Wake turbulence and separation: why there is extra room behind the A380

You are standing under the approach, an Airbus A380 glides over you – and then, for a while, nothing happens. No A320 right behind it, no tight chain like usual. A few seconds later there is a rushing sound over your head, even though the giant touched down long ago. Both have the same cause: wake turbulence. This guide explains how the vortices behind every aircraft form, which rules air traffic control uses to separate traffic because of them and what you actually notice of it at the fence.

Wake turbulence and separation: the short answer

Every aircraft that generates lift leaves behind two counter-rotating vortices. They are stronger the heavier the aircraft is, and they can make a following, lighter aircraft roll violently around its longitudinal axis. To prevent that, air traffic control divides all types into wake turbulence categories and keeps set minimum spacings between them – in nautical miles on approach, in minutes on takeoff. The bigger the weight difference between the leading and following aircraft, the bigger the gap.

How wake turbulence forms

A wing generates lift because the pressure on its upper side is lower than on its underside. At the wingtip, the air wants to equalize this difference and flows from below to above around the edge. Because the aircraft is moving forward at the same time, this flow rolls up into a vortex – one at each wing, with opposite rotation.

The time span matters: the vortices form from the moment the nose gear lifts off on takeoff and end only when it touches down again on landing. So if you stand at the runway end, you are dealing with vortices from the rotation point to landing.

How strong the vortices are depends mainly on three factors:

• Weight: The most important factor. The more lift the wing has to deliver, the stronger the vortices. A fully loaded long-haul jet at takeoff produces significantly more than the same type empty.
• Speed: Slow-flying aircraft need a higher angle of attack for the same lift – on approach and shortly after liftoff, the vortices are therefore especially pronounced.
• Configuration: Extended flaps change the shape and distribution of the vortices. The wing shape and the size ratio to the following aircraft also play a role.

What happens to the vortices afterward

Wake vortices do not stay put. They sink slowly – according to the technical literature, at higher altitude by roughly 90 to 150 meters per minute – and settle a few hundred feet below the flight path of the aircraft that created them. Near the ground they hit the surface, then spread out sideways and lose their danger in the process. A crosswind carries them away from the approach path – usually an advantage for the follower. Dangerous, on the other hand, is a light crosswind that holds a vortex right over the runway centerline.

The vortices survive longest in calm, stably stratified air with little wind. Turbulent air and strong wind speed up the decay. A typical lifetime is one to three minutes.

The four ICAO categories

So that controllers do not have to calculate anew for every pair of aircraft, ICAO divides all types into wake turbulence categories. The basis is the maximum takeoff weight (MTOW), not the actual weight on the day.

Category	Code	Criterion	Examples
Light	L	MTOW up to 7,000 kg	Cessna 172, Pilatus PC-12
Medium	M	over 7,000 kg and under 136,000 kg	Airbus A320neo, Boeing 737-800, Embraer E190, ATR 72-600
Heavy	H	from 136,000 kg	Airbus A350-900, Boeing 777-300ER, Boeing 787-9, Boeing 747-8
Super	J	types explicitly listed as Super	Airbus A380

The Super category exists because the A380, with a maximum takeoff weight well over 500 tons, sits far above all other passenger aircraft. Wake vortex measurements around its certification led to larger spacings behind it than behind an ordinary Heavy. A borderline case is the Antonov An-124: the UK aviation authority additionally classifies it as Super, ICAO does not. You can find more about the giants of the sky in the article on the largest aircraft in the world.

“Heavy” and “Super” on the radio

You can even hear the category: crews of Heavy types add the word “Heavy” to their callsign on first contact with a radio station; for the A380 it is “Super.” That way the controller and following crews know immediately what they are dealing with. What distinguishes a callsign from a flight number is explained in the article Callsign and flight number. What the law says about listening in is covered in Listening to air traffic radio: the legal situation.

Spacing on approach and takeoff

On approach, air traffic control separates by distance. The following values are the classic ICAO minimum spacings for radar separation on final approach, given in nautical miles (NM, 1 NM = 1.852 km). Where the table does not provide a special wake turbulence spacing, normal radar separation applies.

Leading ↓ / Following →	Light	Medium	Heavy	Super
Super (A380)	8 NM	7 NM	6 NM	normal radar separation
Heavy	6 NM	5 NM	4 NM	normal radar separation
Medium	5 NM	normal radar separation	normal radar separation	normal radar separation
Light	normal radar separation	normal radar separation	normal radar separation	normal radar separation

On takeoff, air traffic control works with waiting times. A Medium or Light behind a Heavy waits at least two minutes; behind an A380 it is three minutes for Medium and Light and two for a Heavy. If the lighter aircraft departs from a taxiway intersection further down the runway, the waiting time gets longer, because it lifts off earlier and is therefore longer at the altitude of the aircraft ahead.

