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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 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 are, 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.

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.
  • 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

  • A319 / B744, en-route near Oroville WA USA, 2008 - On 10 January 2008, an Air Canada Airbus A319 en route over the north western USA encountered unexpected sudden wake vortex turbulence from an in trail Boeing 747-400 nearly 11nm ahead to which the pilots who then responded with potentially hazardous flight control inputs which led to reversion to Alternate Control Law and aggravated the external /disturbance to the aircraft trajectory with roll up to 55° and an unintended descent of 1400 feet which with cabin service in progress and sea belt signs off led to cabin service carts hitting the cabin ceiling and several passenger injuries, some serious.
  • A320, en-route, North East Spain 2006 - On 28 May 2006, a Vueling Airbus A320 encountered sudden significant turbulence at FL325 and, during a temporary loss of control, was forced down to FL310 before recovery was achieved. Seven occupants sustained minor injuries and there was some internal damage caused by an unrestrained cabin service cart. The origin of the disturbance was found to have been wake vortices from an Airbus A340-300 which was 10nm ahead and 500 feet above on the same airway but the Investigation found that the crew response had been inappropriate and could have served to exacerbate the effects of the external disturbance.
  • A306, vicinity JFK New York USA, 2001 - On November 12, 2001, an Airbus Industries A300-600 operated by American Airlines crashed into a residential area of Belle Harbour, New York, after take-off from John F. Kennedy International Airport, New York. Shortly after take off, the aircraft encountered mild wake turbulence from a departing Boeing 747-400.
  • B733, en-route, Santa Barbara CA USA, 1999 - On 2 September 1999, a United Airlines Boeing Boeing 737-300 in the cruise at FL240, experienced severe turbulence due to an encounter with the wake vortex from a preceding MD11 on a similar track which had climbed through the level of the B737 with minimum lateral separation, 1.5 minutes earlier.
  • B735, en-route, North East of London UK, 1996 - On 5 September 1996, a Boeing 737-500 operated by British Midland, encountered severe wake turbulence whilst in the hold over London. The wake was attributed to a B767 some 6 nm ahead.
  • C185, Wellington New Zealand, 1997 - On Monday 3 March 1997 at 1014 hours, privately owned and operated Cessna 185 encountered wake turbulence from previous departing aircraft, the pilot lost control of the aircraft at a height from which recovery was not possible and the aircraft descended to the ground.
  • E170, en-route, Ishioka Japan, 2014 - On 29 April 2014, an Embraer E170 being operated in accordance with ATC instructions in smooth air conditions suddenly encountered an unexpected short period of severe turbulence which led both members of the cabin crew to fall and sustain injury, one a serious injury. The Investigation concluded that the turbulence encountered, which had occurred soon after the aircraft began descent from FL110, was due to an encounter with the descending wake vortex of a preceding Airbus A340 which had been approximately 10 nm and 2 minutes ahead on the same track and had remained level at FL 110.
  • P28A / S76, Humberside UK 2009 - On 26 September 2009, a Piper PA28-140 flown by an experienced pilot was about to touch down after a day VMC approach about a mile behind an S76 helicopter which was also categorised as 'Light' for Wake Vortex purposes rolled uncontrollably to the right in the flare and struck the ground inverted seriously injuring the pilot. The Investigation noted existing informal National Regulatory Authority guidance material already suggested that light aircraft pilots might treat 'Light' helicopters as one category higher when on approach and recommended that this advice be more widely promulgated.
  • WW24, vicinity John Wayne Airport Santa Ana CA USA, 1993 - On 15 December 1993, the crew of an IAI Westwind on a domestic passenger charter flight failed to leave sufficient separation between their aircraft and the Boeing 757 ahead on finals in night VMC and lost control or their aircraft which crashed killing all occupants and destroying the aircraft in the impact and post-crash fire.

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

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. [Note: not all NAAs fully adopt ICAO Recommendations in this matter]
  • 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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