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En-route Wake Vortex Hazard

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Category: Wake Vortex Turbulence Wake Vortex Turbulence
Content source: SKYbrary About SKYbrary
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Definition

Wake vortex encounter (WVE) is often recognised as a safety issue for aircraft on initial climb and final approach. However, wake turbulence can also be encountered en route, where the risk of injury to occupants will be increased since seat belt signs will often be off and/or cabin service in progress. This article discusses the safety issues related to en-route wake vortex hazard.

What is Wake Vortex?

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.

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. Once formed, vortices descend until they decay. Certain meteorological conditions, such as stable atmosphere and temperature inversion, sustain wake vortices and make wake encounter more likely.

Effects

The potential for significant wake vortex turbulence in the cruise is greatest where an aircraft is following a similar track to another heavier one ahead that is either at the same level or the next available level above. The extent of the ‘in trail’ case can be extrapolated to climb and descent but it all cases, the important thing to note is that, especially for following aircraft two categories or more below that of the leading aircraft, significant wake turbulence can be encountered at greater separation than is required in en route airspace where radar control service is provided. In the case of crossing tracks, it has been found that the time interval necessary to avoid creating a wake vortex hazard is 3 minutes.

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. 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

ATC traffic separation standards in controlled airspace will not necessarily prevent significant encounters with wake turbulence and a greater risk of injury exists because passengers and/or Cabin Crew will often not be secured in their seats. However, it is unlikely that any loss of control will be more than very brief and easy to 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, listening out on RTF and by use of the TCAS display; if similar track climbing or, when level, one-level-above traffic is observed up to 20 nm ahead and known to be a significantly heavier aircraft type, then the seat belt sign can be switched on and an appropriate Cabin announcement made. At the discretion of the aircraft commander, cabin crew may also be advised to temporarily cease any cabin service and secure themselves and their equipment.

ATC awareness of the persistence of wake turbulence at en route altitudes, beyond required traffic separation minima, is sometimes poor.

Typical Scenarios

  • A 50 seat regional jet level at FL 370, with cabin service in progress and the seat belt signs off, encounters unexpected wake turbulence from an overtaking B777 also level at FL370 as it rejoins the same track 7nm ahead. 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.
  • A Medium-category jet aircraft at FL360 encounters wake turbulence from a Heavy-category type aircraft descending 8 Nm ahead from FL380 through the same level. This induced an uncommanded large roll, up to about 60°, with altitude variation. The Captain immediately switched off Autopilot, turned the control wheel 43 degree to one side and after 2-3 seconds 15 degree to other side. Due to pilot action aircraft was brought back to wings level position.
  • A Medium-category jet aircraft enroute at FL320 had had just been cleared to climb to FL360. A Heavy-category type, enroute at FL330, had just passed in the opposite direction. The Medium-category jet aircraft suddenly rolled and the autopilot disconnected. The first officer assumed manual control of the aircraft, levelled the aircraft and re-engaged the autopilot. Both flights continued to their destinations for safe landings, however, 4 people on board the encountering aircraft received minor injuries as result of the wake turbulence encounter.

Contributory Factors

  • Leading aircraft weight - Heavy category types, in particular with MTOW above 350T (incl. AIRBUS A-340-500, AIRBUS A-340-600, AIRBUS A-380-800, BOEING 747-400 (international, winglets), BOEING 747-8, BOEING 777-300ER) induce strongest wake turbulence levels.
  • Relative size of leading and following aircraft;
  • Relative tracks, positions and lateral/vertical separation of proximate aircraft - the risk is greater for aircraft following the same track/profile than for the cross-track case aircraft which are climbing or descending behind a Heavy category aircraft in level flight or an aircraft in level flight with a Heavy category aircraft climbing or descending ahead;
  • flying below the tropopause - the atmospheric conditions are generally favourable for the wake vortex to remain strong for a longer period of time, and the wake vortices may potentially descend one flight level lower;
  • Wind velocity relative to the track being flown by the generating aircraft - cross-track wind reduces the risk to in-train aircraft

Accidents and Incidents

The following events on the SKYbrary A&I database feature Wake turbulence encountered in trail:

  • 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.)
  • 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.)
  • 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.)
  • 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.)
  • … further results


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