You can do the math yourself on how many seconds a gap in miles amounts to at the runway end. An approach at 140 knots ground speed covers 6 NM in about two and a half minutes – enough time to readjust the camera after the A380.

RECAT-EU and time-based separation

The classic system has a weak point: a Heavy is not the same as a Heavy. An aircraft just above the limit is treated the same as a Boeing 777-300ER with full tanks. That costs capacity.

EUROCONTROL’s RECAT-EU therefore divides the fleet into six groups instead of three or four. Medium and Heavy are each split into an upper and a lower group, with Super and Light added. The basis was a data set of over 100,000 wake vortex measurements. For many pairings the spacings drop as a result, without compromising safety. The system was introduced in Paris in 2016, among other places, at Charles de Gaulle Airport, and in March 2018 at London Heathrow Airport. Capacity gains of up to eight percent are cited.

Heathrow also introduced time-based separation, live since March 24, 2015. The idea: in a strong headwind, all aircraft fly slower over the ground, so a fixed spacing in miles takes ever longer in time, and the landing rate drops. At the same time, the vortices decay faster in strong wind. If you set the spacing in seconds instead of miles, the landing rate stays stable. According to the project, on windy days an average of just under three more aircraft per hour land.

When it goes wrong: incidents that shaped the rules

The separation rules were not created at a desk but also grew out of accidents and incidents. Two cases show why they are taken so seriously:

• American Airlines Flight 587 (2001): An Airbus A300-600 crashed shortly after takeoff in New York after flying into the wake of a Boeing 747 ahead. The US investigation agency NTSB, however, saw the cause not in the vortex itself but in excessive rudder inputs in response to it, which overloaded the vertical stabilizer.
• A380 over the Arabian Sea (2017): A Challenger 604 business jet crossed the flight path of an Emirates A380 about 300 meters below it. It flew into the wake, rolled several times around its longitudinal axis and lost considerable altitude before the crew could recover it.

The second case also shows that wake turbulence is not only an airport topic. In cruise it usually causes only a short, hard jolt – but a good reason to stay buckled up on the plane.

What you notice of it as a spotter

Wake vortices are invisible, but not unnoticeable. At the right spots and in suitable weather, you experience them with all your senses.

A classic for such observations is Myrtle Avenue at Heathrow, where approaches come in very low over the road. At the Zeppelinheim viewing platform in Frankfurt or the Munich Airport visitors’ hill (Besucherhügel), too, you can observe the spacings between Heavies and Mediums well. How the final approach works technically and why all aircraft come in on the same line is explained in the guide to the ILS approach. Which side landings are made on is decided by the runway direction in use.

Wake turbulence over residential areas

Under the approach paths of large airports, wake turbulence is also an issue for residents. When low-flying vortices hit buildings, they can lift or loosen roof tiles. Around Frankfurt Airport, this has been reported in connection with places under the approach such as Flörsheim; many roofs there were subsequently secured with additional clips against such damage. For you as a spotter, that means: where vortices can move tiles, unsecured tarps, umbrellas or light bags will blow away too.

What to watch out for

• Category by takeoff weight, not by size. Whether a type counts as Medium or Heavy depends on maximum takeoff weight. That explains some surprising classifications.
• Headwinds make gaps bigger. In strong wind, aircraft arrive more slowly over the ground – fixed spacings in miles thus take longer in time, unless the airport uses time-based separation. How to read wind from the weather report is shown in Reading METAR and TAF.
• Parallel runways are a special case. If runways lie very close together, vortices can drift onto the neighboring runway. Additional rules then apply there. How runways are named is explained in the article on runway designations.
• Noise is not the same as vortices. The rushing sound after the flyover is something different from the engines’ exhaust jet. Jet blast hits you behind departing aircraft, vortices under landing ones.
• Never cross the barrier. Exciting vortex effects are no reason to get closer to the runway. What is allowed at the fence is covered in Is plane spotting allowed?.

Conclusion

Wake turbulence is the invisible companion of lift: two powerful, counter-rotating vortices that sink behind every aircraft, drift with the wind and decay after a few minutes. Because they can make lighter followers roll dangerously, air traffic control separates by weight category – with the largest spacings behind the A380. Modern procedures such as RECAT-EU and time-based separation win back capacity without giving up safety. For you at the fence, that means: the gaps in the approach chain follow a logic, and on humid days you can even see and hear the vortices.