Wake turbulence: strength and behaviour
Wake turbulence (CAR 101.01): turbulent air behind an aircraft. Sources:
- Wingtip vortices
- Rotor-tip vortices
- Jet thrust stream
- Rotor downwash
- Prop wash
Every aircraft in flight makes it, helicopters too (PHAK ch.14; PSTAR 7.03, 7.08).
Wingtip vortices:
- By-product of lift.
- Two counter-rotating cylinders trail each wingtip.
- Higher angle of attack: stronger (PHAK ch.5).
Strength (TC AIM AIR 2.9):
- Strongest behind heavy, clean (flaps and gear up), slow aircraft.
- Weight most significant.
- Dirty configuration hastens decay (PHAK ch.14).
- Most often met on approach and departure (PHAK ch.5).
- Strength directly proportional to weight, inversely proportional to span and speed (PHAK ch.5).
Generation and motion:
- Starts at rotation (nosewheel lift-off), severe just after it. Ends when a landing aircraft's nosewheel touches down.
- Maximum: just before touchdown for a landing aircraft, just after lift-off for a departing one (TP 1102).
- Sink: immediate. 400 to 500 ft/min for large heavy aircraft, less for smaller ones. Always under 1,000 ft total in 2 min. At 1,000 ft vertical separation, sink has little operational effect.
- Spread and drift: outward at about 5 kt, drifts with wind.
Wind effects:
| Wind | Effect |
|---|---|
| Crosswind | Decreases upwind vortex's lateral movement, increases downwind vortex's |
| Crosswind 3 to 7 kt | Can hold upwind vortex in touchdown zone, or push downwind vortex onto another runway |
| Light crosswind | Can hold a vortex over the runway |
| Tailwind | Can push a landing aircraft's vortices forward into touchdown zone |
(Crosswind rows: TP 1102.)
Calm or light wind, vortices can (TC AIM AIR 2.9.1):
- Stay in touchdown area
- Drift from a nearby runway
- Sink into a crossing runway's takeoff or landing path
- Sink into the traffic pattern
- Sink into a VFR flight path at 500 ft AGL and below
Near the ground:
| Height | Effect |
|---|---|
| Within about 200 ft AGL | Ground effect and wind can drift a vortex laterally, even back to where it started |
| Below 100 ft AGL | Vortices separate and break up faster |
Dissipation:
- Little in first 2 min, then varies (TC AIM AIR 2.9).
- More atmospheric turbulence: faster decay (TC AIM AIR 2.9).
- Still air: hazardous wake may persist 5 min or more (PSTAR 7.02, exam bank).
- Cruise: for 2 min after a heavy passes, hazardous vortices dissipate very slowly (PSTAR 7.05, exam bank). Higher cruise speed leaves less time to decay (PHAK ch.14).
- Not forecastable: size, weight, speed and air conditions vary. Vortices move down when shed, then outward near the ground (cushion effect) (TP 1102).
Trap: thinking wake rises or stays on flight path. It sinks and drifts.
References
- TC AIM AIR 2.9
- TC AIM AIR 2.9.1
- TP 1102 Take-off / Approach and Landing: Wake Turbulence
- CAR 101.01
- PHAK ch.5, ch.14
- PSTAR 7.02, 7.03, 7.05, 7.08 (exam bank)