Refer to figures.
WAKE TURBULENCE

The strength and size of the vortices produced by an aeroplane are dependent on the amount of lift produced. Strong vortices are produced by large transport aeroplanes that create a significant amount of turbulence behind the aeroplane along the path of the aircraft.

The disturbance so caused can extend for a considerable distance and last for several minutes. This is wake turbulence that can seriously affect the controllability of following aeroplanes if they are too close.

The severity of the turbulence so created is categorised to enable following pilots to assess the difficulties that may be encountered. The larger, heavier aeroplanes create the worst turbulence that will cause problems to following pilots particularly when close to the ground during take-off or landing. Pilots are advised by the aviation authorities of the minimum distance and/or time separation from the aeroplane ahead deemed to be safe, to avoid having control problems.

The minimum separation distance and/or time is based on the relative sizes of the aeroplanes concerned. Wake turbulence is affected by the height of the aeroplane and the prevailing wind conditions. The downwash of the airflow behind the aeroplane causes the vortices to descend to one thousand feet below the aeroplane. However, at low height, when the aeroplane is in ground effect, the vortices move outward from the path of the aeroplane over the ground. But any cross wind between 5 kt and 10 kt will prevent this happening to the upwind vortex and will keep it on the aircraft’s track. It is particularly important during the take-off phase that the following aircraft complies with the advised separation because of this.

Aircraft mass, speed, aspect ratio and trailing-edge flap setting influence the strength of the vortices. The vortex strength is greatest for heavy aeroplanes at low airspeed and in the clean configuration, which is just after rotation at take-off. Vortices only cease when the nose-wheel touches down during landing ground run.