🟢 In 60 seconds
Wake turbulence forms behind wings as counter-rotating wingtip vortices. Strongest when heavy + slow + clean (approach / take-off). Vortices descend 400–500 fpm to ~900 ft below the generating aircraft and persist 2–3 min in calm air. Major risk for following light aircraft.
Wake turbulence forms behind wings as counter-rotating wingtip vortices. Strongest when heavy + slow + clean (approach / take-off). Vortices descend 400–500 fpm to ~900 ft below the generating aircraft and persist 2–3 min in calm air. Major risk for following light aircraft.
Wake Turbulence Avoidance
Wake turbulence consists of counter-rotating vortices generated at the wingtips of an aircraft as the wing produces lift. They can cause severe loss of control for the following aircraft.
Source: ICAO Doc 4444 PANS-ATM Chapter 5 (Separation); EUROCONTROL RECAT-EU (Re-Categorisation, since 2015).
Physics of wake turbulence
- Cause: lower pressure on the upper surface and higher pressure on the lower surface of the wing → air flows from high to low at the wingtip → two counter-rotating vortices.
- Onset during flight: vortices begin as soon as the wing produces lift — that is, the moment the aircraft lifts off the ground (nose / main gear leaves the runway) at rotation and the wing carries the weight. They cease when the wing no longer produces lift — i.e. at touchdown.
- Strength: proportional to weight (more weight = more lift = stronger vortices) and inversely proportional to wingspan (shorter span = more concentrated vortices) and speed (slow speed → higher AoA → stronger vortices).
- Especially strong near the ground and in slow, heavy configuration (e.g. heavy, slow, flaps out — i.e. take-off or final).
- Behaviour in air:
- Vertical: descend at about 400–500 fpm for 1–2 min, then stabilise about 500–900 ft below the generator.
- Horizontal: drift with the wind; with a crosswind they move to the leeward side at wind speed.
- Lifetime: typically 2–5 minutes; longer near the ground and in wind (sticks to the surface).
Special case: vortices stay on the runway
- With very light crosswind (or wind aligned with the runway) vortices barely drift sideways — they can remain on or directly next to the runway, almost stationary.
- → Greatest risk during take-off immediately behind a Heavy/Super with calm or near-aligned wind.
- With stronger crosswind the vortices drift off, but may land on parallel runways or taxiways.
ICAO wake-turbulence categories
Traditional (ICAO Annex 6):
| Category | MTOM |
|---|---|
| HEAVY (H) | ≥ 136 000 kg (300 000 lb) |
| MEDIUM (M) | 7 000 – 136 000 kg |
| LIGHT (L) | < 7 000 kg |
Special:
- SUPER (J): Airbus A380, Antonov An-225.
EUROCONTROL RECAT-EU (since 2015) has 6 categories (A through F) for finer discrimination — primarily for ATC separation.
Standard separation (behind Heavy / Super)
On approach (final) — IFR separation in NM:
| Ahead | Following | Minimum |
|---|---|---|
| Super (A380) | Heavy | 6 NM |
| Super | Medium | 7 NM |
| Super | Light | 8 NM |
| Heavy | Heavy | 4 NM |
| Heavy | Medium | 5 NM |
| Heavy | Light | 6 NM |
| Medium | Light | 5 NM |
Source: ICAO Doc 4444 Table 5-8 (Standard Wake-Turbulence Separation Minima for Arriving Aircraft).
Take-off — time separation:
- 2 minutes behind an aircraft of the next higher wake-turbulence category.
- 3 minutes behind a Heavy/Super when Light/Medium follow, or for mid-runway departures (intersection take-off ahead of the Heavy rotation point).
Vortices while taxiing — taxi spacing
- When taxiing behind an aircraft of the next higher wake-turbulence class, keep a minimum distance of about 200 m (recommendation; see FAA AIM 7-3-7).
- Reason: jet blast and vortices on the taxiway can act even when stopped; for light aircraft jet blast can tip the aircraft.
Avoidance — VFR practice
Take-off scenarios
- Behind a Heavy on the same runway:
- Wait 2-3 minutes before take-off roll.
- Rotate before the predecessor's rotation point (when possible) — stay above the wake; lift off before the lift-off point of the Heavy.
- Climb at a steeper angle (Vx) → keep trajectory above the Heavy's.
- Crossing a runway with a recent Heavy take-off: wait 2-3 min.
Landing scenarios
- Behind a Heavy on approach:
- Stay above the predecessor's approach path.
- Touch down beyond the predecessor's touchdown point — e.g. beyond the point where the Heavy's nose gear touched down (typically past the first third of the runway).
- Steep approach and long landing — deliberately touch down well past the standard touchdown point.
- Wake vortices drift with wind — account for crosswind: vortices of the preceding aircraft drift in the leeward direction.
En-route
- Following a larger aircraft: 1000 ft below, 5 NM offset.
- Overtaking: pass above (vortices sink).
- Behind a descending aircraft: stay above its descent path.
Special case: helicopters
- Helicopters hovering or taking off near a runway produce strong downwash and turbulence — similar to wake vortices but broader and more stationary.
- A light aircraft pilot should wait before take-off on a runway near a hovering/departing helicopter until the helicopter has flown away or the downwash has dispersed.
- Source: FAA AC 90-23G and FAA AIM Section 7-4 Helicopter Vortices.
Effect of an encounter
Severity depends on the size ratio of follower/generator:
- Light behind Heavy: severe roll tendency, control resistance, possibly uncontrollable bank.
- Medium behind Heavy: noticeable roll, manageable with aileron + power.
- Heavy behind Heavy: light roll, rarely critical.
Main rule
Avoid visually combined with ATC separation. If ATC cannot separate (e.g. VFR at uncontrolled fields), the pilot is personally responsible for maintaining spacing.
🎯 Exam essentials
- ICAO categories: Light (< 7 000 kg), Medium (7 000–136 000 kg), Heavy (≥ 136 000 kg)
- Min separation behind Heavy: Light = 6 NM or 3 min; Medium = 5 NM or 2 min
- Take-off in same direction behind Heavy: wait 2–3 minutes
- Approach avoidance: stay above the Heavy, land beyond its touchdown point
- Helicopters generate especially strong turbulence (downwash + vortices)