Plane & Pilot
Tuesday, March 6, 2012

Combating Crosswinds

Ten Things The Textbooks Don’t Tell You

Here's an aviation fact that you can take to the bank knowing it's almost always true: Neither the wind given to you by the tower, nor that shown on a mid-field wind sock, is likely to be what you actually experience when landing. There are lots of reasons for that, and understanding those reasons will cut down the number of surprises and the amount of drama in your crosswind landings.

1 A Steady Wind Isn't Even Remotely Steady If wind had color, so we could actually see how it's shaped, we'd see that like a river, it's interrupted by swirls and eddies. It's constantly changing, flowing smooth for a few seconds, then changing direction and strength, only to go smooth again. So when we're in the approach and, even more important, when we're in flare and floating down the runway, we're experiencing an ever-changing combination of wind-induced bumps and wiggles. Every several hundred feet down the runway promises a new experience.

2 Wind Is Made Up Of Layers When thinking about fighting wind, we often think of nothing but crosswinds, which is a two-dimensional, left-right way of thinking. However, wind, like airplanes, is three-dimensional, and is usually composed of several layers. It's quite common to turn final and find that our glideslope has changed. At first we looked high, then for no apparent reason, we're low (the numbers are moving up the windshield), and a few more ponies are needed to make the runway. Or, just the reverse happens. This is because the wind at pattern altitude is much less affected by either the friction wind develops with the ground, or the topography and other ground-bound obstacles that the wind has to work around. Also, thermal effects are generally more intense closer to the surface. So, don't be surprised to find that the wind on the ground is grossly different than what was experienced on downwind.

3 Topography Can Really Affect Wind On Short Final Not all runways are in the middle of a mile-square, billiard-like wheat field. In fact, in many parts of the country, runways are perched on, carved out of, clinging to or stuffed into real estate that's everything but flat. And that being the case, the wind's path can be pretty screwy because of what it's curling over and around.

A curl is developed when the wind has to avoid an object and, in effect, wraps itself around the corner, e.g. a runway with a noticeable drop-off at the end. When the wind comes whistling down the runway and the pavement is suddenly no longer there, the wind curls downward. If we're on short final, the curl will try to take us with it.

The reverse of the downward curl is when we're coming over an obstacle at the threshold directly into the teeth of the wind that's trying to climb over that obstacle. It will pick us up, then just as quickly, decide it no longer wants us, and where we were fighting an upper, we're suddenly falling out of the air.
Another variation of curl occurs when we're landing in a crosswind—a time when we really don't need any more complications—on a runway with a ridge or tree line alongside it. This time, the curl is coming over the trees, and it may try to slam you into the ground. It may try to pick you up. It may just pound you senseless, then smooth out, as if saying, "Okay, I've screwed with you enough."

4 Wind Socks At The Beginning Of The Runway Are Best We always try to land at the beginning of the runway (if we're doing our jobs anyway). Unfortunately, most airports think we land in the middle of the runway because that's where they put the wind sock. Socks can't be that expensive! Every runway should have a sock at each end, so we actually know what the wind is doing where we'll be touching down. The further down the runway a wind sock is located, the closer to fantasy it becomes.


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