[hobbes1]

Boarding a flight could mean standing on line, waiting to be boarded, which, at that time of day, would probably not take too long, as the airport would not be too crowded.Showed up late, to further lessen the chance of being screened tightly, and got on. This is why they boarded in Portland, instead of Boston. They would have been inside the airport, transferring, not checking in in Boston, which probably would have had better security.

[aristeides]

But they were taking a big chance then of missing their flight (if, say, they got caught in traffic.) I wonder what their plans would have been if they had missed it.

I think this means the cell-phone communications among the hijackers in the different planes were critical for whether the operation was allowed to go forward.