Figure 5-12.  Wingtip vortices.  Figure 5-13.  Avoid following another aircraft at an altitude within 1,000 feet.  5-9  As the air curls upward around the tip, it combines with the  downwash to form a fast-spinning trailing vortex. These  vortices increase drag because of energy spent in producing  the turbulence. Whenever an airfoil is producing lift, induced  drag occurs and wingtip vortices are created.  Just as lift increases with an increase in AOA, induced  drag also increases. This occurs because as the AOA is  increased, there is a greater pressure difference between the  top and bottom of the airfoil, and a greater lateral flow of air;  consequently, this causes more violent vortices to be set up,  resulting in more turbulence and more induced drag.  In  Figure 5-12,  it is easy to see the formation of wingtip  vortices. The intensity or strength of the vortices is directly  proportional to the weight of the aircraft and inversely  proportional to the wingspan and speed of the aircraft. The  heavier and slower the aircraft, the greater the AOA and the  stronger the wingtip vortices. Thus, an aircraft will create  wingtip vortices with maximum strength occurring during  the takeoff, climb, and landing phases of flight. These  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  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.  Wingtip vortices are greatest when the generating aircraft is  “heavy, clean, and slow.” This condition is most commonly  encountered during approaches or departures because an  aircraft’s AOA is at the highest to produce the lift necessary  to land or take off. To minimize the chances of flying through  an aircraft’s wake turbulence:  vortices lead to a particularly dangerous hazard to flight,  wake turbulence.  Avoiding Wake Turbulence  • Avoid flying through another aircraft’s flight path.  • Rotate prior to the point at which the preceding aircraft  rotated when taking off behind another aircraft.  • Avoid following another aircraft on a similar flight  path at an altitude within 1,000 feet.  [Figure 5-13]  • 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 5-14]  V or t e x  AVOID  Nominally  500–1,000 ft  Sink rate several hundred ft/min 