Every wing that makes lift also makes a pair of trailing vortices, and wake turbulence separation is the system that stops a following aircraft flying into the invisible, rolling air a heavier one leaves behind.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Where the wake comes from
Wake turbulence is not engine exhaust. It is the trailing vortex that forms whenever a wing produces lift: higher pressure under the wing spills around the tip to the lower pressure above, rolling into two counter-rotating cores that stream behind the aircraft.
Their strength depends on the wing loading and how it is flying. Vortex intensity rises with weight and angle of attack and falls with speed, so the strongest wake comes from an aircraft that is heavy, slow and clean, with little flap or gear to spread the load. That is exactly the configuration on take-off and on short final, which is why most wake encounters happen near the ground.
Once shed, the vortices sink a few hundred feet below the flight path and drift with the wind. A light crosswind can hold one stationary over the runway, or carry it onto a parallel runway, which is why a calm or light-crosswind day can be the trickiest.
The ICAO categories
To turn this into a workable rule, ICAO Doc 4444 (PANS-ATM) groups aircraft by maximum certificated take-off mass:
- Light (L): 7000 kg or less.
- Medium (M): more than 7000 kg but less than 136 000 kg.
- Heavy (H): 136 000 kg or more.
- Super (J): the largest types, specified as such in ICAO Doc 8643. Today that is the Airbus A380. (Some authorities, including the FAA, additionally treat the Antonov An-225 as super under their own national schemes, but under ICAO the An-225 is Heavy.)
Each aircraft type's category is published alongside its designator in ICAO Doc 8643, and it is the category of the aircraft ahead (the one generating the wake) and the one behind (the one at risk) that together set the spacing.
How separation is applied
The principle is simple: a lighter aircraft following a heavier one needs more room, because it is the one most easily upset. Controllers apply this as a distance behind an aircraft in the air, or as a time interval between departures from the same runway, with the largest gaps behind Super and Heavy types and progressively less behind Medium and Light.
The exact minima are where it gets local. The baseline values are in Doc 4444 and the FAA AIM, but many regions now use recategorisation schemes, such as RECAT-EU and the FAA's RECAT, that divide the fleet into more groups to use airspace more efficiently. Because of that, the figures differ by region and keep evolving, so treat the separation the controller applies, and your operator's procedures, as the authority rather than a memorised table.
Common pitfalls
- A wake can outlast the aircraft. The vortices linger after the generating aircraft has gone, so a clear-looking runway is not always clear air.
- It sinks and drifts. Stay at or above the heavier aircraft's path, and watch the crosswind that can move a vortex where you do not expect it.
- "Caution wake turbulence" shifts the responsibility. When ATC passes that and clears you to follow, accepting visual separation can make the spacing yours to judge.
- Categories are standard; minima are not. Do not assume the numbers from one country apply in another.
In Pilot EFB
Pilot EFB keeps a quick wake-turbulence separation reference so the categories and the principle are easy to recall on the ground during planning. It is a personal reference on your device; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so the separation issued by air traffic control and your operator's procedures are always the authority on the day.