"What is windshear? Can it rip the wings off or otherwise crash a plane?"
One of those buzzwords that scare the crap out of people, windshear is a sudden change in the direction and/or velocity of the wind. Remember that a plane's airspeed takes into account any existing headwind. If that velocity suddenly disappears or shifts to another direction, those knots are lost. It can happen vertically, horizontally, or both, as in the case of a microburst preceding a thunderstorm. A microburst is an intense, localized burst of air from a storm front. Think of it like an upside down mushroom cloud. The potency of windshear runs the range of barely noticeable to potentially deadly. When airplanes are taking off or landing, they operate very close to their minimum allowable speeds, and encountering a strong shear is dangerous. At higher speeds it's not of such concern.
Fortunately, windshear has become easier to forecast, and as a rule it does not appear out of nowhere, flipping a plane upside down without warning. Conditions that propagate shear are generally predictable, and pilots are trained to avoid them. Windshear got a lot of press in the 1970s and 1980s when it was still a misunderstood phenomenon. The crash of Eastern flight 66 in New York in 1975 is considered the watershed accident after which experts began to study it more seriously. The last headline crash attributed to windshear was in Dallas in 1985.
This "rip the wings off" business is something I can't begin to address. It's like asking if a wave can break a ship in half. Theoretically, yes. Practically speaking, no. In the case of windshear, pilots are not worried about losing wings, they are worried about losing speed.
© Patrick Smith 2005. The above text is adapted from ASK THE PILOT. Portions have appeared previously on Salon.com