Wake Turbulence Separation

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 thats 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 greatest vortex strength occurs under conditions of heavy weight, clean configuration, and slow speed.

Aircraft flying directly into the core of a vortex will tend to roll with the vortex. The capability of counteracting the roll depends on the wing span and control responsiveness of the aircraft. When the wing span and ailerons of a larger aircraft extend beyond the vortex, counter-roll control is usually effective, and the effect of the induced roll can be minimized. Pilots of short wing span aircraft must be especially alert to vortex situations, even though their aircraft are of the high-performance type.

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 feet below a large aircraft.

Controllers apply the following wake turbulence radar separation minima between a preceding IFR/VFR aircraft and an aircraft vectored directly behind it and at less than 1,000 feet during any phase of flight. Categories, weight limits, aircraft examples and separation criteria are indicated in the table below.

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