Wake Turbulence
Encountering a sudden uncommanded roll caused by wake turbulence from an aircraft ahead of you can lead to you losing control of your aircraft, with sometimes fatal consequences.
Wake turbulence is caused by counter-rotating wing tip vortices and is a direct and automatic consequence of the generation of lift by a wing; it starts when the nose wheel of an aircraft leaves the ground on take off and stops when the nose wheel touches the ground during landing.
Lift is generated by the creation of a difference in pressure 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 from the wing tips. After the roll up is completed, the wake consists of two counter-rotating cylindrical vortices.
The strength of the vortex depends on the weight, speed, and shape of the wing of the generating aircraft and the largest factor in vortex generation is weight. Whilst you can experience wake turbulence if you fly too close to another GA aircraft, as the vortex strength increases proportionately with the weight of the aircraft, the vortices from an A380 are significantly larger than those from a 737. Vortices typically persist for one to three minutes, lasting longest in stable air conditions with low wind speeds, however a cross-track wind will carry them away from the flight path.
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 encountered most when flying close to the ground in the vicinity of airfields where aircraft are on approach to or departure from the runway 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 response of the pilot to counteract and can lead to the loss of the aircraft.
So, if you've ever flown through wake turbulence, you probably have a new found respect for how quickly things can go wrong, but what can you do about it? AVOID. RECOGNISE. RECOVER.
Avoiding Wake Turbulence during landing
When you're in the circuit with other aircraft, you should pay attention to their track and where they're landing. This is because wingtip vortices slowly sink, dissipate and drift with the wind. So when you're following an aircraft on final, you need to do two things to stay safe:
Stay above the aircraft's final approach flight path
Note the aircraft's touchdown point and land beyond it
By staying above their approach path, you will remain clear of the wake and by touching down beyond their landing point, you guarantee that you won't encounter their wake near the runway (providing the aircraft ahead is carrying out a full stop), that's because when an aircraft touches down, it stops producing wingtip vortices. If the aircraft ahead is carrying out a touch and go, be careful as vortices will still be present after the touchdown point once it rotates and starts to climb away.
Do both, and you'll avoid any wake turbulence problems during landing; however, remember your training. In all cases you should use the UK minimum distance separation shown in the table below (or convert that into a time based separation which is equivalent to the distance separation minima) to provide enough time for any wake turbulence to dissipate.
Avoiding Wake Turbulence during takeoff
Avoiding wake turbulence on takeoff isn't quite as straightforward as it is during landing. That's because if the aircraft in front you has better climb performance, you're going to have a hard time avoiding their wake. Therefore, to avoid wake turbulence on takeoff:
Rotate prior to the point at which the preceding aircraft rotated
Manoeuvre your aircraft to avoid the flight path of the preceding aircraft
Because vortex production starts when an aircraft takes off, it's important for you to lift off prior to the point the previous aircraft did. However, after you've lifted off, you are now subject to being in the wake of the preceding aircraft. GA aircraft don't climb nearly as fast as commercial aircraft, so if you maintain the same heading as the aircraft in front of you, the potential to fly through their wake is high. By manoeuvring left or right of the runway after takeoff, you can ensure you'll stay clear of the vortices, but which direction should you turn?
Wind and Wake Turbulence on takeoff
Wind is a key factor in avoiding wake turbulence because wingtip vortices drift with the wind. Because of this, if possible, you should turn your aircraft upwind after takeoff and away from the vortices of the preceding aircraft. Unfortunately, you may not always be able to manoeuvre upwind after takeoff, especially at busy airfields, so if you are constrained by this, what can you do? The good news is there's one final option: wait it out.
UK minimum separation
The UK conforms, in general, to the ICAO, standards on wake turbulence (see ICAO Wake Turbulence Category). However, experience at those UK aerodromes where an air traffic control service is provided and wake turbulence separation minima are applied, has shown that certain modifications to the relationship between the Maximum Certified Take Off Mass of an aircraft and the wake turbulence separation are advisable for the safety of operations. Therefore, a modification to the thresholds between the MEDIUM and LIGHT wake turbulence categories and the introduction of a separate category (SMALL) for separation purposes has been introduced in the UK (see NATS AIC P 083/2020: Wake Turbulence). The careful observance of these separation minima has prevented loss of control, as a consequence of wake turbulence encounters in the flight phases where they apply, but when they have been ignored, fatal accidents have followed sudden and rapid uncommanded rolls.
Here is a summary of the NATS wake turbulence separation minima for final approach:
and here is a summary of the wake turbulence separation minima for departures:
Rotary downwash
In some cases, getting close to large helicopters such as an A109 or S76 can also generate a loss of control situation due to, in this case, the downwash from the rotors. In a similar way to wake turbulence, you should try to remain clear of any rotary traffic, particularly during the take off and landing phases of flight. Encountering downwash can provide a similar effect to wake turbulence and can result, in some instances, in large uncommanded roll moments which, depending on their severity, may be unrecoverable. Therefore the AVOID. RECOGNISE. RECOVER principles of wake turbulence also apply to following rotary traffic.
Recognising Wake Turbulence
If after taking all precautions you do find yourself experiencing a rapid and prolonged uncommanded roll during landing or take off behind another aircraft, you are likely experiencing wake turbulence.
Recovering from wake turbulence
You must keep the aircraft flying and get away from the ground as a matter of priority. Therefore, if you do find yourself suffering from the effects of wake turbulence you should:
Rapidly roll the wings level
Maintain flying speed
Climb away from the ground at your aircraft’s best rate of climb
Summary
Wake turbulence can affect any aircraft, large or small. The smaller the aircraft, the more susceptible it is to it. But remember, wake turbulence can be caused by a light aircraft too. So, think about your spacing on takeoff and final when there is traffic ahead, both fixed wing and rotary, and if in doubt, revert back to your training and leave the appropriate distance or time between you and the aircraft in front to let the wake turbulence vortices dissipate. However, if you do end up in a severe wake turbulence situation then remember to: AVOID. RECOGNISE. RECOVER.
Related articles and further reading
AAIB Bulletin: 7/2010. PA28 accident, rotor downwash from S76
DA42 Loss of Control on Approach during Runway Lighting Calibration Flight
NATS
AIC P 83/2020: Wake Turbulence
ICAO
EASA
Safety Information Bulletin No. 2017-10: En-route Wake Turbulence Encounters, 22 June 2017.