On an aircraft, what is a Cooper vane?

A Cooper vane (also called a Dan Cooper switch or D.B. Cooper device) is a mechanical aerodynamic wedge that prevents the staircase of an aircraft from being lowered during flight.

In the US, following three hijackings in 1972, the Federal Aviation Administration ordered that Boeing 727 aircraft be fitted with Cooper vanes. The device was named for an unidentified airplane hijacker dubbed D. B. Cooper, who used the rear stairway to exit a Boeing 727 in flight and make a parachute escape. Claiming to be armed with a bomb, he demanded $200,000 in ransom (equivalent to $1,400,000 in 2022), and four parachutes. After releasing the flights passengers in Seattle Washington the hijacker instructed the crew to refuel and begin a second flight to Mexico City. 30 minutes after taking off, the hijacker opened the aircraft's aft door, deployed the staircase, and parachuted into the night over southwestern Washington never to be found.

The Cooper vane is a very simple device: It consists of a spring-loaded paddle connected to a plate that prevents the stairs of an aircraft from being lowered in flight. When the aircraft is on the ramp 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.