A few points about the flying of airplanes: I was a flight instructor for about 4 years combined and while I never even got close to one of the "big guys" I do understand a little bit about flying.
One - the bigger the plane the more stable it is. Therefore, it really did not have to be "flown" but rather "aimed." I was unaware that the first plane had to make such a large circle to get down, but any pilot can manage a large, looping, descending turn. Now, it might be true that in doing so he/she might rip a wing off due to unreasonable stresses, but (unfortunately in this case) those aircraft are certified to stay put together beyond all but the most unthinkable stresses. It's comforting to know when you are at 30,000 feet, but if a wing had fallen off earlier then the tower would have been safe, anyway.
Two - flying a plane is not really all that difficult. Even taking off in a plane is not that difficult. LANDING a plane is extremely difficult. Remember, all these terrorists had to do was make simple course and altitude changes. As long as he kept his airspeed up and stayed within certain parameters of bank, pitch, etc, a plane of that size would have been a pussycat to fly. The greatest threat in any flying is the loss of airspeed (airflow over the wings). As long as they were in a descending profile all they had to do was make sure they did not gain too much speed. A very inexpensive GPS unit would have told them how far out to begin descending to be down to their desired altitude in time. I used one all the time. If they had any training at all in the FMS or onboard computer systems of the airplanes it would have been just that much easier.
Three - disabling the transponder only kept the ATC specialists from knowing the altitude of the airplane. The controllers have ways of identifying "primary" targets, and since the controllers knew where the planes were when they disappeared, all they had to do was highlight the "blip" and follow it. They could determine the speed the plane was traveling over the ground, but would have no idea where it was in relation to altitude. I had many, many up close and personal experiences with aircraft that had no transponders but were identified to me by controllers who had a "primary target only, altitude unknown" in my general area. As I pointed out earlier, each of the planes was followed up until almost the points of impact. It is true that they could not be followed below a certain altitude, but they were almost down by the time that happened.
Thank you Spaw for the "numbnuts" description of how the design of the buildings worked. That visual image is better than all the engineering descriptions combined.
When the designers made provision for a fully loaded 707 to hit the building they were calculating weight X mass. They did not, for whatever reason, take into consideration the resulting fire from the fully loaded fuel tanks. So whether it was a fully loaded 707 or a 767 the weight issue was moot. The buildings withstood the shock with just the expected swaying motion. They would still be up if it were not for the resulting fires. And just a small amount of fuel would have been needed for the huge fireballs that were initially visible. Remember that the planes had tons of Jet-A fuel on board. One gallon of jet fuel weighs approx. 6 pounds (depending on temperature). The heat above those damaged floors must have been like hell itself - remember many people jumped to their deaths rather than face the inevitable death by burning.