What causes most weather-related general aviation accidents? According to a recent compilation of National Transportation Safety Board statistics, 42 percent involve adverse winds. That's just under half of the 3,633 total weather-related accidents listed for that time period. In comparison, low ceilings and visibilities played a part in 24 percent of all weather accidents.
We can assume these numbers refer primarily to landing accidents in which gusty winds, crosswinds, and tailwinds played a factor. The landing phase always claims the most accidents, with loss of directional control on the runway leading the list of causes. Though this type of accident seldom results in fatalities, the 1,526 wrecks and injuries represented by that 42 percent still indicates a real problem.
Most pilots stay on the ground when the wind kicks up. If you don't believe it, just go to your local airport on a windy day. Chances are, the ramp will be fairly quiet. You could say that the ground-bound pilots are demonstrating good judgment by staying home. However, you could also say that they are depriving themselves of a valuable chance to hone their high-wind landing skills. To do this safely, you'll need an experienced instructor.
To understand just when and how adverse winds arise, you'll need to build up a core of wind knowledge.
Conventional texts do a great job of hitting the basics, but they often ignore some critical bits of wisdom. One is that when constant pressure levels and good VFR weather persist in a given area, surface winds tend to pick up around 10 a.m. and diminish sharply just before nightfall. This phenomenon is the result of surface heating, which creates rising thermal bubbles — especially over dark areas like parking lots, stands of trees, and runways.
Due to atmospheric compensating mechanisms, rising air in one area means that air will be sinking somewhere else nearby. That area could be on short final, where downdrafts may dictate a blast of power to keep you on the proper glidepath. Once over the runway, expect a nice shot of rising air from the thermal action. As for crosswinds, they'll be there, thanks to adjoining parcels of air rushing in to fill the vacuum created by the rising, runway-heated air. What's all this mean? If you want to practice crosswind landings or landings in gusty conditions, warm up early in the morning when winds are lightest. After a few times around the pattern, you'll be better prepared for the increasing wind levels that follow.
These kinds of diurnal wind fluctuations also occur at airports located in coastal areas and for much the same reasons. Land masses heat up and cool down much faster than large bodies of water. In the late morning, air temperatures over land rise and draw ocean or lake air onshore. Late in the day, it's the air over the water that retains the day's heating, so winds blow offshore as the warm air entrains the land's cooling air.
Fronts, of course, also bring winds — and wind shifts — with them, but bear in mind that fronts can be very, very different. Textbooks are sure to warn you about fast-moving cold fronts, and they should. But passage of this type of front is relatively easy to time and seldom catches well-briefed pilots unaware. It's the slower moving fronts, the fronts that are weak or that contain little in the way of precipitation or convection, which are harder for meteorologists to predict and can set traps for lackadaisical preflight planners.
A well-defined cold front is preceded by a flow of air (at altitudes below about 10,000 feet) from the southerly quadrants of the compass. There are often gusts to the 20- to 30-knot level, and unseasonably warm surface temperatures are very likely. As the front approaches, you'll notice a drop in the barometer/altimeter setting and an intensification of the wind. After frontal passage (FROPA, in surface observation lingo, posted in the Remarks section), pressure rises, and winds shift to the west or northwest. If the post-frontal temperature drop is great, these winds can blow up to 30 or 40 knots and contain wind- shear-inducing gusts.
Weaker, ill-defined cold fronts and troughs present less of a wind hazard. That's because there's often (never say never when talking about the weather) not enough of a temperature or pressure contrast to initiate a strong flow of wind.
Warm fronts, because they travel slower than the cold variety and spread their effects over a larger area, often aren't considered as great surface wind producers. However, this depends entirely on your location along the warm front and the strength of the parent low. Generally speaking, the weaker the low and the farther your distance from it along the frontal boundary, the weaker the surface wind.
Near a center of deep low pressure, the surface winds accompanying a warm front can be impressive indeed. As with cold fronts, pressure falls, then rises, as the warm front passes, but the shifts in wind direction are quite different. Prior to passage, winds blow out of the east or northeast. This is how you know you're in the low's northeast corner — a place infamous for its rotten weather. Afterwards, winds come from the southerly quadrants.
Though the focus here is adverse surface wind, I can't ignore some new research concerning low-level jet streams — which can be close enough to the surface to affect pattern altitudes. Several studies have documented the presence of winds up to 75 knots at altitudes as low as 2,000 to 5,000 feet. These low-level jets have been found in the warm sectors of midwestern frontal complexes and behind cold fronts east of the Appalachians. The processes that create them are still the subject of speculation, but we'll be certain to learn more about low-level jets in the future.
One bit of evidence is bound to be sharp temperature contrasts between clashing air masses just above the atmospheric layers where air moves freely, away from terrain effects. That's because, in the end, wind is always a function of temperature (and therefore, pressure) contrasts — whether at the surface or at 35,000 feet.
The best expression of the winds caused by a deep low (apart from a hurricane) would be a coastal low of the type often seen off the American East Coast. Their colloquial name — nor'easters — refers to the surface wind directions prior to the passage of the storm's low pressure center. Flying in or near a nor'easter is not a good idea. The winds can reach hurricane strength, and IFR ceilings and visibilities are virtually guaranteed. Weather charts and reports illustrating a typical winter coastal low accompany this article and show a fair amount of support for the low at higher levels of the atmosphere.
The models we've been discussing involve classical behavior patterns of low pressure systems and their attendant fronts. That is, a parent center of low pressure with a cold front extending to the southwest and a warm front extending to the east or southeast. In this model, lows and their frontal complexes move from the west or southwest to the east or northeast.
Where lows and fronts violate this textbook illustration, the usual rules don't apply. Weak surface lows — those without any low pressure aloft to help them deepen — can meander for a day or so, then fizzle out with no appreciable surface wind. Stationary, east-west fronts with small centers of low pressure will likewise create little surface wind. Don't relax, though. These fronts can persist for days and produce widespread heavy precipitation and low ceilings.
Surface winds are also the product of forces on smaller scales. That's why an understanding of valley winds, thunderstorms winds, and gust fronts is so important. The latter two, of course, dictate avoidance by healthy margins — 20- to 30 nautical miles away is a good start.
Valley winds are like other diurnal winds in that wind flows are dictated by cycles of heating and cooling. Winds flow up valleys during periods of maximum heating and flow down in the early evening, as the earth's heat radiates away.
This information is not pure esoterica. Many airports in elevated terrain are located along river banks, with their runways aligned with the river's flow. In the morning, prevailing winds will probably favor the runway heading down the valley. By late afternoon, a change to an up- valley runway will probably be in order. In cases where terrain and other obstacles dictate one-runway operations at all times, an understanding of local valley winds can mean the difference between a safe takeoff or landing and another adverse-wind accident.
But all the advice in the world won't help the ignorant. Conscientious pilots will practice their crosswind technique, work at coping with gusts and wind shear, and know that turbulence nearly always accompanies strong winds. They'll learn all they can and anticipate the worst.
Another set of NTSB statistics singles out lousy pilot attitude as perhaps the root cause of the majority of all weather-related accidents. There was no record of any type of weather briefing prior to 55 percent of the accident flights.
Poor judgment, not high winds, seems to be the biggest problem.
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