The pilot tried to maintain altitude without stalling, but the airplane's propeller clipped a power line; one wing and then the other hit power poles before the airplane came down and stopped against a brick wall.
Another NTSB report describes how an airplane hit the top of a tree on final approach. The pilot landed safely on the grass next to the runway. The pilot reported that the airplane dropped after encountering "wind shear about 1,000 feet from the end of the runway."
Today, the term wind shear is commonly used in reports such as these as well as in news reports for the general public. But until the 1980s, meteorologists were usually the only people who ever talked about wind shear. The term came into general use after being used in news stories about deadly airliner accidents in New Orleans in 1982 and at Dallas-Fort Worth in 1985. These accidents were caused by microburst-induced wind shear. But that sounds complicated. Instead, news reports generally used "wind shear" as a synonym for "microburst." Later reports about other accidents, research into microbursts, and measures taken to reduce microburst accidents continued to use "wind shear" to describe microbursts.
This is probably why the FAA added a question about wind shear to the written examination for private pilot and recreational pilot certificates. Answering the question correctly requires knowing that wind shear can occur at any altitude and that it can be vertical or horizontal. That is, you have to know that there's more to wind shear than microbursts.
While microbursts often cause the most dangerous wind shear because they affect airplanes near the ground, where a pilot has little time or room to recover, other kinds of wind shear can also be dangerous. It's more important that you know about wind shear than that you know the correct answer to a question on the FAA exam.
Wind shear refers to a change in wind direction or speed over a horizontal or vertical distance. Meteorologists describe wind shear in terms of the amount of wind change over a distance. For example, a report might say there's a shear of 25 mph over 1,000 feet.
Meteorologists describe wind shear in vector terms. That is, differences are described in terms of both differences in the direction and speed the wind is blowing at one location compared with the direction and speed of the wind at another location. The locations can be separated vertically or horizontally.
Wind shear is important to weather forecasters. The amount of vertical shear can determine whether a forecast predicts severe thunderstorms or severe thunderstorms with a chance of tornadoes. This is because winds at different levels of the atmosphere from different directions at different speeds can add the twisting motion to the air needed for severe thunderstorms to spawn tornadoes.
Hurricane forecasters sometimes report that wind shear is likely to weaken a storm. This occurs when the winds right above the ocean and the winds aloft are blowing from different directions or with great differences in speed. The resulting shear can rip a hurricane apart by destroying the upward movement of warm, humid air that's needed to keep the storm going. Figure 3 shows how winds blowing from different directions can add a twisting motion to the air. The blue arrows represent winds blowing in different directions while the red arrows represent the spinning set into motion by the winds. Figure 4 shows how winds blowing at different speeds can also cause a spinning movement of the air between them. The short arrow represents a slow wind speed, and the long arrow is a faster wind speed. Both Figures 3 and 4 can represent either vertical or horizontal wind shear. They also show how wind shear creates turbulence. To see how this happens, imagine that both show vertical shear and you're flying an airplane through the up-and-down movements of the red arrows.
High-flying pilots have to worry about the wind-shear-induced turbulence encountered when flying into or out of a jet stream. At lower altitudes, fronts, inversions, and showers or thunderstorms can create wind shear. When a front moves past, the wind direction changes. In other words, the winds are blowing from different directions on the two sides of a front.
Not all fronts create potentially dangerous wind shear, however. In general, only fronts with a temperature difference of 10 degrees or more across the front, or those moving faster than about 30 knots, are likely to cause strong wind shear. Strong inversions, or warm air atop cold air, also can create wind shear. Such inversions are common in valleys during the winter, but they can occur anywhere at any time of the year.
Figure 2 shows a valley filled with cold air (shown in blue) with warmer air above. Upper-level winds in the warm air blow across the top of the cold layer, much in the way the wind blows across the top of the ocean. Winds in the valley can be calm while the wind above the inversion is blowing at 25 kt or faster. Imagine taking off and climbing out of the valley in Figure 2, flying from the left to right. As you fly into the warm air you suddenly encounter a 25-kt tailwind, which causes a loss of lift. As the nose pitches down you regain airspeed and lift, but the shift could give you a few bad moments. You are also likely to encounter turbulence, maybe severe turbulence, because the wind is creating air movements like those in Figure 4.
While any strong wind shear can be dangerous, the more dangerous type is low-level wind shear (LLWS). The pilot might not have time to regain airspeed and altitude if he is close to the ground when the wind shifts. This is why forecasts warn of the possibility of LLWS whenever it's likely. Forecasts also say that thunderstorms imply the possibility of LLWS because thunderstorms can produce microbursts and other forms of low-level shear. By definition, LLWS occurs within 2,000 feet of the ground.
Thunderstorms or even showers can produce downbursts, as shown in Figure 1. Falling rain drags down cool air that spreads out when it hits the ground. If the downward-moving air is concentrated in an area with a diameter of two-and-a-half miles or less it's called a microburst. Microburst winds can be stronger than those from ordinary downbursts because the wind is more concentrated. In a microburst, the winds can curl as shown in Figure 1, creating a "vortex ring" something like a horizontal tornado. The biggest danger is the sudden shift from a head wind to a tail wind.
While microbursts can last up to 30 minutes, they are considered short-lived and cover a small area. This is why they weren't really discovered until the 1980s. Airliner crashes and close calls led to intense scientific studies, including Doppler radar examination of storms be- lieved likely to create microbursts.
One of the strongest microbursts ever recorded hit Andrews Air Force Base on August 1, 1983, only minutes after Air Force One had landed with then-President Ronald Reagan aboard. Peak winds near the air base were measured at 120 kt. But winds only about two miles away peaked at about 10 kt.
Scientific discoveries led to the creation of microburst training programs; low-level wind shear warning systems for airports; Terminal Doppler Radar now installed at major airports; and the development of onboard wind shear detectors for large aircraft.
While microbursts are the most dangerous kind of wind shear, thunderstorms produce weaker shears that can be dangerous. Thunderstorms send air downward that spreads out when it hits the ground. While most winds are relatively weak, they can travel for miles as "gust fronts," which cause the wind to quickly change speed and direction as they pass. Such gust fronts could have caused the accidents that first brought wind shear into the public's vocabulary. In fact, one NTSB report mentioned thunderstorms within 10 miles of the airport.
You don't need to be near a powerful thunderstorm to be caught by wind shear. Understanding that is the first step toward making sure a sudden wind shift never catches you by surprise.
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