How much of the cost of a flight is fuel? What percentage of the mass being moved is passengers and their luggage be freight and the plane and fuel itself?
Better question: what fraction of airline's margins could one explain with the variation of passengers' weights? Airlines' pre-tax margins are low [1]. Almost any factor, meaningless or meaningful, will look tiny relative to revenues or costs.
[1] https://www.statista.com/statistics/225856/ebit-margin-of-co...
In terms of flight operating costs (FOC) per hour, about 16-36% according to the numbers for a 737-500 across for Continental/United/Southwest a few years ago.
It's very dependent on utilization. Fixed ownership (i.e. depreciation/leasing) costs start to dominate at low utilization rates.
Boeing 737‐500 FOC per block hour (~2017):
Crew Fuel Maint. Ownership Hrs/day
Continental $510 $430(19%) $ 651 $698 3.9
United $927 $487(16%) $1048 $510 4.3
Southwest $388 $537(36%) $ 251 $350 8.2
The same for the 737MAX.
So an extra 100 lbs on every passenger is a drop in the bucket!
https://courses.washington.edu/ie337/Boeing%20Tour%20Facts.p...
Edit: the plane holds 328-550 depending on seat configuration per the above link. I went with the upper end of that range per consideration of the “safety considerations” indicating it is the upper end of that range that is being reconsidered.
Is the average American 100 lbs heavier than the average person from the rest of the world? Probably nowhere near that.
The problem case for weight are the regional jets (E190 etc) and to a lesser degree the 737/A320 family. Those tend to be weight limited enough that you need to offload passengers or baggage if it's too warm outside.
Of course, fuel is no different than any other weight, so the same rule would apply to the revenue load (people, bags, freight)—for every three pounds it cost you one pound of fuel.
Incidentally, I've always felt ticketing should be pro-rata wrt weight. Well, where 'always' is defined as 'since I obtained my FAA dispatcher license.'
/Acey