Thanks PF.
In the event of no-showing passengers then the aircraft has more than enough fuel on, better to have too much than too little fuel!
Yes, I understand that. What I meant was: if the aircraft is significantly lighter than expected (because a substantial number of passengers have not shown up and/or fewer bags have been loaded than expected), would this not mean that the V1 / Vr figures etc. need to be re-calculated? A lighter aircraft takes off at a slower speed, right? How "late in the day" can you do this?
Unless I am mistaken (please correct me if so), it is not possible for an airline to know the definitive number of passengers it will have on a particular flight until they present themselves at the gate, especially where online check-in is possible (rather than going with the figures at close of check-in) and said passengers do not check-in any luggage.
I apologise if I'm missing something fundamental here - it just seems that with the ever-increasing "ease" of flying, it makes knowing actual numbers harder. Perhaps the odd no-show simply has no effect in terms of flight performance calculations, but does present a security risk, as you say.