Avoidance

Trailing vortices have characteristics which, when known, will help a pilot visualize the wake location and thereby take avoidance precautions. Vortex generation starts with rotation (lifting off of the nosewheel) and will be severe in that airspace immediately following the point of rotation. Vortex generation ends when the nosewheel of a landing aircraft touches down.

Because of ground effect and wind, a vortex produced within about 200 feet AGL tends to be subject to lateral drift movements and may return to where it started. Below 100 feet AGL, the vortices tend to separate laterally and break up more rapidly than vortex systems at higher altitude. The vortex sink rate and levelling off process result in little operational effect between an aircraft in level flight and other aircraft separated by 1,000 feet vertically. Pilots should fly at or above a heavy jet’s flight path, altering course as necessary to avoid the area behind and below the generating aircraft. Vortices start to descend immediately after formation and descend at the rate of 400 to 500 feet per minute for large heavy aircraft and at a lesser rate for smaller aircraft, but in all cases, descending less than 1,000 feet in total in 2 minutes.

Vortices spread out at a speed of about 5 knots. Therefore, a crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus, a light wind of 3 to 7 knots could result in the upwind vortex remaining in the touchdown zone for a period of time or hasten the drift of the downwind vortex toward another runway. Similarly, a tailwind condition can move the vortices of the preceding landing aircraft forward into the touchdown zone.

Since vortex cores can produce a roll rate of 80° per second or twice the capabilities of some light aircraft and a downdraft of 1,500 feet per minute which exceeds the rate of climb of many aircraft, pilots should be particularly alert in calm or light wind conditions where the vortices could:

Remain in the touchdown area
Drift from aircraft operating on a nearby runway
Sink into takeoff or landing path from a crossing runway
Sink into the traffic pattern from other runway operations
Sink into the flight path of VFR flights at 500 feet AGL and below

On the Ground

Before requesting clearance to cross a live runway, wait a few minutes when a large aircraft has just taken off or landed and when holding near a runway, expect wake turbulence.

Takeoff and Landing

Pilots must consider the following procedures during takeoff:

When cleared to takeoff following the departure of a large aircraft, plan to become airborne prior to the point of rotation of the preceding aircraft and stay above the departure path or request a turn to avoid the departure path.
When cleared to takeoff following the landing of a large aircraft, plan to become airborne after the point of touchdown of the landing aircraft

Pilots must consider the following procedures during landing:

When cleared to land behind a departing aircraft, plan to touchdown prior to reaching the rotation point of the departing aircraft.
When behind a large aircraft landing on the same runway, stay at or above the preceding aircraft’s final approach flight path, note the touchdown point and land beyond this point if it is safe to do so.
When cleared to land behind a large aircraft on a low approach or on a missed approach on the same runway, beware of vortices that could exist between the other aircraft’s flight path and the runway surface.
When landing after a large aircraft on a parallel runway closer than 2,500 feet, beware of possible drifting of the vortex onto your runway. Stay at or above the large aircraft’s final approach flight path, note his touchdown point and land beyond if it is safe to do so.
When landing after a large aircraft has departed from a crossing runway, note the rotation point. If it is past the intersection, continue the approach and land before the intersection. If the large aircraft rotates prior to the intersection, avoid flight below the large aircraft’s flight path. Abandon the approach unless a landing is assured well before reaching the intersection.