Wx Watch: Working the Wind

Pilots can view strong winds as either a curse or a blessing. Fifty-knot tailwinds aloft for that long cross-country flight? Great! Twenty gusting to 30 knots, blowing at 90 degrees to your destination's 2,000-foot-long, 50-foot-wide active runway? Bad, bad news. For these and so many other reasons, pilots cultivate a strong interest in any weather report or forecast involving the wind's behavior. That's because, in the final analysis, adverse winds can represent a lot more than a rough ride and unscheduled fuel stops. According to an AOPA Air Safety Foundation Safety Review, General Aviation Weather Accidents: An Analysis and Preventive Strategies, loss of control while landing or taking off in crosswinds caused 987 accidents between 1982 and 1993, making botched crosswind landings one of the biggest categories of general aviation accidents. Most of those accidents didn't involve fatal injuries, but there were plenty of minor and serious injuries, and the insurance claims must have added up to a pretty penny indeed.

What causes wind?

To be brutally simple, wind is caused by differences in atmospheric pressure. Let a low-pressure system move up against a high-pressure system, and the compression of air between them can set up a steep pressure gradient. This compression shows up on surface analysis and winds aloft charts as closely spaced isobars (surface charts) (winds aloft charts). Either way, the result is the same. The more closely packed the isobars or height contours, the greater the pressure differential and the stronger the wind.

Temperature contrasts between two differing air masses can also play a big part in creating wind. Warm air is lighter and rises, while cold air is denser and sinks. Nature abhors a vacuum, so when one parcel of air rises, the air around it rushes in to take its place. It's this simple dynamic that causes land and sea breezes along shorelines, valley winds, and surface winds where convection is present.

Think about that temperature-wind relationship the next time you fly. Ever wonder why surface winds so often begin to pick up in midmorning, reach their peak at the warmest time of day, and then die down by late afternoon or early evening? Absent any frontal passages or storm systems, it's because solar energy heats up the surface, which in turn heats up the air above it, which in turn causes thermals, which in turn cause the air nearby to rush in to fill the "vacuum" left by the rising air. This diurnal (daily) process also plays a huge role in the shoreline breezes just mentioned.

The strongest winds

Pilots should know when to anticipate strong winds — especially strong surface winds. Sure, a good weather briefing ought to mention adverse surface winds via an Airmet Tango. This type of airmet, like all the rest, shows up at the beginning of an area forecast, right after the synoptic summary, and is certain to be given top billing by a flight service weather briefer. Airmets Tango warn of nonconvective turbulence, low-level wind shear, and strong (sustained winds of 30 knots or greater) surface winds. In addition, the outlook portions of a terminal aerodrome forecast (TAF) should forecast any sustained surface winds greater than 15 knots, gust value changes of 10 knots or more, and wind direction changes of 30 or more degrees.

But a well-educated pilot should be able to look at a few charts and decide for him- or herself if there's a setup for adverse wind. Here's what to look for:

• A high- and a low-pressure system in close proximity. This creates a squeeze play for the pressure gradient, so winds aloft and at the surface can reach high levels. This is what happens, for example, when high pressure settles over Northern California and low pressure deepens over the Southern California deserts. The systems are comparatively close together, and the clockwise flow around the high teams up with the counterclockwise flow around the low to generate winds up to 60 mph or more at the surface. In Southern California, these are called Santa Anas — warm winds caused by compressional heating of the air as it flows downhill from the high desert to the coastal lowlands.
• Ahead of a strong, fast-moving cold front, expect strong winds out of the south. It may sound odd that the harbinger of colder air is frequently a daylong blast of warm, humid air from the Gulf of Mexico, but that's the way it goes east of the Rockies. That southerly flow is the leading edge of the counterclockwise flow around the cold front's parent low, so beware if you're heading out on a southbound cross-country. In a situation like this there will be strong winds aloft, a good chance of thunderstorms ahead of the front, and a very good chance of strong surface winds and low-level wind shear.
• Behind a strong cold front. Once this type of front has passed, the wind flow typically will be out of the west or northwest. This is in large part the clockwise flow around the leading edge of an oncoming area of high pressure, and conditions can be very windy, turbulent, and gusty.
• In and near thunderstorms. Rising and falling air currents can create microbursts, squall lines, and wind shear that can — and has — felled the largest, most powerful airplanes.
• At ridge height and in mountain passes. The winds may be tolerable at the surface, but at altitude they're nearly always stronger. High terrain can disturb and intensify the winds aloft, making for a rough ride and making any landings at high-elevation airports a real challenge of your skill and nerves. Mountain passes can act as venturis, so any winds flowing through them will be stronger than those in any surrounding areas. It's something to think about if you're trying to outclimb rising terrain with a tailwind.

Crosswind landings

This is really a subject for another article — nay, a book! — but it's certainly appropriate to review the basic techniques for landing in crosswinds. Most instructors seem to favor the crab-and-slip method.

