All aircraft generate a wake while in flight. This disturbance is caused by a pair of counter-rotating vortices trailing from the wingtips. The vortices from larger aircraft pose problems 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 flightpath accordingly.

 During ground operations and during takeoff, jet-engine blast (thrust stream turbulence) can cause damage and upsets at close range. For this reason, pilots of small aircraft should consider the effects of jet-engine blast and maintain adequate separation. Also, pilots of larger aircraft should consider the effects of their aircraft’s jet-engine blast on other aircraft and equipment on the ground.
 

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 the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. After the rollup is completed, the wake consists of two counter-rotating cylindrical vortices. Most of the energy is within a few feet of the center of each vortex, but pilots should avoid a region within about 100 feet of the vortex core. [Figure 6-17] Vortex Strength  The strength of the vortex is governed by the weight, speed, and shape of the wing of the generating aircraft. The vortex characteristics of any given aircraft can also be changed by the extension of flaps or other wing configuration devices as well as by a change in speed. The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow.

Vortex Behavior

 Trailing vortices have certain behavioral characteristics that can help a pilot visualize the wake location and take avoidance precautions. Vortices are generated from the moment an aircraft leaves the ground, since trailing vortices are the by-product of wing lift. The vortex circulation is outward, upward, and around the wingtips when viewed from either ahead or behind the aircraft.

Tests 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. [Figure 6-18]

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 or 3 knots. A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. A tailwind condition can move the vortices of the preceding aircraft forward into the touchdown zone.

Vortex Avoidance Procedures

• Landing behind a larger aircraft on the same runway—stay at or above the larger aircraft’s approach flightpath and land beyond its touchdown point.
• 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 flightpath and note its touchdown point.
• Landing behind a larger aircraft on crossing runway—cross above the larger aircraft’s flightpath.
• Landing behind a departing aircraft on the same runway—land prior to the departing aircraft’s rotating point.
• Landing behind a larger aircraft on a crossing runway—note the aircraft’s rotation point and if past the intersection continue and land prior to the intersection. If the larger aircraft rotates prior to the intersection, avoid flight below its flightpath. Abandon the approach unless a landing is ensured well before reaching the intersection.
• Departing behind a large aircraft, rotate prior to the large aircraft’s rotation point and climb above its climb path until turning clear of the wake.
• For intersection takeoffs on the same runway, be alert to adjacent larger aircraft operations, particularly upwind of your runway. An intersection takeoff clearance is received, avoid headings that will cross below the larger aircraft’s path.
• If departing or landing after a large aircraft executing a low approach, missed approach, or touch and go landing (since vortices settle and move laterally near the ground, the vortex hazard may exist along the runway and in your flightpath, particularly in a quartering tailwind), it is prudent to wait 2 minutes prior to a takeoff or landing.
• 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 your position laterally and preferably upwind.