here is a link that speaks to the possibility: http://www.public-action.com/911/noradsend.html
. . ."Controlling the aircraft from the ground is nothing new. The military has been flying obsolete high performance fighter aircraft as target drones since the 1950s. In fact, NORAD (the North American Air Defense Command) had at its disposal a number of U.S. Air Force General Dynamics F-106 Delta Dart fighter aircraft configured to be remotely flown into combat as early as 1959 under the auspices of a program known as SAGE. These aircraft could be started, taxied, taken off, flown into combat, fight, and return to a landing entirely by remote control, with the only human intervention needed being to fuel and re-arm them."
Re-read that final sentence in the above quote:
"These [NORAD] aircraft could be started, taxied, taken off, flown into combat, fight, and return to a landing entirely by remote control … "
Given over 40 years of institutional experience, flying remotely controlled "suicide" jets into the World Trade Center towers would have been a piece of cake for NORAD.
(sorry, the link doesn't seem to be 'clickable'... copy and past web address to your browser)
First of all, how are you going to convince American Airlines and United Airlines to take their aircraft out of service for many months for the purpose of modifying those aircraft? Secondly, how are you going to integrate such a modification into the systems of the B-767 and B-757, and do in such a way that the pilots can't detect the modification during their systems checks? Remember, we are not talking about older generation airliners. Thirdly, how are you going to modify the aircraft under the watchful eyes of mechanics and inspectors?
Now, if you order hardware and engineering technical data for such a modification, that will leave a paper trail that can be easily tracked.