In a few cases pilots have been rendered unconcious or killed and their planes have flown on as 'ghost' planes.
So imagining a situation where the pilots of an airliner suddenly die and the passengers are unable to break their way into the cockpit, but this plane is on a direct collision course for a major place like Times Square or the Whitehouse.
Is it possible for the military to legally shoot down a plane over a rural area before it reaches a location where it will kill possibly hundreds?
Unequivocally yes.
An Australian judgement asserted this by (happily) adopting the reasoning of the US courts:
The matter also came up for discussion in the Communist Party case where Justice Dixon adopted the U.S. view that:
... it is within the necessary power of the Federal government to protect its own existence and the unhindered play of its legitimate activities. And to this end, it may provide for the punishment of treason, the suppression of insurrection or rebellion and for the putting-down of all individual or concerted attempts to obstruct or interfere with the discharge of the proper business of government ...
A plane (or ship, or train etc.) that is not responding to hails and acting to put citizens at risk is a legitimate target of military force. It doesn't matter if it is acting that way because of a deliberate decision of its controllers (e.g. terrorists), because it's not under control or because it's controllers are idiots.
