My point was that they always seem to be carrying an excess of what they need.
And my point is that the 747 had more than enough fuel to get to Paris without the center fuel tank. TWA isn't going to fill up with thousands of pounds of additional fuel just to haul it to Paris.
Airlines fuel a plane for the destination plus a safety factor.
IIRC the safety factor is enough fuel for an alternate airport plus 1 hour.
The weight of the passenger load plus fuel load must be within a certain weight. If the fuel load to get to the destination plus safety factor plus passengers is too much, then passengers are bumped from the flight to get the weight down.
Airport altitude plays a role also.
I recall in the ‘old days’ when prop planes were grounded at Denver on a hot summer afternoon as there wasn’t enough ‘lift’.in the thin air.
My son works for one of the airlines and makes those calculations. He told me that it’s still a problem.
The short answer is commercial airplanes don’t usually take off with a full load of fuel.
Just at a guess, if you’re trying to land in an emergency then you might want as little of the stuff that might burst into flames onboard as possible.
My point was that they always seem to be carrying an excess of what they need.
So, because airplanes don’t carry EXACTLY the estimated amount of fuel on board, you infer that they carry full tanks everywhere they go? Deep thinking. Really.