Page 16 - TNT Dec 2017

The airspace near Wichita, Kansas has
an Alert Area that warns pilots about extensive visual traffic – including KC-135R heavy tankers – at McConnell Air Force Base.
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Air Force KC-135’s based at McConnell Air Force Base are a common sight in the airspace around Wichita, Kansas.
Wake Turbulence Advisory
FAA Advisory Circular (AC) 90-23G provides specific information about wake turbulence formation and behavior, as well as suggesting avoidance techniques. Let’s summarize and comment on the highlights of this guidance.
Lift is generated by pressure differential above and below the wing surfaces...the lowest pressure over the upper wing surface and the highest pressure under the wing. This pressure differential results in swirling air masses trailing downstream of the wingtip. The wake consists of two counter-rotating cylindrical vortices (AC-90-23G figure 1). The strength of the vortex is governed by the weight, speed, wing shape and wingspan of the generating aircraft. The extension or retraction of flaps, slats or other wing configuring devices will change the vortex characteristics of an aircraft. However, most significantly the vortex strength increases 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 (flaps and slats retracted) since the turbulence from a “dirty” aircraft configuration hastens wake decay.
Flying around Wichita with a KC-135 in the McConnell pattern? Until it turns final, the tanker is fairly heavy, flying fairly slow, and has its flaps and other wing devices retracted, generating its greatest vortex strength, according to the FAA. AC 90-23G continues:
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 another aircraft.
Since both a traffic pattern encounter and an en route, high- altitude confrontation may result in significant vortex strength, we need to know how wake turbulence behaves after it leave the airplane’s wingtips.
14 • TWIN & TURBINE
December 2017