Apparently there were groups of 4-5 on each aircraft armed with nothing more than knives (at least on the one which crashed near Pittsburgh). Faced with men who trained for this sort of operation and our day to day lower level of security, I honestly doubt whether there is anything you can do to prevent this type of act.
Whatever you do deter terrorists acts, groups like these will train to exploit the weak points. We can try and reduce the number of these, starting with carry-on baggage for one, but there will always be a risk.
Their weapons may have been nothing more than plastic knives or spikes. Even these could probably be hidden by professional terrorists. They will always have the element of surprise and therefore the upper hand.
Depressing I know but I'm trying to be realistic.
[ 12 September 2001: Message edited by: knackered ]