Seem to remember discussing this in ground school, where we were told that in general you can use a "standard" weight. But if you're flying an aircraft that's been chartered to take a load of rugby players on tour, then the chances are their average weight will be considerably above average - and in this case, you must weigh the passengers. No idea if anyone actually does this or not - comments from the pros?
Also worth noting that this only applies to big aircraft. In light aircraft, the weight of each individual passenger is taken into account. In my aircraft I can carry full fuel and two average sized men, assuming no baggage. Given that I'm roughly average sized, maybe slightly larger than average, if I have a large passenger, I can't fill the fuel tanks up.
FFF
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