Although the dangers of wake turbulence are well known, the A380-slipstream incident over the Arabian Sea (http://bit.ly/2nUJ1ly), that rendered a German-registered Challenger 604CL damaged beyond economical repair last year, triggered EASA in publishing a safety information bulletin on wake turbulence (http://bit.ly/2H6fyNn).

Upside down

The 7 January 2017 incident occurred as the aircraft was flying at 34,000 ft. between Male, Maldives and Abu Dhabi’s Al-Bateen airport, when a UAE-registered A380 flying at 35,000 ft. from Dubai to Sydney, Australia, passed overhead. Around 45 seconds later, the Challenger began to roll, reaching 42 deg bank angle in one second. The airplane continued to roll despite control inputs by the crew and it completed three rotations around its longitudinal axis. According to the BFU’s interim report (http://bit.ly/2EED31X), flight data recorders showed vertical accelerations up to 3.2G, a height loss of 8,700 ft. and airspeeds up to 330kts before the pilots managed to regain control and diverted to Muscat, Oman. One passenger suffered from head injuries and a broken rib; another fractured a vertebra. The other passengers and the flight attendant sustained minor injuries.

It was by no means the first event involving wake turbulence from an A380:

• On 14 September 2012, a Virgin Australia Boeing 737-800, en route from Denpasar to Brisbane, encountered wake turbulence south of Bali from an A380 travelling in the opposite direction 1000 ft above, about 2 nm behind, and slightly left of the Boeing’s track. The aircraft rolled initially to the right, then to the left, with a maximum left-bank angle of about 40 degrees.
• On 16 October 2011 over Germany, a westbound British Airways A320 on climb from FL320 encountered severe wake turbulence from an eastbound Qantas A380 at FL330. The A320 was subjected to forces of up to +1.93G and rolled between -26 degrees and +32 degrees. Four passengers suffered minor injuries.
• On 3 November 2008, the crew of a Rex Saab 340B reported a temporary loss of control while about 7 nm from touchdown and turning onto final approach for runway 34R at Sydney. The Saab rolled 52 degrees to the left, then 21 degrees right, pitched down 8 degrees and lost between 300–400 ft in altitude. One person suffered minor injuries. The Australian Transport Safety Bureau (ATSB) found that the cause was wake turbulence which had drifted in a westerly crosswind from an A380 operating about 3.4 nm ahead on the parallel runway 34L. The ATSB calculated that with winds of about 35 knots at 2400 ft, the A380’s wing vortices took 72 seconds to cover 1300 metres.

What is wake turbulence?

Wake turbulence is an inevitable by-product of flight. It’s the result of differential pressure between the upper and lower surfaces of a fixed or rotating aerofoil. The turbulence is caused by the roll up of airflow behind the wingtips, creating a clockwise vortex behind the left wingtip and an anticlockwise one behind the right wingtip.

The vortices are generated the whole time an aircraft is airborne. While generally only a few metres in diameter, they can be very intense, depending on the aircraft’s weight, wingspan, configuration and attitude. Size really does matter, though there are exceptions; some aircraft, such as the Boeing 757, have a reputation for producing particularly intense vortices.

What makes wake vortices particularly dangerous is that they can persist some distance behind, and below, the aircraft generating them. En route, an aircraft’s wake can extend for more than 25 nm, and descend very slowly downwards and outwards—levelling off around 1000 ft below the generating aircraft.

This means encounters can occur when an aircraft passes below the flight path of another aircraft—even though ATC vertical separation is being applied. In the terminal environment, the concentration of aircraft of different sizes increases the risk of exposure to wake vortices.

What to do?

The downward or upward force from a wake turbulence vortex is likely to cause a following aircraft to initially roll in one direction. The natural reaction is to try to counter the roll with opposite aileron and rudder. Counterintuitively, however, this is likely to make matters worse.

This is because as the aircraft enters the vortex, it is likely to experience a stronger roll in the direction of the vortex opposite to that encountered initially as shown by Airbus flight tests (http://bit.ly/2BW8BiD). This second roll would be amplified by any force applied by the pilots to correct the initial roll. It is thought that such overcorrection contributed to the severity of the A380/Challenger encounter.

In the absence of specific procedures in the aircraft flight manual, the advice is to avoid abrupt reverse control inputs, and instead allow the aircraft to pass through the vortex and then recover. Pilots should not use rudder to counteract the effects of wake turbulence, because this can create forces beyond the aircraft’s structural limits. The advice is also to leave the autopilot engaged, but be ready to resume manual control if it disengages itself.

Remember:

• Initially just wait (Do not voluntarily disconnect the autopilot)
• Release controls (Resist the urge to immediately move the controls)
• Do not use the rudder to counteract the effects
• Only once clear of turbulence: start recovery control inputs

How to avoid?

The most obvious solution is to avoid the vortices from another aircraft in the first place.

En route, offset upwind from the route centreline – after obtaining ATC clearance – or request a level change.

In a controlled environment, air traffic controllers will apply wake turbulence separation between instrument flight rules (IFR) aircraft in flight and, for take-offs, between all aircraft. However, if the pilot of an IFR flight accepts a clearance to visually follow a preceding aircraft, the pilot is responsible for both separation and wake turbulence avoidance.

Controllers will provide a wake turbulence cautionary advice to all controlled flights if, in the controller’s opinion, wake turbulence may have an adverse effect.

Whenever possible, try to visualise the wake vortices (e.g. contrails).

If unstable on final, go around.

Take-off and landing

En-route

New ATC separations (RECAT)

The ICAO separation minima (see PANS-ATM table below) are used worldwide except in two countries, USA and UK. These two Countries apply a different classification with different weight limits and separations.

However due to increased demand the USA and Europe have started a re-categorization (RECAT). Its objective is to reduce the approach and departure separation between some aircraft pairs, without safety degradation, to improve the landing capacity of a given runway or runway pair. The aim is that RECAT should not create a scenario that is worse than that which exists today with ICAO separations.

While the FAA (RECAT 1 FAA) decided to reclassify the aircraft by MTOW and wingspan, Europe’s RECAT 1 EU (see table below) takes into consideration not only the strength of the wake vortex of the leader aircraft, but also the resistance of the follower. The RECAT EU was approved by EASA end 2014, and implemented at CDG in 2016 among other airports worldwide. Implementation is not mandatory and only the most important European airports will use it, the other ones will keep the ICAO separations.

RECAT 2/3 or “pair-wise” separation, taking into account leader and the follower types, possibly by groups of aircraft, will be implemented in the coming years. Read more on Skybrary: http://bit.ly/2stft2W.

References

• European Aviation Safety Agency (2017) SIB 2017-10: En-route Wake Turbulence Encounters – http://bit.ly/2CgWQPn
• Rosenkrans W. (2013), Outmaneuvered Airflow – http://bit.ly/2suHNCqOsborne T. (2017) German Challenger Totaled After A380 Wake Turbulence – http://bit.ly/2nUJ1ly
• Bundesstelle für Flugunfalluntersuchung (2017) Interim Report BFU17-0024-2X – http://bit.ly/2EED31X
• Airservices Australia (2017) Aeronautical Information Circular H30/17 – http://bit.ly/2Bpqqpf
• Federal Aviation Administration (2017) Circular 90-23G – http://bit.ly/2EEDaup
• Airbus (2016) Safety First Magazine Ed. January 2016 pp. 42-51 – http://bit.ly/2BW8BiD
• Skybrary (2016) RECAT – Wake Turbulence Re-categorisation – http://bit.ly/2stft2W