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Wake turbulence and the "Heavy" callsign
The invisible hazard that sets the spacing on every final approach.
Every aircraft that flies leaves two spinning tornadoes of air behind it. A wing makes lift by having higher pressure underneath than on top, and at each wingtip the air curls from below to above, rolling up into a pair of counter-rotating wingtip vortices that trail behind the aircraft for miles. Fly into one and it can roll a smaller aircraft faster than its controls can counter. That is wake turbulence, and it is the reason air traffic controllers space aircraft further apart behind a large one than behind a small one.
When it is worst
A wake is strongest behind an aircraft that is heavy, clean and slow — in other words, an airliner taking off or landing, flaps not yet out or speed low and weight high. The vortices sink a few hundred feet per minute and level off roughly 500 to 1,000 ft below the generating aircraft's flight path, drift with the wind, and can linger for a couple of minutes in calm air — which is exactly where the next aircraft on the approach will be. Close to the ground they spread sideways and can drift onto a parallel runway.
The wake categories
For separation purposes ICAO puts every aircraft type into a wake turbulence category by its maximum take-off mass. The letter is filed in item 9 of the flight plan and shown on the controller's flight strip.
| Category | Maximum take-off mass | Examples |
|---|---|---|
| J — Super | Specifically designated types | Airbus A380 |
| H — Heavy | 136,000 kg or more | Boeing 777, 787, 747, Airbus A330, A350 |
| M — Medium | Over 7,000 kg, less than 136,000 kg | Airbus A320 family, Boeing 737, Embraer E-jets |
| L — Light | 7,000 kg or less | Light aircraft, small business aircraft |
Heavy and Super flights add the word to their callsign on first contact — "Emirates two zero one Heavy", "Qantas one Super" — so that everyone on the frequency knows what is ahead of them.
Separation behind a heavier aircraft
When one aircraft follows another at the same level or within 1,000 ft below it — on final approach above all — the ICAO radar separation minima grow with the leader's category:
| Leader | Follower | Minimum |
|---|---|---|
| Super (A380) | Heavy | 6 NM |
| Super (A380) | Medium | 7 NM |
| Super (A380) | Light | 8 NM |
| Heavy | Heavy | 4 NM |
| Heavy | Medium | 5 NM |
| Heavy | Light | 6 NM |
| Medium | Light | 5 NM |
Every other pairing uses the normal radar minimum, typically 3 NM in a terminal area. Departures are spaced by time instead: a lighter aircraft taking off behind a heavier one usually waits two minutes, and longer behind an A380, so that the wake has time to sink and drift away.
RECAT and time-based separation
Four categories are a blunt instrument — a 737 and a 757 are both "Medium", yet one makes a far stronger wake. Modern schemes known as RECAT (in Europe, RECAT-EU) split aircraft into more categories with distances tailored to each pair, letting airports such as Paris and London fit more landings into an hour without losing safety. Heathrow went further in 2015 with time-based separation, which spaces arrivals by time rather than distance: in a strong headwind, aircraft fly slower over the ground and the wake blows away faster, so a fixed time keeps the runway flowing where a fixed distance would not.
On the frequency
Controllers warn crews whenever wake might be a factor — "caution wake turbulence, the preceding landing aircraft is an Airbus A350" — and a pilot who wants more room can always ask for it. Pilots also fly to stay out of wakes: above the preceding aircraft's glide path, and landing beyond its touchdown point.
In the simulator
Every flight strip in the ATC Radar Simulator shows the aircraft type and wake category — B77W/H, A320/M — exactly as on a real strip, and the mix of heavies and mediums is what makes sequencing a stream onto the ILS interesting. The simulator enforces a single minimum gap on final rather than the full wake table, but the strips let you practise the real habit: check what is ahead, and leave the extra miles behind a heavy. See also holding patterns for where the extra spacing comes from on a busy day.