Anyone who took some flying lessons?  Because pilots were trained to follow a terrorist's orders, similarly to how bank tellers today are told "no heroics, just give a robber the cash; we'll round him up eventually".  

And FYI the idea literally was written into a Tom Clancy novel years before (although IIRC it was Japan that attacked) so the terrorists did not even need to be creative.

That same novel put forth the idea of combined physical/economic warfare as well -- crash the stock market just before you attack.