ok.. i have the answer from here
http://www.guidetopsychology.com/airfacts.htmAlthough turbulence is generally not dangerous, it would be a lie to say that it is never dangerous. So let’s sort out the dangerous aspects of it.
Commercial aircraft prefer to fly “straight and level” (at a constant heading and altitude) because it is convenient. First of all, air traffic controllers have to keep aircraft from flying into each other, and having pilots fly at predictable paths during each segment of a flight makes a controller’s job possible. Second, people tend to get airsick when an airplane moves erratically, so straight and level flight makes things more comfortable for the passengers and crew. And third, because the shortest distance between two points is a straight line, straight and level flight makes for economy. (Actually, since the earth’s surface is curved, aircraft routes are always curves, not straight lines—but that doesn’t matter to this discussion.)
When an aircraft flies through turbulent air, though, it will tend to rock its wings and dip and bob, all because the air in which it is flying is moving every which way. There is nothing dangerous about this because there is no physical law that says an airplane has to fly in a straight line at a constant altitude and at a constant airspeed. If you watch an aerobatic airshow sometime you will realize that airplanes can fly in all sorts of positions—even backwards, for a short time—and still be safe. So, even though level flight may be preferred, if an airplane enters turbulent air, its erratic flight poses no real safety issue to the airplane itself. Usually.
more info...
http://www.guidetopsychology.com/airfacts.htm