[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.

[Demidog]

But there would be no way for the President to know their true intent unless they stated it as such.

Right. Which would make it wrong for him to take the lives of innocents. Furthermore, his insistence that they be unarmed and defensless puts their blood on his hands no matter what occurs.