They might at least have made the the plane crash short of its target.

which is quite possibly what happened with the 4th aircraft.

I believe - from latest reports - that tsuwm's surmise is correct. LAte reports say at least two men aboard that plane that crashed in Pennsylvania made an effort to foil the hijackers. They both made cell or airphone calls to family and implied that was what they were going to try to do.

Re terrorists flying plane : Pilots say the type aircraft involved "are hell to take off and land but easy to fly." Further, flight instructions manuals in Arabic were discovered at Logan Airport, Boston.