On November 25th 1971 the first aircraft hijacking occurred. DB Cooper parachuted from the rear air stairs of a 727. In order to prevent subsequent attempts all 727 aircraft were quickly retrofitted with a DB Cooper latch. This was simply a spring loaded sliding bar that was actuated by airflow outside the aircraft and prevented the door from opening. The latch is attached to the outside of the aircraft and made it impossible from anyone inside the aircraft from being able to open the air stairs in flight. It quickly became known as the DB Cooper latch or more properly the Cooper vane. The Cooper vane was also installed on all DC 9 aircraft, the only other aircraft in use at the time that had an aft air stair.

Sky divers thought it would be fun to repeat DB Cooper's feat of exiting an aircraft into the exhaust of three jet engines. In order to do this they must remove the DB Cooper latch and reinstall the four bolts without the latch.

More Info: en.wikipedia.org