Yesterday, I saw an interesting Lufthansa training film from the 1970s explaining how aircraft crews should respond to hijacking. (In the early 1970s, hijackings occurred weekly.) Most of the recommendations from security experts turned out to be exactly wrong for 9/11: Don't attempt anything against the hijackers. Do everything the hijackers demand. Passengers should follow the instructions of the hijackers and should not try to be heros. On the other hand, they described locked cabin doors with TV monitors.
https://www.youtube.com/watch?v=8pzIbUW8jIw