All aircraft generate wake turbulence during flight. This disturbance is caused by a pair of counter-rotating vortices trailing from the wingtips. The vortices from larger aircraft can pose a significant hazard to encountering aircraft. The wake of these aircraft can impose rolling moments exceeding the roll-control authority of the encountering aircraft. Also, the turbulence generated within the vortices can damage aircraft components and equipment if encountered at close range. For this reason, a pilot must envision the location of the vortex wake and adjust the flight path accordingly.
Vortex Generation
Lift is generated by the creation of a pressure differential over the wing surface. The lowest pressure occurs over the upper wing surface and the highest pressure under the wing. This pressure differential triggers the rollup of airflow aft of the wing, resulting in swirling air masses that trail downstream from the wingtips. After the rollup is completed, the wake consists of two counter-rotating cylindrical vortices. Most of the energy lies within a few feet of the center of each vortex. [Figure 1]
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| Figure 1. Vortex generation |
Vortex Strength
Terminal Area
Wake turbulence has historically been thought of as only a function of aircraft weight, but recent research considers additional parameters, such as airspeed, wing characteristics, wake decay rates, and the encountering aircraft’s resistance to wake effects. The vortex characteristics of an aircraft change with the extension of flaps or other wing configuration devices, as well as with changes in airspeed. However, as the basic factors are weight and speed, vortex strength increases with an increase in aircraft operating weight or a decrease in aircraft speed. The greatest vortex strength occurs when the generating aircraft is heavy, slow, and clean, since the turbulence from a “dirty” aircraft configuration hastens wake decay.
En Route
En route wake turbulence events have been influenced by changes to the aircraft fleet mix resulting in more Super and Heavy aircraft operating in the NAS. There have been wake turbulence events in excess of 30 NM and 2,000 feet below the wake-generating aircraft. Air density is also a factor in wake strength. Even though the speeds are higher in cruise at high altitude, the reduced air density may result in wake strength comparable to that in the terminal area. In addition, for a given separation distance, the higher speeds in cruise result in less time for the wake to decay before being encountered by a trailing aircraft.
Vortex Behavior
Trailing vortices have certain behavioral characteristics that can help a pilot visualize the wake location and take avoidance precautions.
Trailing vortices are generated whenever an aircraft is producing lift. During normal flight operations, wake vortices begin as the aircraft becomes airborne and continue until it touches down because they are a byproduct of wing lift. [Figure 2]
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| Figure 2. Vortex behavior |
The vortex circulation is outward, upward, and around the wingtips when viewed from either ahead or behind the aircraft. Tests with large aircraft have shown that vortices remain spaced a bit less than a wingspan apart, drifting with the wind, at altitudes greater than a wingspan from the ground. Tests have also shown that the vortices sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft.
When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the ground at a speed of 2 to 3 knots. A crosswind decreases the lateral movement of the upwind vortex and increases the movement of the downwind vortex. A light quartering tailwind presents the worst-case scenario because wake vortices may remain along a significant portion of the final approach and extended runway centerline, rather than only near the touchdown zone.
Vortex Avoidance Procedures
The following procedures can help pilots avoid wake vortices in common operating scenarios.
- Landing behind a larger aircraft on the same runway—stay at or above the larger aircraft’s approach flight path and land beyond its touchdown point. [Figure 3A]
- Landing behind a larger aircraft on a parallel runway closer than 2,500 feet—consider the possibility of drift and stay at or above the larger aircraft’s final approach flight path and note its touchdown point. [Figure 3B]
- Landing behind a larger aircraft on a crossing runway—cross above the larger aircraft’s flight path.
- Landing behind a departing aircraft on the same runway—land prior to the departing aircraft’s rotation point.
- Landing behind a larger aircraft on a crossing runway—note the aircraft’s rotation point and, if that point is past the intersection, continue and land prior to the intersection. If the larger aircraft rotates prior to the intersection, avoid flight below its flight path. Abandon the approach unless a landing is ensured well before reaching the intersection. [Figure 3C]
- Departing behind a larger aircraft—rotate before the preceding aircraft’s rotation point and climb above its flight path until clear of the wake.
- For intersection takeoffs—be alert to larger aircraft operating on the same or nearby runways, particularly when their wake may drift toward the intended departure path.
- If departing or landing after an aircraft that has executed a low approach, missed approach, or touch-and-go landing, remain alert for wake vortices that may settle and drift laterally near the runway. Comply with applicable ATC wake-turbulence separation requirements and delay the operation when necessary to avoid the wake.
- En route, it is advisable to avoid a path below and behind a large aircraft, and if a large aircraft is observed above on the same track, adjust the flight path laterally and, when practical, move upwind of the generating aircraft’s track.
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| Figure 3. Vortex avoidance procedures |