Monday morning quarterbacking regarding the possibility of having been able to prevent what happened in New York, DC, and Pennsylvania on 9/11 amounts to nothing but a waste of precious oxygen .if one is talking about preventive action that could have been taken on that infamous day.
Much could have been done in the months and years that led up to 9/11 to prevent the holocaust. But the palpable absence of that ounce (pound/ton) of prevention comes under the heading of presidential (especially 1993-2001 presidential)/legislative incompetence/malfeasance/accommodation, to which the American public either was not privy, or simply turned a blind eye.
If terrorists hijack an airliner again, you can bet that they are going to do it (just as they did last time) so that the time interval between taking over the controls and plowing into whatever strategic/populated target they plan to destroy will be minimal. There will be no window of opportunity to make the myriad of decisions as to how/when/where best to shoot the aircraft down. If we again allow hijackers/terrorists seats on a commercial American jet, we may as well record one more plane (and one more target) as destroyed (unless passengers of the Todd Beamer type somehow manage to avert the disaster).
With that said, I dont believe the next terrorist act on American soil will involve aircraft at all. So by focusing our attention on frisking little old ladies (with lethal hatpins in their possession) at our airports, we are not only harassing our senior citizens (and other innocents unfortunate enough to find themselves victims of the harassment quota du jour), but we are diverting our attention from other, more logical defensive, precautionary actions. (Its kinda like placing five hundred armed guards, shoulder-to-shoulder, around the perimeter of a bank that was held up yesterday (and making a determined point of randomly intimidating a certain percentage of the customers), while the robbers huddle, unnoticed and unhindered, a few miles away, planning to hold up the jewelry store in the next town).
As for President Bush issuing an order to shoot down any of the four doomed 9/11 flights, not only is there no precedent (thank God) for such a ridiculous move, but, had he had the opportunity and the inclination to do so, the media and the enemies of America (many of whom sit in DC pretending to govern the nation they despise) would have made the post-Watergate presidential crucifixion look like a romp in the park.