See following for 'A Pilot's view' from 2001
The Hijackings: A Pilot's View
The theory put forward then was that the hi-jackers had taken the flights before. This was later confirmed by T. Carter who was an American Airlines Flight Attendant.
Scroll through to 11:11 for T. Carter on the following video link.
She reveals that some of the hi-jackers had conducted prior reconnaissance flights on the routes and had made video tapes as well as visiting the cockpits.
T. Carter - parapolitical investigator and 911 stewardess
In addition to prior intelligence gathering flights the hi-jackers obviously chose a fine weather day.