[Demidog]

There is no simple answer for that question

Yes. There is a simple answer. Unless you want to argue that it is perfectly OK to murder in which case you'd have to explain why every state has murder laws. Your scenario is one of self-defense. I wouldn't agree that it would be OK to shoot down that airplane unless it was a known fact that the plane was to be used as a weapon. In which case, if the passengers were unarmed due to the fact that the government had demanded they be disarmed, thus taking away their right to defend themselves, the government would be guilty of murder.

[Bush2000]

I wouldn't agree that it would be OK to shoot down that airplane unless it was a known fact that the plane was to be used as a weapon.

But there would be no way for the President to know their true intent unless they stated it as such. As we know from the 9/11 hijackings, no such intent was every overtly mentioned; therefore, the only thing at the President's disposal now is the knowledge of what hijackers did previously. So you see: It's not a clear cut case of self-defense. The President would have to use his best judgement over whether any hijacking posed a greater threat.