft, 

causing possible injury to occupants and damage to 
aircraft. Pilots should always be aware of the 
possibility of a wake turbulence encounter when 
flying through the wake of another aircraft, and adjust 
the flight path accordingly. 

7

4

2.  Vortex Generation 

a. 

The creation of a pressure differential over the 

wing surface generates lift. The lowest pressure 
occurs over the upper wing surface and the highest 
pressure under the wing. This pressure differential 
triggers the roll up of the airflow at the rear of the 
wing resulting in swirling air masses trailing 
downstream of the wing tips. After the roll up is 
completed, the wake consists of two counter

rotating 

cylindrical vortices. (See FIG 7

4

1.) The wake 

vortex is formed with most of the energy concentrated 
within a few feet of the vortex core. 

FIG 7

4

Wake Vortex Generation 

b. 

More aircraft are being manufactured or 

retrofitted with winglets. There are several types of 
winglets, but their primary function is to increase fuel 
efficiency by improving the lift

to

drag ratio. 

Studies have shown that winglets have a negligible 
effect on wake turbulence generation, particularly 
with the slower speeds involved during departures 
and arrivals. 

7

4

3.  Vortex Strength 

a. 

Weight, speed, wingspan, and shape of the 

generating aircraft’s wing all govern the strength of 
the vortex. The vortex characteristics of any given 
aircraft can also be changed by extension of flaps or 
other wing configuring devices. However, the vortex 
strength from an aircraft increases proportionately to 
an increase in operating weight or a decrease in 
aircraft speed. Since the turbulence from a “dirty” 
aircraft configuration hastens wake decay, the 
greatest vortex strength occurs when the generating 
aircraft is HEAVY, CLEAN, and SLOW. 

b.  Induced Roll 

1. 

In rare instances, a wake encounter could 

cause catastrophic inflight structural damage to an 
aircraft. However, the usual hazard is associated with 
induced rolling moments that can exceed the 
roll

control  authority of the encountering aircraft. 

During inflight testing, aircraft intentionally flew 
directly up trailing vortex cores of larger aircraft. 
These tests demonstrated that the ability of aircraft to 
counteract the roll imposed by wake vortex depends 
primarily on the wingspan and counter

control 

responsiveness of the encountering aircraft. These 
tests also demonstrated the difficulty of an aircraft to 
remain within a wake vortex. The natural tendency is 
for the circulation to eject aircraft from the vortex. 

2. 

Counter control is usually effective and 

induced roll minimal in cases where the wingspan 
and ailerons of the encountering aircraft extend 
beyond the rotational flow field of the vortex. It is 
more difficult for aircraft with short wingspan 
(relative to the generating aircraft) to counter the 
imposed roll induced by vortex flow. Pilots of short 
span aircraft, even of the high performance type, must 
be especially alert to vortex encounters. 
(See FIG 7

4

2.) 

Wake Turbulence 

7

4