Ask any pilot — new student or grizzled vet — what's most likely to cause a pre-landing rise in blood pressure, and the answer is likely to be the same: rip-roaring surface winds. More specifically, rip-roaring crosswind components. Strong surface winds can range in effect from merely bothersome to outright dangerous. There are several reasons for this.
One is that any strong wind usually implies turbulence. Typically, the stronger the wind, the more turbulent the situation. This is especially true when high terrain is nearby. Terrain and obstacles can amplify turbulence by causing the air flowing past them to tumble and create eddies and vortices that can translate into a very rough ride. Even such seemingly small protuberances as hangars, towers, and buildings near an airport can disturb the air blowing past and around them, causing turbulent gusts and wind shifts both in the landing pattern and on the ground.
A phenomenon known as wind shadow is one type of obstruction-related wind disturbance that can be present in even light winds. This is most often associated with a prominent tree line situated very close to a runway. Wind flows at treetop height may be strong enough to cause the crowns of the trees to sway in the breeze. But downwind and below treetop altitude, wind speeds can fall to near zero in the trees' "shadows." What can happen when you descend to a runway ringed by trees? You can lose all or part of any headwind component that you may have had above treetop height, and then experience a high sink rate close to the ground. We've all known airports that seem to be plagued by potholes or localized areas of sink on final. Wind shadow explains many of them.
Wind shear is another consideration when flying near the surface in extremely windy conditions. Air masses at lower altitudes seldom move in a steady, streamlined manner. Instead, they move in pulses that ebb and flow, however slightly. Some of this variation in speed occurs because of surface friction and is a function of altitude. Fly at 10,000 feet or so and the ride is apt to be mostly smooth. But descend to land and you pass through layers of the atmosphere having ever-diminishing wind strength. In high-wind conditions, this drop in wind speed is by no means gradual. One second you're in air moving at 35 knots; the next, you have descended into air blowing at 25 kt. The result? A smack-your-head-into-the-headliner jolt and a squeal from the stall warning horn as the airplane penetrates the shear zone.
This kind of excitement is sporty enough at altitude. The airspeed and lift variations associated with descending into shear zones on final approach and touchdown can be enough to cause control problems, or worse.
You can expect strong surface winds whenever a well-developed low pressure center moves into town, when a front passes through, when you're sandwiched between a high and a low, and when local effects such as sea breezes and mountain winds go into high gear.
A briefer should certainly tell you about these scenarios as part of the synoptic discussion portion of a standard weather briefing. If you're looking for forecast information, then a more elaborate description of the forces making high winds can also be found in the meteorological discussion at the end of the 48-hour low-level significant weather prognosis (prog) charts. But if you're trying your hand at self-briefing over the Internet or via DUATS, then look for the following signs among the National Weather Service's graphic products:
- Any nearby front that's forecast to move through your intended takeoff or landing airport should raise concern — not just for wind problems, but for the possibility of IFR or marginal VFR conditions. The worst case would be the passage of a fast-moving cold front aligned along a north-south axis. Before the cold front arrives, expect healthy winds (gusts to 30 kt and beyond wouldn't be unusual) out of the southerly quadrants of the compass. After the front passes, winds will shift to a westerly or northwesterly direction — and bring plenty of gusts and turbulence. A front aligned east-west isn't nearly as dynamic and is more likely to produce several days' worth of low ceilings and visibilities. In fact, most stationary fronts run east-west, and these are notorious producers of instrument weather.
- A deep low pressure center, indicated by closely spaced isobars encircling the low's L symbol on a surface analysis or prog chart, can be another signpost of strong surface winds. This is where the wind field closes in and rushes to the low. Because of the circulation around the low, two or more fronts can be in close proximity. Incidentally, a falling barometer together with winds switching to an east or northeasterly direction is a surefire indicator that a low is on its way. (This rule is also true as an indicator of the approach of those deepest lows — hurricanes.)
- The appearance of a high pressure center (a large H symbol on weather charts) and a low pressure center in close proximity is another warning of high surface winds. On a recent surface chart for the western United States, a high was centered over northern California. A low was parked farther south over the Gulf of California. Between the two, Santa Ana winds of up to 60 kt ripped through southern California. Why? Because the clockwise-flowing air off the east side of the high teamed up with the counterclockwise flow coming around the north of the low. It was a squeeze play that forced plenty of high-speed air from the east in a venturi-like dynamic. If you were flying in Southern California on that day, I hope that you were landing on Runway 9.
Sea breezes and mountain winds act on diurnal (day/night) cycles. With sea breezes, air flows from sea to land during the heat of the day, when rising air over land draws ocean air inland. At night, when land masses cool off, air flows back to the comparatively warmer sea.
Mountain winds tend to flow up valleys during the day, then blow downhill at night as falling temperatures cause the surrounding air masses to sink.
Another source of diurnal winds has to do with surface heating. As with sea breezes and mountain winds, daytime maximum temperatures — which reach their highest between 10 a.m. and 6 p.m. local time — cause rising air masses. As warm bubbles of air lift from the surface, adjoining air masses rush in to fill the void that they've left behind. This is what usually causes the type of daytime surface winds that occur under high pressure, fair-weather conditions.
We all know how this type of wind works. The day dawns in a dead calm, with perhaps some ground fog. About 10 a.m. the first breezes start and the fog burns off. By 1 p.m. surface winds turn gusty, and the air remains turbulent until 6 p.m. or so, when the day's heat starts to wane.
When any of these phenomena crop up, it's important for pilots to have a solid handle on crosswind and high-wind takeoff and landing techniques. A full discussion of the issues surrounding crosswind and high-wind landing procedures is beyond the scope of this article, but suffice it to say that for takeoffs and landings in high winds, your job boils down to these points:
- Knowing your airplane's capabilities. This means knowing the maximum demonstrated crosswind component, conducting a careful preflight calculation of your takeoff and landing distances, and having an alternate airport in mind — one with runways more favorably aligned into the wind if the forecast winds turn out to be too strong or too cross for a comfortable and/or safe landing. Big headwinds are seldom a problem — as long as they're not kicking up a lot of turbulence. A strong tailwind, however, can spell big trouble during takeoffs or landings, especially when high density altitudes prevail. Your airplane's performance charts can tell you exactly how much extra takeoff or landing distance a tailwind can produce.
Takeoffs and landings in strong winds are tricky, but don't forget that taxiing in high winds also poses potentially serious problems. High-wing and tailwheel airplanes are especially prone to weathervaning, and if the surface winds are strong enough, taxiing with any degree of precision may be impossible. In the worst of cases, taxiing airplanes can be overturned by gusts or strong steady-state winds. - Knowing your capabilities. Proficiency in crosswind and high-wind takeoffs and landings is a must. But there are levels of proficiency, and you must be honest in evaluating your skill and comfort levels. Can you perform a good wing-low, opposite-rudder touchdown at the proper airspeed, with no drift from the runway centerline, and in a nose-high attitude? If so, with how much of a crosswind component? At what level of crosswind component will you throw in the towel and divert to an airport with more favorably aligned runways? And what about gusts, shifting winds, turbulence, or wind shear on final approach? How much deviation in airspeed, descent profile, and sink rate will you tolerate before deciding that an approach in high-wind conditions is uncomfortably unstable?
High levels of proficiency and knowing the answers to the above questions are just as important to safe high-wind operations as knowing the weather signposts that bring them about in the first place.
E-mail the author at [email protected].
Related Articles
