Ignoring the possibility that a pilot received weak crosswind flight training (a subject for yet another soapbox), the main reason the windsock instills fear is that we often don't understand what the sock and surrounding wind indicators are telling us. The general feeling is that the best sock is a limp sock, which definitely is not the case.
First of all, there is wind and then there is wind. All winds are not created equal, and there's a lot more to the difference than simply direction and velocity. Spend 30 seconds studying the windsock on several successive days and you'll realize that there are as many different types of winds as there are variations in rivers: from small and lazy to huge and angry. A quick glance at the sock will give you the "big two" (direction and speed), but a few more seconds studying the sock's behavior will give you a much better understanding of the character of the wind you're about to challenge.
Remember that windsocks are accurate only within a small radius. If an airport has only one windsock and it's alongside the midpoint of the runway, the only thing of which you can be certain is that the wind at either end of the runway won't necessarily match. This is why so many airports have socks at both ends of the runway. The sock in the middle tells traffic which runway to use, but the sock at the end of the runway tells you what to expect on approach.
From the air you have to squint hard and circle the airport a couple of times if you really want to study the sock. Before takeoff, however, 15 seconds spent watching the sock gives you valuable information, some of which can literally make or break your flight.
Direction
It's rudimentary that the more wind there is on the nose (within limits), the easier an airplane will land because it's moving more slowly across the ground. Conversely, the farther the wind is off the nose (a crosswind) the more difficult it becomes. In fact, every model has demonstrated that it has the control authority required to land in a 90-degree crosswind of a given velocity; that maximum demonstrated crosswind component is clearly displayed on a visible placard or in the pilot's operating handbook. Past that speed, it'll probably handle more wind, but the manufacturer isn't willing to go out on a limb and guarantee it.
However, if the airplane is placarded at 20 knots, that's also saying it can easily handle more than 20 kt if the wind is less than 90 degrees off the nose. Wind charts show that an airplane placarded for a 20-kt, 90-degree crosswind can handle 32 kt at 40 degrees off the nose--or an amazing 60 kt at 20 degrees' deflection.
Speed
Is there a foolproof way to accurately read the velocity represented by a windsock?
The answer is yes. And no. Windsocks are manufactured and calibrated in such a way that a given sock will always have a certain amount of "hang," or bend, until the wind reaches a specific speed--at which point the wind sock is blowing arrow-straight. For most windsocks, that's 15 kt, but some larger ones are designed to indicate higher wind speeds when fully inflated.
If the windsock, regardless of its operational limits, is standing out straight, we know we're going to have to work to get the airplane back in. However, the velocity of the wind isn't nearly as important as certain other aspects of its personality.
Gust character
We're all familiar with gust spread: The wind will suddenly increase or decrease in velocity. Because airplanes have inertia, they can't react to a gust change by instantly speeding up or slowing down. So, if the wind suddenly drops 10 kt, for an instant, the airspeed drops at least a percentage of that amount until the airplane can react to the change. The effect may only last for a second or so, but during that time, if we're landing, the airplane may get planted on the runway much more firmly than we'd prefer when a dying gust jerks the lift from under us. Sudden gusts are why the airspeed needle bounces around on extremely rough days.
Some gusts are hardly worth worrying about; however, the character of gusts can go from mildly irritating to flat-out scary. Once again, as speed is the most obvious factor of wind, the speed differential of the gust is the most obvious factor there too: It's gusting from 10 to 17 kt so there's a seven-knot gust spread. Is something as small as a seven-knot gust spread something to worry about and how does it show up on the windsock?
The gust spread becomes worrisome when it represents a sizeable percentage of the stall speed of the airplane. If an airplane stalls at 45 kt, as many do, then a seven-knot spread is only 15.5 percent. If, however, the airplane stalls at, say, 30 kt--something like a Piper Cub or Taylorcraft--then the same gust is 23 percent of the stall speed and the effects can be much more dramatic. Anyone who has tried to land a Taylorcraft, one of the world's true floaters, in even a moderate gust spread can vouch for the fact that gusts can alternately balloon them or slam them onto the runway. This is the reason for making wheel landings in taildraggers: They are flown at a higher speed so the effect of the gusts is diminished.
But, how does the windsock show we're dealing with a sizeable gust spread? It's intuitive that when the windsock changes from being droopy to straight and back to droopy again that the wind is gusting, and the degree of slack between straight and droopy indicates the relative size of the gusts. Is it possible to assign an actual number to the amount of change visible? No, but just knowing that the wind is gusting "a little" or "a lot" is important. However, if you sit and stare at the sock for a short while, you'll get a feeling for what percentage of "straight" the peak of the gusts represents and that gives you at least a feeling of how close they are to the max indication of the sock.
