Wednesday, March 4, 2009

Wake Vortex Turbulence -Skybary-


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Definition

Turbulence which is generated by the passage of an aircraft through the air.

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, they are 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.

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 generally persist for between one and three minutes, with survival 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 occurs much more quickly at low level 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 with close spacing. 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. 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 Wake Turbulence Category at [[1]].
  • 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 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.

Typical Scenarios

  • An aircraft turns onto the final approach behind a heavier aircraft and encounters wake vortex turbulence which induces such a high rate of roll that the AP disconnects requiring prompt manual recovery action by the flight crew.
  • A small turboprop aircraft departing behind a widebody jet after applying the prescribed separation timing to the large aircraft start-of-roll 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 from a heavier aircraft on approach ahead and experiences a violent but transient pitch disturbance.
  • A 50 seat regional jet encounters wake turbulence from an overtaking B777 also 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.

Contributory Factors

  • Leading Aircraft Weight
  • Leading Aircraft speed.
  • Leading Aircraft wing configuration (Flap setting etc.)
  • Relative size of leading amd following aircraft.
  • Relative position and lateral/vertical separation of aircraft.
  • Closeness to the ground.
  • Wind velocity relative to the track being flown

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).

Related Articles

Further Reading

  • FAA Aeronautical Information Manual, Chapeter 7 (Safety of Flight), Section 3 wake turbulence

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