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Wake Vortex Turbulence

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Category: Wake Vortex Turbulence Wake Vortex Turbulence
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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
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 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.

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-train 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.

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.

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 and is described in detail in the article on ICAO Wake Turbulence Category.
  • 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.

Typical Scenarios

  • An Airbus A319 turns onto final approach behind a Boeing 747 with the recommended spacing but still encounters wake vortex turbulence which induces such a high rate of roll that the autopilot disconnects requiring prompt manual recovery action.
  • A small turboprop aircraft departing behind a widebody jet after applying the prescribed separation timing to the large aircraft start-of-roll time instead of the rotation time then encounters severe wake vortex turbulence immediately after take-off which causes an uncommanded rapid roll leading to a 60 degree bank angle and near impact with the terrain below.
  • An aircraft on base leg encounters the vortices which have drifted sideways from a heavier aircraft on final approach ahead and experiences a sudden but transient pitch disturbance.
  • A business jet which has accepted a visual approach with self positioning behind an A320 ahead on finals fails to apply the recommended separation and experiences a violent uncommsanded roll from which recovery is only just achieved before terrain impact.
  • A light aircraft making an approach to a grass strip parallel to the main runway independent of APP ATC is not flying the recomended vertical profile of greater than 3 degrees and encountersa the wake turbulence generated by an overtaking Boeing 737-900 causing a sudden wing drop from which control is only just regained.
  • A 50 seat regional jet level at FL 370 encounters wake turbulence from an overtaking B777 also level at FL370 as it rejoins the same track 7nm ahead but, despite uncommanded autopilot disconnect, recovery to wings level is quickly achieved by PF, although not before one of the Cabin Crew has been thrown sideways and seriously injured by impact with a bulkhead and a passenger using the toilet has been slightly injured after being thrown against the compartment wall.

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 velocity relative to the track being flown by the generating aircraft - cross-track wind reduces the risk to in-train aircraft
  • Wind velocity - light winds delay decay
  • Turbulence, from sources other than wake vortex, accelerates vortex decay

Solutions

  • 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.
  • 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).

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