One of the most incredible things to come out of the investigations into the 9/11 attacks is the amount of planning and preparation Al Qaeda put into the hijacking of the four planes that day. In addition to living among us here in the US for months and years and securing basic flight training on commercial jets, the hijackers also took several "test flights" before their day of action. Yes, some of the terrorists actually flew the routes they would later hijack in order to learn what they might face on 9/11.
They checked out the passenger loads on those trans-continental flights--realizing that most were lightly-traveled--meaning fewer potential threats to their operations. They found out what potential weapons could be taken through security without raising concerns. They observed the movements of the cabin crews--when service carts would not be blocking the aisle to the cockpit and when fewer stewards would be at the front of the plane. They even noted the landmarks on the ground they would have to use to navigate the planes back to their intended targets--since they would not be able to reprogram the flight computers themselves. Of course, all of that planning and preparation paid off--as the terrorists went four-for-four in taking over the planes--and three-for-four in reaching their targets.
And that is why we should be deeply disturbed by what has happened to Malaysian Airlines Flight 370. Even if there is no direct connection with anyone on-board the missing flight, what happened to the plane is providing those who might want to take over another flight in the future with a pretty good playbook.
We (and those with bad intent) have learned that it's not that hard to board a flight with a stolen passport (as the two Iranian nationals did). We have learned that there are gaps in the air traffic control system (as flight 370 "disappeared" during the handoff from Malaysian controllers to Vietnamese controllers). We have learned that turning off a plane's "radar and satellite signatures" is not that hard--while everyone has found out that there are still some systems that continue to maintain contact (much to the surprise of even veteran pilots). However, we have learned that such data is not "instantaneous" and takes several days to decipher. We have learned that evading radar detection is still possible (if you know what you are doing)--and even if you are picked up as an "unidentified aircraft", some countries won't do anything about it (and they won't even report it unless they are "asked to" after the fact).
And the biggest lesson we have learned is that despite all of the "No Fly Lists", the body scans, the taking off your shoes, the ban on liquids in carry-ons and extra pat-downs of 80-year old women--all it takes is one pilot with his own agenda to turn a commercial airliner into a guided missile. Perhaps we should rethink those armored doors to the cockpit that were installed as the "last line of defense" after 9/11.