1. The terrorist pilots lacked the skill and training needed to fly jetliners into their targets
 
This is an especially popular contention with respect to American flight 77.  Hijacker pilot Hani Hanjour was a notoriously untalented flier who never piloted anything larger than a four-seater. Yet he is said to have pulled off a remarkable series of aerobatic maneuvers before slamming into the Pentagon.  The pilots of American 11 and United 175 also had spotty records and had flown only private planes.  They should have had great difficulty navigating to New York City, and even greater difficulty hitting the twin towers squarely. To bolster this idea that the hijackers were Oswaldian pawns, the conspiromongers often invoke impressive-sounding jargon and fluffery about high-tech cockpits, occasionally trundling out testimony from pilots.
 
Reality: The cabal’s feats did not require in-depth technical knowledge or a high degree of skill.  The attackers, as private pilots, were completely out of their league in the cockpits of those 757s and 767s; however they were not setting out to perform single-engine missed approaches or Category-3 instrument landings with a failed hydraulic system – or to land at all.  They were setting out to steer an already airborne jetliner, in perfect weather, into the side of a building.  Though, for good measure, Mohammed Atta and at least one other member of his group did buy several hours of simulator training on a Boeing 727 (this was not the same type of jet used in the attacks, but it didn’t need to be).  Additionally they obtained manuals and instructional videos for the 757 and 767, available from aviation supply shops.
 
Hani Hanjour’s flying was exceptional only in its recklessness.  If anything, his loops and spirals above the nation’s capital revealed him to be exactly the shitty pilot he by all accounts was.  To hit the Pentagon squarely he needed only a bit of luck, and he got it.  Striking a stationary object — even a large one with five beckoning sides — at high speed and from a steep angle is very difficult.  To make the job easier, he came in obliquely, tearing down light poles as he roared across the Pentagon’s lawn.  If he’d flown the same profile ten times, seven of them he’d probably have tumbled short of the target or overflown it entirely.
As for those partisan pilots known to chime in on websites, take them with a grain of salt.  As somebody who flies 757 and 767s for a living, I think my testimony carries some weight.  Ask around and you’ll discover that the majority of professional pilots feel the way I do.