Causes

Wake turbulence has its greatest impact on departure and arrival procedures, however, pilots should not assume that it will only be encountered in the vicinity of aerodromes. Caution should be exercised whenever a flight is conducted anywhere behind and at less than 1,000 ft below a large aircraft. Wake turbulence is caused by wing-tip vortices and is a by-product of lift. The higher air pressure under the wings tries to move to the lower air pressure on top of the wings by flowing towards the wing tips, where it rotates and flows into the lower pressure on top of the wings. This results in a twisting rotary motion that is very pronounced at the wing tips and continues to spill over the top in a downward spiral. Therefore, the wake consists of two counter-rotating cylindrical vortices.

The strength of these vortices is governed by the shape of the wings, and the weight and speed of the aircraft; the most significant factor is weight.

The strength of the vortex shows little dissipation at altitude within 2 minutes of the time of initial formation. Beyond 2 minutes, varying degrees of dissipation occur along the vortex path, first in one vortex and then in the other. The break-up of vortices is affected by atmospheric turbulence; the greater the turbulence, the more rapid the dissipation of the vortices.