13.wake-turbulence-avoidance. Wake Turbulence Avoidance
Wake turbulence is a byproduct of lift. Whenever an airfoil produces lift, air flows from the high-pressure region beneath the wing around the wingtip into the low-pressure region above the wing. This circulation rolls up into two counter-rotating cylindrical vortices trailing from each wingtip. These wingtip vortices are the primary component of wake turbulence, and they can be violent enough to roll, pitch, or even break apart smaller aircraft that encounter them.
Vortex Generation
Vortex generation begins the moment the nosewheel lifts off on takeoff and ends when the nosewheel touches down on landing. The strength of the vortex is governed by the weight, wingspan, and speed of the generating aircraft. The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow — for example, a large jet on approach in a landing configuration but with flaps retracted, or shortly after rotation on takeoff.
Vortex strength can be expressed conceptually as:
Vortex strength ∝ (Weight) ÷ (Wingspan × Airspeed)
A fully loaded widebody at low speed in clean configuration produces the most hazardous wake.
Vortex Behavior
Once generated, vortices behave predictably:
- They sink at roughly 300–500 feet per minute behind the generating aircraft.
- They level off and dissipate approximately 500–900 feet below the flightpath of the generator.
- They persist for several minutes, longer in stable air.
- In calm wind, vortices tend to drift with any prevailing wind at 2–3 knots.
- A light quartering tailwind (about 1–5 knots) is the most dangerous wind condition. It can hold the upwind vortex over the runway and push the downwind vortex from a parallel runway onto your runway.
- Near the ground (within about a wingspan of the surface), vortices tend to move laterally outward at roughly 5 knots.
Avoidance Procedures
The pilot in command is responsible for wake turbulence avoidance. ATC separation standards exist, but when a pilot accepts a visual approach or operates VFR, the pilot is responsible. General avoidance principles:
- Stay above and upwind of the larger aircraft's flightpath whenever possible.
- Visualize the vortex sinking behind and below the generator and plan the flightpath to avoid that region.
- When in doubt, request additional spacing.
Specific scenarios:
- Landing behind a larger aircraft, same runway: Note the larger aircraft's touchdown point and land beyond it.
- Landing behind a larger aircraft, parallel runway closer than 2,500 ft: Treat as the same runway — stay at or above the larger aircraft's final approach path and land beyond its touchdown point.
- Landing behind a larger aircraft, crossing runway: Cross above the larger aircraft's flightpath.
- Landing behind a departing aircraft, same runway: Land before the larger aircraft's rotation point.
- Landing behind a departing aircraft, crossing runway: Land prior to the intersection if the larger aircraft rotated past it; otherwise avoid flight below its flightpath.
- Departing behind a larger aircraft: Rotate before the larger aircraft's rotation point and climb above and upwind of its climb path until turning clear.
- Intersection takeoffs, same runway: Avoid headings that cross below the larger aircraft's path. If a heavy departed from the full-length recently, an intersection takeoff is not advisable.
- Departing or landing after a heavy/large aircraft executing a low-approach, missed approach, or touch-and-go: Ensure at least 2 minutes have elapsed before takeoff or landing.
- En route VFR: Avoid flight below and behind a larger aircraft's path. If passing a larger aircraft, do so upwind and laterally well clear.
ATC Separation
ATC applies wake turbulence separation when radar-identifying or sequencing aircraft based on the ICAO/FAA weight categories: Super (A380, AN-225), Heavy (max takeoff weight 300,000 lb or more), Large (more than 41,000 lb up to 300,000 lb), and Small (41,000 lb or less). For example, a Small landing behind a Heavy on the same runway requires 6 NM spacing; behind a Super, 8 NM. ATC will issue the advisory "CAUTION WAKE TURBULENCE" but the responsibility to avoid the wake remains with the pilot.
Helicopters and Tiltrotors
Helicopters in forward flight generate vortices similar to fixed-wing wake. In a hover or hover-taxi, they produce strong rotor downwash that extends roughly three rotor diameters outward. Avoid operating within three rotor diameters of a hovering helicopter.
Recovery from a Wake Encounter
If you inadvertently fly into wake turbulence, the encounter typically lasts only a few seconds but can produce sudden, large rolling moments. Apply coordinated control inputs to recover, avoid abrupt opposite-aileron applications that could overstress the airframe, and exit the wake by climbing and turning upwind. Report the encounter to ATC so following traffic can be advised.
Understanding vortex behavior — heavy, clean, slow aircraft generate the worst wake, vortices sink and drift with the wind, and a quartering tailwind is the most hazardous — is the foundation of effective wake turbulence avoidance.