Definition

Wake Vortex Turbulence is defined as turbulence which is generated by the passage of an aircraft in flight. It will be generated from the point when the nose landing gear of an aircraft leaves the ground on take off and will cease to be generated when the nose landing gear touches the ground during landing. Where another aircraft encounters such turbulence, a Wake Vortex Encounter (WVE) is said to have occurred.

Description

Potentially hazardous turbulence in the wake of an aircraft in flight is principally caused by wing tip vortices. This type of turbulence is significant because wing tip vortices decay quite slowly and can produce a significant rotational influence on an aircraft encountering them for several minutes after they have been generated. Jet Efflux and Prop Wash can also hazard the control of an aircraft both on the ground and in the air but, whilst these effects are often extreme, their effects are more short-lived.

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.

Wake Vortex Turbulence

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.

Vortices typically persist for between one and three minutes, with their survival likely to be longest in stable air conditions with low wind speeds. Such conditions can extend their survival at higher cruise altitudes beyond that at low level because of the lower air density there. Once formed, vortices will, in almost all cases, likely descend until they decay or in the low level case until they reach the ground if this comes first. Decay of low level vortices will occur more quickly over land because of the boundary layer effect. An across-track wind direction can carry them away from the flight path which the aircraft generating them has followed.

Effects

The potential for hazardous wake vortex turbulence is greatest where aircraft follow the same tracks - i.e are 'in trail' and closely spaced. This situation is mostly encountered close to the ground in the vicinity of airports where aircraft are on approach to or departure from particular runways at high frequencies. Sudden uncommanded roll moments may occur which, in extreme cases, can be beyond the absolute power of the flying controls or the prevailing response of the flight crew to counteract. The high rate of roll may cause uncommanded disconnection of the Autopilot and transient or terminal loss of control can result in terrain impact in rare cases. En route in-trail uncommanded roll can be similarly caused to smaller aircraft by the effect of larger ones, which may be ahead at a higher level. Note that if the generating aircraft is climbing or descending rapidly (greater than 1000 fpm) then a significant wake vortex may persist across several flight levels. If the generator aircraft is descending, this means that a WVE can occur above the position of the generator aircraft at the time of the encounter. The greater longevity of vortices at higher cruise altitudes can lead to encounters at much greater in track separation than ATC separation minima if the prevailing wind speeds are low.

A cross-track encounter en route is likely to lead to only one or two sharp 'jolts' as the vortices are crossed. In either en route case, injuries to unsecured occupants can result, both passengers and cabin crew. Since most operators ensure that passengers are secured during intermediate and final approach and during initial climb after take off, it is Cabin Crew who will be most at risk of injury if they are not yet secured during the later stages of an approach.

Contributory Factors

• Leading aircraft weight
• Leading aircraft speed
• Leading aircraft wing configuration (Flap setting etc.)
• Relative size of leading and following aircraft
• Relative tracks, positions and lateral/vertical separation of proximate aircraft - the hazard is greater for aircraft following the same track/profile is greater than for the cross-track case
• Closeness to the ground - vortex ceases to be hazardous when ground contact occurs
• Wind direction relative to the track being flown by the generating aircraft - a cross-track wind reduces the risk to in-train aircraft
• Wind speed - light winds delay decay
• Turbulence, from sources other than wake vortex, accelerates vortex decay

Defences

Take off and Landing

• ATC provide standard separation for all departing aircraft and for IFR traffic on approach. Separation depends on the relative size of the aircraft. Traditional separation is described in detail in the article on ICAO Wake Turbulence Category and newer separation standards in effect at some US and European aerodromes are discussed in the article RECAT - Wake Turbulence Re-categorisation. Organizations should ensure that ICAO recommended separation minima for aircraft on approach and departure are understood and applied by both ATC and pilots with appropriate training inclusion, including, for pilots, periodic recovery practice during simulator training. Also, procedural documentation for both pilots and ATCOs to include the ICAO separation recommendations for arrival and departure (as well as any more restrictive national or local arrangements) should be available.[Note: not all NAAs fully adopt ICAO Recommendations in this matter].
• For VFR arrivals, vortex spacing is the responsibility of the pilot and pilots are advised to apply the appropriate recommended spacing. This will often also be advised by ATC.

En route

ATC traffic separation standards in controlled airspace will not necessarily prevent significant encounters with wake turbulence and the greater risk of injury because both cabin crew and some passengers will probably not be secured in their seats. However, it is unlikely that any loss of control will be more than very brief and easy recover from if at least minimum ATC separation standards are maintained.

The only available direct defence against occupant injuries is for the flight crew to maintain situational awareness by monitoring other traffic in the vicinity by listening out on RTF and by use of the TCAS Display and then use the seat belt sign and direct communication with Cabin Crew to temporarily secure all occupants if in-train climbing or one-level-above traffic is observed up to 10 nm ahead and confirmed with ATC as being a significantly larger aircraft type.

ATC awareness of the persistence of wake turbulence at en route altitudes beyond reqired traffic separation minima is sometimes poor. Reliable ground system support functions to inform and warn Air Traffic Controllers of potentially hazardous wake encounters are not yet in operational use. When an En-route Air Traffic Controller identifies a traffic proximity situation with risk of a potentially hazardous wake encounter, he/she may provide traffic information to the trailing aircraft, including a caution for potential wake turbulence and when possible, may propose a change of lateral or vertical flight path, as appropriate.

Accidents and Incidents

A list of all wake-related accidents and incidents on SkyBrary can be found here.

Further Reading

EASA

• Safety Information Bulletin No. 2017-10R1: En-route Wake Turbulence Encounters, 2 September 2021

FAA

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

UK CAA

• Safety Sense Leaflet No 15c - Wake Turbulence
• UK AIC P092/2017 - Wake Turbulence
• UK Wake Turbulence Categories, UK CAA website

HindSight Articles

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

Airbus

• Wake Vortices, C. Lelaie, Airbus Safety First Magazine No. 21, pp. 42-50, January 2016
• Airbus FOBN - Wake Turbulence Avoidance

Other

• 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)