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Proficiency: In the vortex

The truth about wake turbulence

Wake turbulence is serious business.  Vortex speeds of almost 300 feet per second (or 178 knots) have been recorded; that circulating air can cause rolling moments that exceed the capability of an aircraft to counteract. To make matters worse, wake turbulence encounters are most likely to occur around airports when pilots are at their most vulnerable—flying slowly at low altitudes.
P&E December
Photography by Paul Bowen

Once air traffic control says, “Caution, wake turbulence,” ATC generally provides no additional information to keep you away from these potentially nasty swirls of air. Furthermore, you are responsible for your own wake turbulence avoidance after accepting instructions to follow another aircraft, traffic information, or a visual approach clearance. Good to know.

The FAA shares information and advice such as this in the Aeronautical Information Manual as well as Advisory Circular 90-23G, Aircraft Wake Turbulence. While well-intentioned, these publications contain errors that imply vortices are not present in places they do exist. Relying on the advice in these publications to maneuver around vortices could spell trouble.

The publications effectively convey the hazards of an encounter with wake turbulence, but they fall short when they offer advice to avoid such an encounter in the first place.

Aircraft Wake Turbulence describes vortex behavior in the air and near the ground to “help pilots visualize the wake location and movement and take appropriate avoidance actions.” But vortices are present in more places than reported in this advisory circular. AC 90-23G reports that, “An aircraft generates vortices from the moment it rotates on takeoff to touchdown, since trailing vortices are a by-product of wing lift.” Not exactly. Vortices begin early in the takeoff roll and can continue after touchdown.

Consider a general aviation aircraft, say a Cessna 152, on its takeoff roll. As soon as the throttle is increased and the airplane starts moving, air is flowing across the wings. The cambered wings, as well as their angle of incidence, ensure the wings create lift while the aircraft is still on the ground. Even before rotation and liftoff, the flow across the top of the wing is faster than that below, so the higher-pressure air below begins to curl upward about the wing tip.

I filmed a video in my Cessna 152 on its takeoff roll to correct misconceptions about aircraft wake turbulence (www.aceaerobaticschool.com). The cones attached to the wing tips start to rotate behind the wing and show that the vortex action begins early in the takeoff roll. The idea that you’ll be protected from a wake turbulence encounter by using exclusively the part of the runway on which a generating aircraft was recently operating is wrong.

Furthermore, AC 90-23G emphasizes, “The wake of larger aircraft requires the respect of all pilots.” That is certainly true, but even smaller aircraft can generate considerable wake turbulence. Glider pilots practice a maneuver called “boxing the wake” in which the glider pilot, while still attached to the towplane, maneuvers around a rectangular path that contains the wake turbulence of the towplane. Hitting the wake of the towplane shows that it does not take a large airplane to produce wake turbulence.

Airplanes departing and approaching an airport are especially vulnerable to a wake turbulence encounter because lower airspeeds mean reduced control authority. And, despite best efforts at avoidance, an inadvertent encounter is always possible. In that case, I teach my flight students that in a roll upset, they should try to counteract the roll by staying upright but, “If you can’t beat it, then join it” by continuing the roll until upright once more. Never pull back on the yoke when upside-down. Instead, push forward on the yoke when inverted to minimize altitude loss.

The exact nature of wake turbulence behavior is sensitive to the type of generating aircraft as well as the precise wind conditions, and is nearly impossible to predict. Thus, attempting to avoid vortices by imagining their location is a tall and risky order. After all, vortices are invisible. The best advice for avoiding the wake of another aircraft is to be patient and wait. Vortices dissipate over time and gusty air breaks them up even sooner. Plan on at least two minutes between landing or departing operations and extend that window to three minutes if the winds are calm and/or the generating aircraft is large. As pilot in command, it is our prerogative and our duty to make decisions that ensure the safety of flight.

Catherine Cavagnaro owns Ace Aerobatic School in Sewanee, Tennessee (www.aceaerobaticschool.com), and is a professor of mathematics at Sewanee: The University of the South. Cavagnaro is the 2018 FAA Safety Representative of the Year.

P&E December
At the end of the wing tip, higher-pressure air curls around to the lower-pressure environment on top of the wing and forms wingtip vortices (top). As the angle of attack of the wing increases, the vortices become stronger (above).
P&E December
With flaps down, the Cessna 152 features four main vortices (top). With flaps up, the flap vortices vanish and the wingtip vortices strengthen (above).

Catherine Cavagnaro

Catherine Cavagnaro is an aerobatics instructor (aceaerobaticschool.com) and professor of mathematics at Sewanee: The University of the South.

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