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Most of the time, pilots want the least amount of drag possible on their plane, so it can fly faster and farther, more efficiently. However, drag is quite useful for helping an aircraft slow down. Pilots can lift special flaps on the airplane's wing to increase the drag and help brake the plane. 

The Space Shuttle would deploy a parachute (also called a drag chute) when landing to help slow it down, as re-entry speeds were so fast, it needed help from extra drag to slow down all the way to a stop.