A Cooper vane is a mechanical aerodynamic wedge that prevents the airstair or rear stairway of an aircraft from being lowered in flight. Following three hijackings in 1972, Boeing 727 aircraft were ordered by the Federal Aviation Administration to be fitted with Cooper vanes. The device was named for famed airplane hijacker D. B. Cooper, who used the rear stairway to exit a plane in flight and make his escape via parachute. The Cooper vane is a very simple device consisting of a spring-loaded paddle connected to a plate. When the aircraft is on the tarmac, the spring keeps the paddle perpendicular to the fuselage, and the attached plate does not block the stairway. As the aircraft takes off, the airflow pushes the paddle parallel to the fuselage and the plate is moved underneath the stairway, preventing it from being lowered. Once the airflow decreases on landing, the spring-loaded paddle returns to its initial position, thereby allowing the stairs to be lowered again. Although this device was intended to prevent hijackings aboard the 727 and other aircraft with an airstair, many airlines sealed the airstair entirely, never to use it again.
