Description

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.

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 flight controls or the prevailing response of the flight crew to counteract. The high rate of roll may cause uncommanded disconnection of the Autopilot and result in transient or, in rare cases, terminal loss of control resulting in terrain impact.

While en route, 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.

Instances in which wake vortex has caused structural failure have been rare. The greatest hazard of a WVE is induced roll and yaw. This is especially dangerous during takeoff and landing when there is little altitude for recovery. Short wing span aircraft, such as most general aviation types, are most susceptible to wake turbulence. The wake vortex induced roll rates can be extreme and countering roll rates may be difficult or impossible even in a high performance aircraft with excellent roll control authority.

Immediate Actions

In the event of a Wake Vortex Encounter, pilots should immediately take the following actions:

• Power - Whenever you are low and slow as will be the case in a WVE during approach, add the power - you'll need it
• Push - Unload the wings or "push" on the yoke until you are slightly "light in the seat". This reduces the angle of attack of the wings which gives better roll control with the ailerons. It simultaneously reduces the drag on the aircraft for better acceleration and, if you are rolling over, slows your descent towards the ground.
• Roll - If aircraft controlability allows a choice, roll (unloaded) to the nearest horizon. If there isn’t a nearest horizon, or if you have significant rolling momentum, continue to roll (unloaded) in that direction to the horizon.
• Go Around - Do not try to salvage a landing after a WVE during approach. Depending upon the atmospheric conditions, there is a risk of re-entering the wake vortex from the preceeding aircraft later in the approach at a lower altitude where a recovery might not be successful.

Further Reading

ICAO

• ICAO Doc 9426 Air Traffic Services Planning Manual, Part II, Section 5 gives detailed characteristics of wake vortices and their effect on aircraft.

EASA

• Safety Information Bulletin No. 2017-10: En-route Wake Turbulence Encounters, 22 June 2017

Other

• Wake Votrices, C. Lelaie, Airbus Safety First Magazine No. 21, pp. 42-50, January 2016
• Good Aviation Practice - Wake Turbulence by Civil Aviation Authority of New Zealand
• 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)