Vortices are greatest when the generating aircraft is clean, and slow.” This condition is most commonly during approaches or departures because an AOA is at the highest to produce the lift necessary take off. To minimize the chances of flying through wake turbulence:
• Flying through another aircraft’s flightpath.
• Prior to the point at which the preceding aircraft rotated, when taking off behind another aircraft.
• Following another aircraft on a similar flightpath altitude within 1,000 feet. [Figure 4-11]
Approach the runway above a preceding aircraft’s path when landing behind another aircraft, and touch down after the point at which the other aircraft wheels contacted the runway. [Figure 4-12]
A hovering helicopter generates a down wash from its main rotor(s) similar to the vortices of an airplane. Pilots of small aircraft should avoid a hovering helicopter by at least three rotor disc diameters to avoid the effects of this down wash. In forward flight this energy is transformed into a pair of strong, high-speed trailing vortices similar to wing-tip vortices of larger fixed-wing aircraft. Helicopter vortices should be avoided because helicopter forward flight airspeeds are often very slow and can generate exceptionally strong wake turbulence.
Wind is an important factor in avoiding wake turbulence because wingtip vortices drift with the wind at the speed of the wind. For example, a wind speed of 10 knots causes the vortices to drift at about 1,000 feet in a minute in the wind direction. When following another aircraft, a pilot should consider wind speed and direction when selecting an intended takeoff or landing point. If a pilot is unsure of the other aircraft’s takeoff or landing point, approximately 3 minutes provides a margin of safety that allows wake turbulence dissipation. For more information on wake turbulence, see Advisory Circular 90-23.