But first things first. Well before you enter an airport's traffic pattern, you should learn about the surface wind conditions from flight watch (on 122.0 MHz), approach control, AWOS, ASOS, ATIS, or unicom. This gives you a good idea of the strength of the crosswind component affecting the airport's runway(s), and allows you to recollect your airplane's maximum demonstrated crosswind component. This figure, published in the Normal Procedures section of late-model pilot operating handbooks (POHs), is frequently listed at the head of the chapter, among the "airspeeds for safe operation." This figure — let's say, 17 knots — is not a limitation. Your skill level may be such that you could safely land in a 25-knot direct crosswind. It's just that on the day the manufacturer scheduled the crosswind tests, the winds happened to have the posted crosswind component. Even so, the maximum demonstrated crosswind component is a good warning sign. That sign says, "Go ahead and try this at home, but our test pilots may not have landed in conditions any stronger than this." So if you're looking at landing in a 25-knot crosswind, it's time to ask yourself, first, "Have I landed in these conditions before, in this airplane, and am I skilled, practiced, and confident enough to handle them?"

If the answer is "yes" then give it a try, but be ready for a go-around. If it's no, then it's time for that second question: "Do ya feel lucky?"

Anyway, after knowing the surface wind, confirming it by observing the windsock, and — if you're landing at an airport so equipped — hearing from the tower of any low-level wind shear alerts, you're ready for the approach. In the crab-and-slip method, the steps are as follows:

• Complete your normal prelanding checklist. Keep an eye on the windsock, any other wind indicators, any available glidepath indicators (e.g., VASI, PAPI, and ILS), and your target touchdown point.
• Once established on final, fly a heading that lets you hold a straight ground track along the extended runway centerline. This means you'll be crabbing into the wind, and dealing with the crosswind component that way — for now. Use power to compensate for any sink or lift you encounter on approach.
• Consider using partial flaps, or none at all, and adjust your approach airspeed accordingly. Consult your POH for these airspeeds, and add one-half the gust factor (e.g., if the winds are 15 gusting to 25, add five knots to the recommended approach airspeed). If the wind is strong enough, the extra lift generated by a full-flap condition may be enough to cause directional control problems during the landing roll, and lift a wing after you've landed.
• On short final, use rudder to "kick out" any crab you've been holding. Make sure the airplane's longitudinal axis is lined up with the runway centerline, and use rudder pressure to keep it there.
• Simultaneously, lower the upwind wing with aileron pressure. This prevents the airplane from drifting sideways. Use enough aileron force to stop the drift. Just think, "wing down, opposite rudder" if you need a reminder as to the combination of forces. Bear in mind that from now on you may have a constant juggling act. You should expect to constantly be changing aileron and rudder pressures as the wind speed and direction change on approach; count on it in gusty and/or shifting winds. If you cannot stop the drift or maintain alignment with the runway, perform a go-around and fly to an airport that has runways more favorably aimed into the prevailing wind.
• In the flare, you should have power off and no drift. The nose of the airplane should be pointed straight down the runway. The upwind main gear should touch down first, in view of the aileron pressures you're holding.
• After touchdown, maintain directional control using the same combination of control forces while you brake to stop the airplane, and use ground steering to keep the airplane on the centerline. As the airplane slows, it will be necessary to add more and more aileron pressure to keep the upwind wing from rising in the crosswind. This is because aileron effectiveness decreases as the airplane's speed decreases. Accept no deviations from the centerline; that goes double for tailwheel airplanes. If the POH, your training, and the wind's strength sanction it, consider raising the flaps after touchdown to shed lift and transfer as much weight as possible to the wheels. If you choose this option in retractable-gear airplanes, be extra careful not to raise the landing gear by mistake.
• Continue to use the proper control pressures as you taxi.

Maintaining directional control of tailwheel airplanes is much more difficult than steering those with nosewheels. This is because a tailwheel airplane's center of gravity is behind the main gear, rendering it much, much more susceptible to weathervaning into the wind. Apart from this consideration, tailwheel airplane landings in strong or gusty winds can be conducted the same as those of a nosewheel-equipped airplane. Some may prefer wheel landings (touching down with the main gear first, then allowing the tail to drop as the airplane slows), but this technique is only for the skilled. You'll be doing a foot-blurring tap dance on the rudder pedals in a crosswind, and the penalty for getting out of step is usually a mean ground loop. It's better to touch down in a three-point attitude with the tailwheel on terra firma, where it can better help the rudder and ailerons in slowing and steering.

Landing in strong, gusty crosswinds is never much fun, but you can become good at it. As with every other challenging maneuver, the best way to earn and keep proficiency is to tackle hairy crosswinds with an adept instructor, maintain your proficiency by taking on crosswinds on your own, and seek out recurrent instruction. No doubt about it — high winds mean hard work!

Links to additional information on flying with the wind may be found on AOPA Online ( www.aopa.org/pilot/links/2001/links0103.shtml). E-mail the author at [email protected].