Page 17 - TNT Dec 2017

Where Does It Go?
The horizontal tornadoes that roll off an airplane’s wingtips when lift is generated have been found to exhibit predictable behavior:
The vortex circulation is outward, upward, and around the wing tips when viewed from either ahead or behind the aircraft. The vortices remain spaced slightly less than the distance of the generating airplane’s wingspan apart, drifting with the wind. Vortices sink at a rate of several hundred feet per minute (fpm), slowing their descent and diminishing in strength with time and distance behind the wake-generating aircraft. Atmospheric turbulence hastens decay.
The worst-case atmospheric conditions are light winds, low atmospheric turbulence and a stable atmosphere. En route in these conditions, vortices can descend more than 1,000 feet. In rare cases, wake turbulence can rise in an updraft or when it bounces off the top of a strong inversion layer.
If the air is indeed stable, the wake turbulence behind a heavy airplane will take as much as 5 miles to make the 1,000-foot descent. The vortices will then linger at that lower altitude, drifting downwind, until the turbulence eventually dissipates. AC-90-23G Figure 5 shows some of the behavior of wingtip vortices.
Caution: Wake Turbulence
Air traffic controllers will provide a “Caution: Wake turbulence” advisory call any time they feel wake turbulence will be an adverse factor for your airplane. This is normally a one-time advisory – the controller only tells you once, and you’re expected to heed the warning thereafter.
We usually expect to hear the wake turbulence advisory when following a larger airplane for takeoff or landing; they’re not commonly heard away from the immediate airport area. That’s because, as stated at the beginning of this article, if a pilot is in visual conditions it’s that pilot’s responsibility to visually detect other aircraft and to then predict and avoid the location of its vortices as well.
A few notes from the FAA about wake turbulence avoidance:
Whether or not a warning or information has been given, ATC expects the pilot to adjust aircraft operations and flightpath as necessary to preclude wake encounters.
When any doubt exists about maintaining safe separation distances between aircraft to avoid wake turbulence, pilots should ask ATC for updates on separation distance and groundspeed.
If a larger aircraft is observed above on the same track (meeting or overtaking), adjust your position laterally, preferably upwind.
Fly at or above the preceding aircraft’s flightpath, altering course as necessary, to avoid the area behind and below the
This illustration visualizes the behavior of wingtip vortices.
some time, pilots were encouraged to learn to command a roll in the direction of turbulence-induced roll, to turn an upset into a full, 360-degree roll to recover upright, most sources now recommend against rolling recover because most airplane will lose a substantial amount of altitude in the process (consider the Canadair Challenger experience).
Instead, most experts recommend letting the airplane enter an unusual attitude during the encounter, and then recovering from that attitude. AC90-23G states:
It may be better to allow the aircraft to transition through the wake and then recover from any resultant unusual attitude, rather than aggressively trying to control the aircraft during the wake encounter. If an autopilot is engaged and remains engaged, it may be better to allow the autopilot to recover from the wake vortex encounter rather than disconnecting the autopilot and using manual control inputs. However, be prepared to assume manual control of the aircraft if the autopilot disengages.
Prior experience or training that emphasizes use of rudder input as a means to maneuver in roll may not apply to all aircraft operations. Using the rudder to counter roll rate during a roll upset may lead to an undesirable aircraft response. Large, aggressive control reversals can lead to loads that can exceed the structural design limits.
Specialized upset recovery training in aerobatic airplanes is still a superb idea for all pilots. If you attend training intended to give you the skills needed to best recover from a strong wake turbulence encounter, expect it to emphasis unusual attitude recovery, and not commanding a roll with the vortex until you are again upright, as your best response to an upset.
Back Under the Pattern
When I’m departing or inbound to my home airport and see a KC-135R in the McConnell Air Force Base pattern above me, or any time I am operating in the vicinity of a larger airplane away from the immediate runway environment, I consciously visualize a pair of wingtip-vortex•tornadoes extending behind the airplane, gradually drifting down to about 1,000 feet below the airplane 2 to 5 miles behind it and hovering there, drifting with the prevailing wind. When I see the traffic, I also see its wake so I can see and avoid the entire turbulence complex, not just the airplane that generates it. T&T
generating aircraft.
Recovery
The proper technique for recovering from a strong wake turbulence encounter may seem counterintuitive. It also differs from what was taught about upset recovery for many years. For
December 2017
TWIN & TURBINE • 15
Thomas P. Turner is an ATP CFII/MEI, holds a master's Degree in Aviation Safety, and was the 2010 National FAA Safety Team Representative of the Year. Subscribe to Tom’s free FLYING LESSONS Weekly e-newsletter at www.mastery-flight-training.com.