You cannot see it. You can only feel it — usually a fraction of a second too late.
Wake turbulence is one of the few hazards in aviation that leaves no trace on a METAR and no symbol on a screen. It is pure aerodynamics: the invisible price every wing pays for producing lift. And because it sits right at the intersection of Principles of Flight (081) and Operational Procedures (070), EASA loves to test it.
Master the physics once and you answer three different exam questions with a single mental model. Here is that model.
Why every wing sheds a vortex
Lift exists because the pressure under the wing is higher than the pressure above it. That pressure difference does not stop politely at the wingtip. At the tip, high-pressure air from below spills around the end of the wing toward the low-pressure region on top.
The result is a rotating mass of air — a wingtip vortex — trailing from each tip. Seen from behind, the two vortices counter-rotate: they drive air downwards between them (downwash) and upwards outboard of each tip (upwash).
This is not a side effect you can engineer away. As long as a wing makes lift, it makes vortices.
The hidden link: vortices and induced drag
Here is the reasoning EASA wants you to connect. The downwash created by those vortices tilts the effective lift vector slightly rearward. That rearward component is induced drag — the drag due to lift.
Two consequences fall straight out of the physics, and both are exam favourites:
- Induced drag rises as speed falls. Slow down and you need a higher angle of attack; the vortices strengthen and induced drag increases. That is why induced drag dominates the low-speed end of the drag curve and is greatest just above the stall.
- Winglets and high aspect ratio reduce it. Anything that weakens the tip vortex — a winglet, or a long, narrow, high-aspect-ratio wing — cuts induced drag. That is the whole reason winglets exist.
Same vortex, two topics. If a question asks why a glider has long thin wings, or why induced drag peaks at low IAS, you are really being asked about the diagram above.
When the vortex becomes a hazard
Behind a large aircraft, those trailing vortices are violent enough to roll a following aeroplane faster than its ailerons can recover. Three factors set their strength, and EASA students remember them as Heavy, Clean and Slow:
- Heavy — more weight means more lift, which means stronger vortices.
- Clean — flaps and gear up concentrate the vortex; a “dirty” configuration spreads and weakens it.
- Slow — low speed and a high angle of attack produce the strongest circulation.
The worst offender is therefore a large aircraft in the take-off or landing phase: heavy, relatively clean, and slow. The vortices then sink below the generating aircraft’s flight path — typically a few hundred feet per minute, levelling off several hundred feet below — and drift with any crosswind. A light crosswind is dangerous precisely because it can hold one vortex over the runway centreline instead of blowing it clear.
EASA wake turbulence categories
Because the hazard scales with mass, ICAO — and therefore EASA operations — sort aircraft into wake turbulence categories by maximum certificated take-off mass (MTOM):
- Light (L): 7,000 kg or less.
- Medium (M): more than 7,000 kg but less than 136,000 kg.
- Heavy (H): 136,000 kg or more.
- Super (J): a special category, in practice the Airbus A380-800.
At Europe’s busiest hubs you will also meet RECAT-EU, a six-category refinement (from Super Heavy down to Light) that uses both MTOM and wingspan. It is already in daily use at Paris Charles de Gaulle, London Heathrow, Vienna and Barcelona, and it generally allows slightly tighter — but still safe — spacing. For the theory exam, know the classic four categories and recognise that RECAT-EU exists and why.
Separation: it protects the follower
The golden rule: wake turbulence separation always protects the lighter, following aircraft. A Heavy behind a Light needs no wake spacing; a Light behind a Heavy needs plenty.
On radar final approach, ICAO minima add distance as the mass gap grows — for example a Medium following a Heavy is separated by 5 NM, and a Light behind a Heavy by 6 NM, against a baseline of 3 NM where no wake requirement applies. On departure, controllers apply time — commonly a 2-minute spacing behind a heavier aircraft, increased to 3 minutes from an intermediate point on the runway. Exact figures live in ICAO Doc 4444 and the local AIP; the principle — bigger mass difference, bigger gap — is what the exam tests.
Flying clear of it
The avoidance techniques come straight from that sink-and-drift behaviour:
- Landing behind a heavier aircraft: stay at or above its approach path and land beyond its touchdown point — the vortices begin where it rotated and then sink behind it.
- Departing behind a heavier aircraft: rotate before its rotation point and climb above its path, turning upwind of the track.
- En route or crossing beneath: keep vertical distance and cross on the upwind side.
How this shows up in the exams
Expect it three ways: an 081 question on why induced drag is highest at low speed; a 070 or 010 question on which aircraft carries the separation responsibility, or on the mass boundaries above; and a scenario question on the safest place to touch down behind a landing Heavy. All three reduce to one picture — pressure differential, roll-up, sink and drift.
Learn the mechanism rather than the trivia, and wake turbulence turns into free marks. Want to drill exactly these Principles of Flight and Operational Procedures questions until they are automatic? Practise them at atpltraining.io.
Frequently asked questions
What causes wake turbulence?
Wake turbulence is caused by wingtip vortices — rotating air that forms when high-pressure air beneath the wing spills around the tip to the low-pressure region above. Any wing producing lift generates them.
What are the EASA wake turbulence categories?
By maximum certificated take-off mass: Light (7,000 kg or less), Medium (more than 7,000 kg but less than 136,000 kg), Heavy (136,000 kg or more) and Super (in practice the Airbus A380-800). Major European airports also apply the six-category RECAT-EU scheme, which additionally uses wingspan.
Why is induced drag greatest at low speed?
At low speed the wing needs a higher angle of attack to produce the required lift, which strengthens the wingtip vortices and their downwash. Because induced drag comes from that downwash, it peaks at low speed and just above the stall.
Which aircraft is responsible for wake turbulence separation?
Separation protects the following — usually lighter — aircraft, and the required spacing increases as the mass difference between the leading and following aircraft grows.
