Vortex Generation

The origin of counter-rotating wing tip vortices is a direct and automatic consequence of the generation of lift by a wing. 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 roll up of the airflow aft 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. For further detail, see the separate article: Induced Drag.

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 extension of flaps or other wing configuring devices as well as by change in speed. However, as the basic factor is weight, the vortex strength increases proportionately.

Vortex Decay

Vortices usually persist for between one and three minutes, with survival often greatest at low level in calm or very light wind conditions and at higher altitudes in thinner air. Once formed, vortices descend until they decay (or reach the ground). Decay is usually rapid and occurs more quickly in windy conditions over land because of the greater variation in both components of wind velocity which are induced by frictional effects of terrain. Cross-winds can carry a vortex away from the flight path which the aircraft generating them has followed.

Contributory Factors

• Aircraft weight
• Aircraft speed
• Aircraft wing configuration (Flap setting etc.)
• Closeness to the ground - vortex ceases to be hazardous when ground contact occurs
• Wind velocity - light winds delay decay
• Turbulence, from sources other than wake vortex, accelerates vortex decay

Further Reading

EASA

• Safety Information Bulletin No. 2017-10: En-route Wake Turbulence Encounters, 22 June 2017

FAA

• FAA Aeronautical Information Manual, Chapter 7 (Safety of Flight), Section 4, Aircraft Wake Turbulence, 2022
• FAA AC 90-23G - Aircraft Wake Turbulence
• FAA "Pilot and Air Traffic Controller Guide to Wake Turbulence"
• Wake Turbulence Training Aid Navigator, FAA website

UK CAA

• Safety Sense Leaflet No 15c - Wake Turbulence
• UK AIC P092/2017 - Wake Turbulence

HindSight Articles

• Crash Follows Encounter with Boeing 757 Wake Vortex;
• Wake Vortex Turbulence - The role of the Air Traffic Controller;

Other

• Wake Votrices, C. Lelaie, Airbus Safety First Magazine No. 21, pp. 42-50, January 2016
• Airbus FOBN - Wake Turbulence Avoidance
• New Zealand Air Force 'Good Aviation Practice' Booklet on Wake Turbulence
• Wake Vortex Influence in General Aviation, a BFU note, 2016
• Video of practical tests to document the effects of wake vortices on aircraft, by the DLR (external link)