Figure 14-45.  Vortex generation.  Vortex Strength  Terminal Area  En Route  Vortex Behavior  Wake turbulence has historically been thought of as only  a function of aircraft weight, but recent research considers  additional parameters, such as speed, aspects of the wing, wake  decay rates, and aircraft resistance to wake, just to name a few.  The vortex characteristics of any aircraft will be changed with  the extension of flaps or other wing configuration devices, as  well as changing speed. However, as the basic factors are weight  and speed, the vortex strength increases proportionately with  an increase in aircraft operating weight or 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 wake turbulence events have been influenced by  changes to the aircraft fleet mix that have more “Super”  (A380) and “Heavy” (B-747, B-777, A340, etc.) aircraft  operating in the NAS. There have been wake turbulence  events in excess of 30NM and 2000 feet lower than 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.  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 (until it touches down), since trailing vortices are the  byproduct of wing lift.  [Figure 14-46]  The vortex circulation  is outward, upward, and around the wingtips when viewed  from either ahead or behind the aircraft. Tests with large  14-27  Figure 14-46.  Vortex behavior.  25  Wake begins  Touchdown Rotation  Wake ends 