Plane & Pilot
Tuesday, March 6, 2012

Combating Crosswinds

Ten Things The Textbooks Don’t Tell You

Some specifics: You're landing on a 5,000-foot runway and the only sock is mid-runway. That's nearly a half-mile from where we'll land. We can fit a lot of wind changes into a half-mile. Especially if the wind is crossed, and there's topography or buildings out there adding their little bit of entertainment to the situation.
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.
5 Control Towers Often Get Wind Information From Off-Airport Sources It's common for the source of wind information broadcast on ATIS, especially at larger airports, to be off of the airport property. So, this time it's an 8,000-foot runway, and the wind information is coming from a tower that's a quarter-mile away: Now, we're landing nearly two miles from the wind information source. Yet another reason why we search out secondary wind information sources, such as wind socks, flags, blowing dirt, trees, etc.

6 Beware The Down-To-The-Surface Wind One of the more unusual winds is the one that ignores boundary layer effects. As a normal rule, the wind right on the surface of the runway is zero, and it builds up to the reported velocity at about 15 feet. However, there are some "rogue" winds that hold much of their velocity right down to the pavement. This is just the reverse of what normally happens. In this case, the airplane is slowing down and losing energy, while the wind is doing neither. Because the wind hasn't decreased, its effect on the airplane is actually stronger. These can be really, really nasty winds because they sneak up on you, and you don't realize until the last second that it takes much more control movement than normal to beat them into submission.

The topography of a runway can change the path of the wind, sometimes quite dramatically. A wind curl is developed when the wind has to avoid an object and in doing so, it wraps itself around the corner.
7 Violent Changes In Direction Can Steal Lift One of the most dangerous winds doesn't even have to be a big one. It can be less than 15 knots and still cause plenty of heartburn. It will have two characteristics that fortunately aren't combined that often, but the combination occurs often enough that it's worth discussing.

The first characteristic is sharp-edged gusts that change direction in a heartbeat. Making it much worse, the sharp peaks of the harder-than-normal gusts are off the main wind heading. As the gust dies, the wind returns to the main heading. So, every time it punches you, it's from a different direction at a higher velocity.

The second characteristic is that the basic direction of the wind is 90 degrees to the runway. When a 90-degree wind is combined with sharp gusts that snap back and forth during flair, the possibility of gusts instantaneously changing from in front of the wing tip to behind the wing tip becomes critical. When wind snaps from in front to behind the wingtip, the airplane doesn't have time to react to the sudden change in airspeed because of inertia. For a moment, it loses so much lift that it becomes dynamic, as opposed to aerodynamic. At that point, it's an ingot, not an airplane. This is when burying the throttle in the panel is called for. Not partial power. Full power, because nanoseconds count.

8 Wind And Turbulence Interact One of the more "fun" wind conditions (read that as teeth-shattering because of the violent ups and downs) is seen in the West more than any other place. This is when a hard-edged, gusty wind is combined with low-level turbulence caused by high ground temperatures and/or topographical effects. The resulting ride can be violent and unpredictable. In these conditions, our primary job is to determine exactly what attitude is required to counter the crosswind, and how to "firmly" maintain it without over doing the corrections. When fighting turbulence in gusty crosswinds, there's a tendency to accidentally bring the ailerons back "past center." By this, we mean that we're supposed to be holding a wing down, but without realizing it, we overcontrol in turbulence and actually put aileron in the other direction. This momentarily picks up the down wing.


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