Chapter

14

Wake Turbulence

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Turbulence Wake 14

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Wake Turbulence

The term wake turbulence is used to describe the effect of the rotating air masses generated behind the wing tips of jet aircraft. Wake vortices are present behind all aircraft but are particularly severe when generated by large aircraft. Wake vortices are most hazardous to aircraft with a small wingspan during take-off, initial climb, final approach and the landing phase. The characteristics of the vortex are determined by the aircraft gross weight, the wingspan, airspeed and attitude; The greatest turbulence being produced by heavy aircraft, flying slowly in a clean configuration. The effects disperse and the localized effect of the vortex spreads and loses intensity. Practically, the vortex patterns from an aircraft may be regarded as two counter-rotating cylindrical air masses trailing from the aircraft. Typically, the two vortices are separated by about three-quarters of the aircraft wingspan and in still air tend to drift slowly downwards and, either level off usually not more than 1000 ft below the flight path of the aircraft, or on approaching the ground move tangentially at about 300 ft/sec from the track of the aircraft (in still air). This decays to an average sideways speed of 5 kt.

Wake vortex generation begins when the nose wheel lifts off the runway on take-off and ceases when the nose wheel touches down again. In a crosswind situation, the outward path of the upwind vortex will be opposed by the crosswind, whilst the downwind vortex will be assisted in the outwards dispersal. If the crosswind component is 5 kt, the upwind vortex will remain effectively stationary virtually underneath the flight path of the aeroplane. Windshear close to the ground can cause the vortices to descend at different rates and even cause one vortex to rise. Atmospheric turbulence and high winds close to the ground cause vortices to decay more quickly, thus conditions of light winds require additional attention.

Wake vortices from helicopters are generated by the down-wash of the main rotor where they are transformed into a pair of trailing vortices similar to those of fixed wing aeroplanes. Evidence suggests that per kilogram of aeroplane mass, helicopter vortices are more intense than those of fixed wing aeroplanes.

ICAO defines wake turbulence categories of aeroplanes by maximum take-off mass as follows:

Category		Max Take-off Mass
Heavy		Equal to or greater than 136 000 kg
Medium		Less than 136 000 kg but more than 7000 kg
Light		7000 kg or less
	Figure 14.1: Wake turbulence categories (ICAO)

Separation minima are applied between aircraft by distance for both take-off and landing and where complex runway arrangements are in use. The following minima shall be applied to an aircraft on approach and departure phase of flight when

•An aircraft is operating behind another aircraft at the same altitude or less than 300 m (1000 ft) below, or

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•Both aircraft are using the same runway or parallel runways separated by less than 760 m, or

•An aircraft is crossing behind another aircraft at the same altitude or less than 300 m (1000 ft) below.

Leading Aircraft	Following	Distance (NM)	Time Equivalent
Heavy	Heavy	4	-
Heavy	Medium	5	2 min
Heavy	Light	6	3 min
Medium	Heavy	3	-
Medium	Medium	3	-
Medium	Light	5	3 min

Figure 14.2: ICAO wake turbulence separation standard for landing

The separation standard for departure is as follows:

			Departure
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Wake		Leading Aircraft	Following Aircraft				Spacing
Turbulence
		Heavy	Medium or Light	same position
				Departing	from	the	2 minutes
		Medium	Light
		Heavy	Medium or Light	Departing	from	an
				intermediate point on			3 minutes
		Medium	Light
				the runway

Figure 14.3: ICAO wake turbulence separation standard for take-off

The above separation minima apply to the categories of aircraft where take-off and landing operations are being conducted on parallel runways (less than 760 m apart), or where the projected flight path of the following aircraft crosses that of the leading aircraft at the same level or within 1000 ft lower.

A separation of 2 minutes is to be applied between arrivals and departures where a medium or light aircraft arriving is following a heavy departing, or between a light arriving following a medium departing, where flight paths cross when a runway has a displaced landing threshold. The reverse situation, a medium or light aircraft departing following a heavy arriving (light aircraft departing following a medium arriving), also requires the 2 minute separation.

A similar 2 minute separation applies to a light or medium departing or arriving after a heavy (light departing or arriving after a medium) has made a low or missed approach in the opposite direction. The same separation criteria will apply for parallel runways less than 760 m apart.

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