Testing Tuesday: Wake Turbulence

The correct answer is “c” – Light quartering tailwind.

Wake turbulence tends to be strongest behind large, heavy aircraft, flying slow, such as landing or departing airliner. All aircraft create a wake, but smaller, lighter aircraft create very minimal, if any, lasting turbulence.

Wake turbulence is a direct effect of the creation of lift – the higher pressure air below the wing will find any way to move to the lower pressure area above the wing, and near the wingtips of any aircraft, this higher pressure air “curls” up and around the wingtip to get there. As it moves around the wingtip, the wingtip continues to move forward with the rest of the aircraft, and the “vortex” created continues to spin, creating a wake on both sides of the aircraft.

The wakes, in still air, tend to sink and move outward from the aircraft slowly, meaning they descend and get farther apart as time goes on. However, just as the wind affects flying aircraft, it also affects the wake created by an aircraft. Strong winds will tend to dissipate a wake, but light winds will simply move them.

So if an aircraft is taking off with a light crosswind, it is possible that, instead of a wake moving outward, away from a runway, the wind may move the upwind wake directly over the runway, meaning that the wake remains a hazard for longer than normal for any aircraft departing or landing on that runway after a heavy aircraft lands or departs ahead of them on that same runway.

Understanding this is critical, so that you can also understand how to avoid wake turbulence if you suspect it might be a factor.

If you are departing after a heavy aircraft departs, you should lift off before the departing aircraft lifted off (wake turbulence is not generated until lift is being generated, so until the plane in front of you has rotated (begun its liftoff), wake turbulence is not a factor). Once you have lifted off, you should also make a shallow bank and move upwind of the departing aircraft’s flight path. Doing this should keep you clear of any wake turbulence generated by the aircraft that departed ahead of you.

If you are landing after a large aircraft lands, you should stay above the large aircraft’s flight path while on the approach, and land after the point the large aircraft landed. Since wake turbulence “sinks,” remaining above the glide path of the large aircraft ahead of you, and landing beyond it’s touchdown point, will keep you above & ahead of any wake turbulence generated by that aircraft.

If you are taking off after a large aircraft has landed, you want to stay on the ground until you passed that aircraft’s landing point. So a rolling takeoff would be recommended here – take the runway and roll forward slowly, and possibly advance the throttle a little more slowly than you normally would, so that your takeoff roll is longer than usual. This will ensure that any wake turbulence you may encounter is while you are still on the ground. A little counter-intuitive, I know, but this is one situation where using more runway than necessary to takeoff is safer.

If you are landing after a large aircraft takes off, your goal is to land before the point where the large aircraft lifetd off the runway, so don’t float and don;t land long. Get on the ground as early as possible, safely.

And of course, you can always choose to wait a few minutes for the wake turbulence to dissipate before you takeoff or land… sometimes tower will make you wait before you depart “for wake turbulence.” And you can always request to do another circuit around the pattern if you are uncomfortable landing behind a large aircraft’s departure or arrival.

For more information on wake turbulence (and wake turbulence avoidance):

• Aeronautical Information Manual, Chapter 7 Section 3
• Pilotfriend.com
• Wikipedia

And watch this video about the topic from YouTube (there are lots of great YouTube videos about wake turbulence):