I think we should all remeber what is that we are trying to prevent the terrorists from doing... our primary concern is not to protect the lives of the people on board, but those on the ground. If the terrorists are determined to kill the 300 people on board (either by executing them one-by-one or by damaging the aircraft to the point it will crash) nobody can stop them, not even armed pilots. But then again, if what terrorist want to do is kill 300 innocent lives they can do that VERY easily by simply driving to a stadium, club, movie theater or any densly-populated area and blow up everybody. Lets say each terrorist can kill 200 people (many movie theaters on saturday night contain even more people), then the 19 terrorists can easily kill 3800 without the need of years of planning, flight instruction and all they had to do to execute the 9-11 attack. In other words, its relatively easy to kill a large number of people, especially if you are very determined and are willing to give your life. Just look at what's happening in Israel. And considering how easy it is to buy guns and build bombs in the US, the terrorist could probably achieve even "better results".
My point is that the 911 attacks were targeting certain american symbols, and were not done to kill the maximum number of people. However, should the terrorist's objectives change, we need to stop them from flying an aircraft into a nuclear power station which could potentially kill millions. This is why our only main concern is to protect the people on the GROUND, not in the air. We can't do more to protect the people in the air in the same way we can't protect the people going to a movie theater... if they want to kill them, and are appropriately trained, they will succeed. period.
The only and only way to protect the people on the ground is prevent the terrorist from acquiring control of the aircraft.
I was thinking about installing a button with close reach of both pilots which can be activated in a matter of split seconds. Once this button is pressed, a previously programmed "hyjack mode" of the FMS takes over the control of the aircraft and at the same time informs ATC. This computer program will first of all take the aircaft away from densely populated areas as well as certain predetermined targets (such as nuclear stations). The aircraft must cruise along a route where, should the hijackers damage the aircaft to the point it will crash, only the people on board would be killed. The second priority for the computer would be to land the aircaft at the closest airport that meets certain criteria such as runway length and the presence of a precision approach that would always keep the aircraft away from densly populated areas. The autopilot would then land the airplane and the good guys would be waiting on the ground.
I know this might sound like a weird idea, but if you think about it, the FMS flies the aircraft most of the time anyways, and it has the capability of performing all of the task I listed above. All we need is a software, thats it. A computer program that will totally exclude every control device in the cockpit and fly the airplane following a previously pre-programmed procedure. At every single stage of the flight this program would always have a "Hijack produre" ready to implement, depending on the location of the aircaft. We all know both Boeing and Airbus have all the computer geeks they need to devise such program.
Of course, an identical copy of the program would be installed in the ATC stations. This way ATC would already know exactly what the entire procedure the aircraft will follow up to the new destination airport including the approach procedure. (without the need of communicating with the aircraft) This means they can easily clear the skies below you with very little consequences on other traffic.
This is very important because it would reduce the problems associated with a false alarm. For example, assuming we have the autistic passenger who starts kicking the door... the pilots can immediately activate the "hijack procedure" knowing that the consequences of a false alarm would only be a couple hours of delay, nothing more. They would not have to shoot and kill a disabled passenger. The same would apply to many other situations where armed pilots might kill someone by mistake.
We all know autopilots can fly the plane (in many cases even better than pilots). So in the event of a hijacking why not let it fly the plane if that could save the lives of thousands, if not millions on the ground? The technology is there, all we need is a bunch of computer geeks to develop a very complex application which takes into considerations all the hundreds of different variables. It can be done, and should terrorists ever think about hijackacking another aircraft, they will know it will be IMPOSSIBLE for them to take control of the aircraft. They might still crash it, but as we said before if a group of determined terrorists wants to kill 300 civilians they will always be able to do so in a society with still has a few civil liberties left. What we want to avoid is having an aircraft blow up a nuclear or chemical plant, and guns in the cockpit cannot prevent that with 100% certainty. However, an effective computer program connected to the FMS would be able to stop them.
palgia