What are the security measures preventing someone from chartering a 747 and using it to commit a terrorist attack?
I haven't spoken to any terrorists, so the only source I have for this is Bruce Schneier, but apparently, they're hard to pull off. Objectively, this makes sense; if they were easy, there would be more of them. 

For the sake of not referencing any actual living or dead person, let's call our supposed terrorist "Hans Gruber". Hans has enough money to charter a 747 and the desire to terrorize someone. How does he go about it?


If he charters the plane himself, takes control of it, and then crashes it into something (the Nakatomi tower, perhaps), and succeeds, then he dies. Surprisingly, I don't know of many people who are (a) willing to commit terror, (b) willing to personally die for their cause, and (c) multimillionaires.
Okay, what if Hans puts up the money but hires someone else to fly?  Well, money comes from somewhere and charter operators tend to remember you when your passenger hijacks and crashes your 747. Suddenly now you have the authorities after you. Plus, there's no guarantee that the person you hired is going to get the job done. 
Okay, but what if Hans is the president of a group of shell companies that can funnel money here and there and disguise the source? Sorry, no good there either. The more people who know about your plan, the more likely the NSA knows about it. At some point (and I would guess that point is not very large) more people in the know results in diminishing returns.
Okay, but what about an opponent who has lots of money, enough willing people to pull off the plan, resources to get them in place and provide the proper training, all while disguising the source? A vast conspiracy? Well, yes, those exist. They're called nations, and other nations go to war with them over things like that. But at that point, it's not really terrorism anymore, is it?