Conspirator 1: Ok, so we're going to hijack Flights 11, 175, 93 and 77, and fly them into...
Conspirator 2: Wait a second. Why don't we use "something else" to hit the towers instead?
Conspirator 3: What for?
C2: Well, I don't think a 767 can fly fast enough.
C1: What do you mean "can't fly fast enough"?
C2: .....
C1: .....
C2: Ok, so anyway, we hijack these planes, then we hit the towers with some other kind of aircraft that looks similar, giving the illusion that it was indeed 767s that hit.
C1: So what do we do with these planes?
C2: Take them "somewhere else"
C3: How do we keep ATC, FBI, NTSB, and other investigators from finding this out?
C1: Oh, we'll figure that out when we get to it.
C2: So wait... why can't we just use the planes again? Isn't this unnecessarily complicated and thus more risky?
C1: .....