Aviation Security and the joke it really is. I bet that 90% of the flying public would be shocked to know how bad aviation security is. Airlines can screen for bombs (it's been done since the 80's) but screening passengers is much more difficult. And in those regards, it's almost impossible untill you start giving everyone a psych evaluation.

The drawback is, in order to take over an aircraft you require alot of power. You must overcome the other passengers on board the aircraft and neutralize the pilots. Both objectives can be done independently, but attempting to do both is problematic. Pre 9/11, the pilots and passengers were not conditioned to defend themselves.

Subdueing the passengers takes a great amount of power. This power can either come from numerical superority or outside influences (weapons of some sort). While numerical superority is possible, it is infeasable due to the sheer number of passengers on board an airliner. Getting 50 or greater terrorists on board a US airliner without any flags being raised would be quite a feat.

This leaves weapons, which is a possibility. While Secure Identification Display Area (SIDA) zones are in theory secure places, it isn't always the case. Weapons can be smuggled in by trusted employees or taken out of checked baggage (you are allowed to check firearms). This would require getting a terrorist inside the SIDA or hiring someone to do it for you. Also, once inside the SIDA, airline travel allows you to stay within the SIDA zone, and thus a breach anywhere in the US would allow you to smuggle these weapons anywhere.

I will go into the pilots later on.