This page covers conspiracy theories relating to all of the planes on 9/11 and individual planes other than Flight 93.
The planes were remotely controlled and thus no hijackers were even needed.
One obvious problem is that Flight 93 didn't fly into its target. So if it was somehow wired up for an auto-pilot takeover, the pilots managed to override it and drive it into the ground. How or why they did this doesn't really make much sense. The real problem here is the logistics of wiring these specific planes up for this kind of technology. "Their control systems are mechanical, not electronic, with cables and hydraulics to move the control surfaces[1]." The only plane that had an electronic setup at the time was the 777, but even with the electronic setup, a pilot could disengage controls easily; the planes that flew on 9/11 were 767s and 757s.
The military has remote controlled planes like the Predator or Global Hawk, but how well do they work? In 2005, General Ronald Keys had this to say[2]:
General Keys: Well, you can control them, but for example, we missed shooting down a MiG-25 during the war because of the latency in the system. We had the Hellfire-armed Predator up and the MiG-25 was coming in to intercept and we had him locked up, but by the time we had fired the missile, he had started his turn and so he broke lock. The reason was there's about a several second delay in the latency.
How much harder would it be to control a commercial jet and to hit specific targets such as the pentagon and the WTC? Even if these systems were to be installed is it even feasible for the remote controllers to carry out these maneuvers?
There are several other problems with the remote control hypothesis. One is the fact that these planes would have needed to be brought in to a somewhere like a factory to have these parts installed. Then they would need to bypass the safety tests, do it when no one else there is looking and finally these installations would have to be hidden somehow so the pilots wouldn't notice.
(Contributed by Dave Sorensen)
The four planes were substituted for other planes
Several issues here.
(Contributed by Dave Sorensen)