Wake Turbulence

(998 millibars). That error represents an indicatedaltitude difference of some 485 feet!

Probably the most noticeable difference between international flight operations and domestic flight operations is radio communication, with phraseology and procedures varying in many respects. In most parts of the world outside the United States, strict adherence to proper terminology is required (“diagonal” versus “slash,” “decimal” versus “point,” “zero” versus “oh”, etc.); however, other areas of the globe operate in a mix of aviation English, local language, and local slang. American slang is generally not understood and local phraseology may not be familiar to many U.S. pilots, so be careful!

International operations should be studied every time you fly out of your home country, not just at recurrent training sessions.

When stepping up to turbine-powered aircraft, you will often be operating at airports with high traffic densities and where the majority of traffic is composed of large aircraft. Most high-density airports have closely spaced parallel or intersecting runways, with approach and departure aircraft strung out 15–75 miles in several directions.

Operating under these conditions you’ll constantly be confronted with situations where understanding of wake turbulence is required. You must know its causes, effects, and how to avoid it in order to give your passengers a smooth and safe ride. This section reviews some common procedures for avoiding wake turbulence hazards.

Wing Tip Vortices

As you know, an aircraft wing creates lift by developing a pressure differential: low pressure over the wing’s upper surface and higher pressure underneath the wing. For most of the wingspan, this pressure differential translates directly into lift. However, something a bit different happens out at the wing tip.

As lift is created, some of the high-pressure air from under the wing rolls up around the wing tip toward the lower-pressure area on top. The result is sort of a mini “horizontal tornado” called a wing tip vortex. These wing tip vortices trail from the aircraft’s wing tips and are the source of the hazard known as “wake turbulence.” Since vortices are byproducts of lift, they are created from the time an aircraft begins to fly until it touches down on landing (Figure 15.8).

Wing tip vortices trail behind and below aircraft flight path and diverge slowly to the sides. aircraft flight path

Wing tip vortices tend to be strongest when associated with a heavy aircraft that’s flying slowly, with its gear and flaps retracted. (Extended gear and flaps tend to break up the vortices, to some degree.) Vortices are most dangerous to a trailing aircraft when the wingspan of that aircraft is less than the diameter of the vortex’s rotational flow. This is especially true if the encountering aircraft flies into the wake vortex of another aircraft on the same heading. In that case the vortex may induce a violent roll, with potentially disastrous consequences if the trailing aircraft is near the ground.

When the wingspan of an aircraft flying into a wake vortex is greater than the diameter of the vortex, the resulting induced roll may be countered effectively by the ailerons. However, the resulting turbulence may be serious or even severe. Either way, pilots must learn to visualize the locations of wing tip vortices and then alter flight paths to avoid them.

Identifying Likely Areas of Wake Turbulence

In order to visualize wake turbulence locations, remember that wing tip vortices are created when an aircraft wing is creating lift. On its takeoff roll an aircraft begins to create wing tip vortices as it rotates and starts to fly. Upon landing, an aircraft ceases to create wing tip vortices around the touchdown point (see Figure 15.9).

Flight tests have shown that wing tip vortices near the generating airplane are spaced approximately a wingspan’s length apart and diverge gradually as distance from the aircraft increases. They tend to sink at a rate of 500–800 feet per minute, slowing and weakening as they descend.

Under no-wind conditions, large aircraft flying near the ground produce vortices that drift outward away from the flight path along the ground at 2–3 knots. Keep in mind, however, that wing tip vortices

rotation point

Wake vortices begin near point of rotation.

touchdown point

Wake vortices end near the touchdown point of landing aircraft.

Most pilots plan rotation and touchdown so as to avoid flight segments of preceding aircraft over the runway. This often means rotating before the takeoff point of a preceding departure and landing beyond the touchdown point of a preceding arrival. Wind drift of wake turbulence must also be considered and, in any case, separation standards observed.

FIGURE 15.9 | Wing tip vortices on takeoff and landing.

drift with the wind (see Figure 15.10). Therefore, a few knots of crosswind may cause upwind vortices to remain on the runway for an extended period of time. That same crosswind can move downwind vortices toward a parallel runway. A head wind or tail wind can easily move wake vortices into touchdown or departure zones, so be aware of the wind and be careful!

Unfortunately, wake turbulence is not so predictable that every pilot can avoid it all of the time. The best wake turbulence avoidance skills are to understand its causes, to be continually aware of traffic activity, and to consider the effects of wind. Following is a summary of some widely practiced wake turbulence avoidance recommendations.

Departure

On departure, note the rotation point of the preceding airplane. If possible, within normal procedures, plan to rotate before that point. After a missed approach by a large aircraft, allow two minutes to elapse before beginning takeoff roll so there’s time for the wing tip vortices to dissipate. Keep in mind the effects an engine failure would have on your aircraft’s performance after departure and what it would do to your projected avoidance strategy.

Approach and Landing

Listen on the radio for the “big picture” of surrounding traffic. Always be aware of the aircraft you are following. What’s the type? Speed and altitude? Check your TCAS (if so equipped); how far behind are you in trail? To avoid wake turbulence, many pilots choose to fly “one dot high” when trailing a heavy aircraft on an ILS (instrument landing system) approach. When landing behind a large aircraft, stay above that aircraft’s flight path and plan to land beyond its touchdown point. Try to spot the touchdown smoke from the aircraft’s tires ahead of you as an indication of which way and how fast the wind is blowing. This will give you some idea of the location of its wake turbulence.

calm

Under calm conditions, vortices diverge from centerline at 3–5 kt.

A light crosswind of 3–5 kt. blows the upwind vortex onto the runway. A stronger crosswind of 8–10 kt. may blow vortices downwind onto parallel runways.

5-kt. crosswind

FIGURE 15.10 | Effect of crosswind on wing tip vortices.