Although the gust spread is important, what's much more likely to get you in trouble is the "character" of the gusts. While watching the gusts, notice what kind of personality the changes have. Does the sock sort of lazily straighten out and then gently droop back down? Does it snap into position and just as quickly drop back down, only to snap back up again? When people talk about "sharp edge" gusts, that's what they're referring to. Gusts that come and go violently will have a much more dramatic effect on the airplane.
Shear potential from directional changes
Along with the relative sharpness of the gusts, the sock also shows us what kind of directional changes are coupled with the gusts and how quickly they happen. One of the nastiest winds is one that has extremely sharp-edged gusts and, as each one hits, the wind direction suddenly changes. It'll be blowing from 030 degrees, suddenly die, and just as quickly snap back into action from 060 degrees. This has dangerous potential.
A wind that changes direction quickly and violently also changes the wind vector that is affecting the airplane. Picture a wind triangle with one of the legs right down the crankshaft. As the wind shifts from 30 degrees off the nose to 60 degrees off the nose, even if it stays the same velocity, the length of the vector going down the nose becomes shorter and the effect is the same as a momentary reduction of airspeed. This is more of an irritation than anything else; your only indication will be the airspeed needle jumping up and down. However, it's a different ball game when the same wind is varying from in front of the wing tip to behind it.
Let's say the wind is snapping from 70 degrees off the nose around to 110 degrees, which puts it behind the wing tip. A violently snapping wind that pops from in front of the wing to behind the wing is a form of wind shear. When the wind was from in front of the wing, it was contributing a certain flow down the nose in which the airplane was maintaining a given airspeed. When it snaps behind the wing, it is taking wind off the nose and putting it on back of the wing, and for a brief instant, the airspeed is less. If this happens quickly and inertia keeps the airplane from reacting, for an instant the airplane loses some airspeed and settles rapidly. This has caused more than one airplane to make a hard landing just when everything seemed to be going well.
When operating in a wind like the above, the throttle hand must be kept at the ready.
The sock doesn't have all the answers
The wind starts somewhere off the field, then flows across it before going elsewhere, picking up characteristics that the windsock won't be able to tell us about. Experience will help. It's like watching for rock-produced eddies or the effects of a riverbank and bottom configuration on the water flow. The difference is that we can't see the effects, and we have to depend on indicators like smoke, flags, grass, and water to help.
Topographic effects on wind
When we talk about the effects of airport topography we have to include not only the hills and hollows around the airport but the buildings on the airport.
If the runway is built up on a slight hill and the ground falls away from the end of the runway, you can count on the wind down the runway making a downward curl at the end. The sharper the dropoff, the sharper the curl. If the runway has a lake at the end, you can see the effect in the water surface as a disturbance in the wave pattern: The wind is pushing all of the water one direction except in the lee of the runway. Most of the time you don't get those kinds of clues and, if landing on an elevated runway in a strong wind, land a little farther down the runway than usual--and be ready with the power.
If the airport has significant hills close to the runway, expect curls off those hills when the wind gets crossways. Look at the trees on top of the hills as you approach and see if limbs move contrary to what the windsock shows. Also, know that some airports develop interesting eddies running across the runway when the wind comes across hangars and other structures. Those same structures can act as flow fences and cause the windsock to give inaccurate indications.
Low-level winds
Once in a while you'll encounter a wind that loses little intensity as it nears the ground. A wind that maintains all of its characteristics right down to the runway can be dangerous, if nothing else because it is so unexpected. Even after you're on the runway, it's trying to get under a wing. These kinds of winds almost always derive from the sharp, gusty conditions we discussed earlier, but the windsock can't tell you they come right down to the deck--so it's wise to assume they do.
The tower doesn't have the answer
Even if a tower-controlled airport has three windsocks, traditionally the tower takes its information off the one mounted midfield. That sock could be nearly a mile from the approach end and wildly inaccurate. Some tower-controlled airports have the ability to read the wind at all three socks, but they don't give that additional wind information unless you ask for it. If you ask, you'll be surprised at the difference between the midfield sock and one at the end of the runway. That's why you never take their word for it. Similarly, the siting of wind sensors for an AWOS or ASOS at a nontowered airport can result in the broadcast of wind information quite different from what you'll experience in the touchdown zone. Carefully analyze the sock closest to your point of takeoff or landing for the most relevant information.
Do what you have to do
Because the narrow little window where we take off and land can easily exist in a micro-climate, we have to do whatever we have to do to make it work. Looking at the windsock, or asking the tower for a wind check while on final, provides information that is only advisory: It's not guaranteed to be what we'll experience right where we land. Regardless of what we see or what we're told, the bottom line is to fly the airplane.
Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
Related Articles
