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Wednesday, September 9, 2015

What you can't see can hurt you: Avoiding wake turbulence

I recently came across a very nice picture of wingtip vortices. (Unfortunately, it was passed to me without any information on who took it or where. If you know, please let me know so I can give them the credit they deserve.)

With the exception of some NASA tests with equipment designed to help make them visible, these are usually invisible. In the picture below, you get a nice view of what these vortices do to the air, and where wake turbulence comes from. (Throughout this post, for simplicity's sake I'll refer to wingtip vortices and wake turbulence interchangeably.)

Notice the airport in the background.
These are behind every airplane that is creating lift, from a 747 to a 172. In fact, in your training you may have done a nice steep turn, and toward the end of it felt a bump seemingly out of nowhere. That was you doing such a good circle that you hit your own wake turbulence.

If they do this to a thin cloud layer, imagine what they'd do to you if your airplane was in that path. Well, the FAA has made sure you have a scary idea of what they could do, as they point out on pages 13-15 and 16:
"The vortices from larger aircraft pose problems to encountering aircraft. The wake of these aircraft can impose rolling moments exceeding the roll-control authority of the encountering aircraft. Also, the turbulence generated within the vortices can damage aircraft components and equipment if encountered at close range."
This is a very dry, boring way of saying that flying into them can flip your butt over and/or rip your wings off. As exciting as falling head over heels for wingtip vortices might sound, their behavior is fortunately very predictable, which means there's an easy way to avoid them.

Since they're part of the air mass, they move along with it. They also tend to sink. Since we usually try to land into the wind, this means they're coming toward you but sinking at the same time. Solution: just stay above the glidepath of the plane ahead that caused them and you'll avoid them.

I use this quite often when I'm flying the Dash-8. Although at 43,000 pounds it's not a particularly small airplane, it's a wee speck compared to the 767s, 747s, and A340s I've landed behind at Newark. In these cases, I intentionally fly half a dot high on the glideslope.

Notice I said half a dot high. This is on a two-dot glideslope indicator. If you have a five-dot indicator, this is less than two dots. The figure below shows a glideslope indicator that's as high as you need.

Don't use more than this; you don't need to stay extremely high to stay above wake turbulence. It's tempting to do so if you think along the lines of "better safe than sorry", but staying way too high will make the final stage of the approach unnecessarily difficult. If you're flying visually or you don't have a glideslope receiver, use the PAPI lights to help you out. All you need for them is 3 white and 1 red. Again, no more.

At 5 miles out, this is only about 100 feet high. As you get closer to the runway, that gets smaller and smaller. By a mile out (further out than you'd be turning base if you were flying a traffic pattern), this is well under 50 feet. The top part of the following diagram shows you what I mean:

Well, that's great if you're coming in to land, but what if you're taking off behind a monster?

In general, the bigger the wingtip vortices, the bigger the plane. This is actually good news, because the bigger the plane, the more runway it tends to use on takeoff. This means that you can probably lift off well ahead of them. However, that's not the full solution, as the chances are very, very good that they can outclimb you by a lot once their wheels do break the ground.

Now what? Remember that wingtip vortices have predictable behavior, and they move along with the air mass. That means if there's a crosswind, they will slide along the side of the runway. Once you're reached a safe altitude, turn a bit in order to keep them on one side of you. If the crosswind is from the right, the vortices will be moving from right to left over the runway. Just turn a bit to the right to keep them on your left and you'll remain clear. If there's a left crosswind, turn to the left.

Most towered fields where GA planes mix in with the big ones understand what it takes to keep the shiny side up under these circumstances. Not only will they not have a problem with you sidestepping, they will expect you to do it on your own. When they say, "Cessna 1234 cleared for takeoff runway 27, caution wake turbulence departing Boeing 737", they are alerting you of the possibility of encountering wingtip vortices AND giving you implicit permission to maneuver within reason to avoid them.

Although they may make for a pretty picture, wingtip vortices can make for an ugly encounter. By keeping this post's tools in your bag, you can look forward to years of smooth flying!

See you next Wednesday!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a FAASafety Team representative and Master-level participant in the FAA's WINGS program. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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