Originally Posted by
MP2086
Another aspect is the national guidance given. Pre-9/11, air crews were trained to cooperate with hijackers and passengers were encouraged to remain passive. The expectation of active resistance to hijackers on planes likely does more to deter them than screening passengers on the ground. My own opinion is that the next 9/11 style attack will occur using either private or cargo jets, so as to remove the wild card of having passengers on board.
Originally Posted by
Rare
Box cutters, pocket knives, and many other things were allowed on planes before 9/11, so no smuggling needed.
Doors were locked during flight, but not "hardened" and therefore potentially easy to breach (which the plotters didnt accomplish but the pax on 77(?) did over PA). But the other answers are more to the point, they were able to take advantage of social engineering to use what were considered benign and acceptable tools as weapons to take control.