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Investigating Microburst Wind Shear

by Rich Coppenbarger

When I first started working at NASA Ames, I was working on accident investigation, specifically accidents due to weather conditions. Whenever there was an accident or incident due to turbulence or other wind-related weather phenomenon we would get the black box data recordings and try to figure out what happened to that airplane.

We often looked at data that involved turbulent conditions occurring at high altitude called Clear Air Turbulence (CAT), which is impossible to see or predict ahead of time. If the turbulence is severe enough the airplane may get thrown around or lose altitude very rapidly. You may have seen or read about this in the news where airplanes may suddenly loose up to 10,000 feet of altitude. Because typical airliners are flying at 30,000 feet they can afford to lose the altitude, but the planes often experience very strong aerodynamic forces. Passengers who are not strapped in can be thrown about the airplane and there have even been fatalities due to CAT. That is why some airlines are now insisting that passengers wear their seatbelts throughout the flight, not just during take-off and landing.

My main research interest while doing accident investigation was something called microburst wind shear. Low-altitude wind shear is caused by downward flowing air, usually as a result of thunderstorm activity. This type of weather phenomenon, often called a downburst, is caused by the same type of conditions that cause tornadoes and occur often in the Midwest. Although downbursts rarely cause damage to homes, they are very dangerous to aircraft that are landing.

Downbursts are dangerous because they often mislead pilots into making the wrong decisions. A lot of the intuitive things a pilot knows to do are wrong when they encounter a microburst. When first entering a downburst, the airplane experiences a head wind which causes the pilot to naturally pull back on the engines. This head wind is followed by a sudden down draft, followed by a tail wind. This means that the pilot must now throttle forward on the engines, but the engines are already at a low power setting due to the pilot's first instinct to throttle back. Occasionally this leaves the aircraft without enough power to climb out of the downburst. A tragic example was in 1985 when an aircraft landing at Dallas-Fort Worth airport during a thunder storm hit a downburst causing it to loose altitude very rapidly and crash, killing people on the aircraft and on the ground. Since the 1970's there have been about 20 accidents due to wind shear downbursts.

What we were trying to do in our program was to understand wind shear phenomena a little more. From blackbox flight data, we were trying to understand what the wind profile of a downburst looked like so that we could develop flight procedures that would help a pilot identify and respond to a wind shear encounter. The goal was to get pilots to recognize this phenomenon and take the appropriate action before the aircraft gets into a dangerous situation. As a result of the enormous amount of research by NASA and other agencies, wind shear related accidents are now much less common than they used to be.

 
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