They would have known from previous trips and assessments of the Round Earth lat/lon readings how much fuel they would need to bring for xxx Round Earth miles.
If you travel 300 miles on a road trip, according to your GPS, and your tank is half empty you have an idea on how much fuel you need.
Again Tom, you naively assume that airplane designers work by a process of trial and error. They don't. They know how much thrust a certain engine produces at a certain speed and altitude LONG before the airplane is off the drawing board. They know how much drag the airplane design creates - again, from LONG before it's off the drawing board. They also know the fuel demands of the engine (depending on altitude, airspeed and thrust) long before the engine is bolted onto an airplane.
Knowing thrust and drag as a function of speed - they can tell you to within a fraction of a percent how much fuel a plane will need over a particular flight - including takeoff, climb-out, cruise, descent and landing.
Your ideas of how airplanes are designed is something out of the 1930's - airplanes haven't been designed and built that way since World War Two.
That they'd fly some route and decide that the airplane needs three times as much fuel as they thought is laughable...especially because the 747-400ER's tanks are totally full for a flight of the airlines' predicted distance. There isn't physically room in the tanks for three times as much.
Also, don't you think they'd wonder why their planes need 100% or 200% additional fuel for flights along some routes than others that they believe are the same distance and duration? Airlines live and die by the amount of fuel they burn...it's around 40% of the ticket price for most routes:
![](https://qph.ec.quoracdn.net/main-qimg-2b03e66072eb6d582af7136d00a99e20)
If those routes that are stretched out by the FE map were true - then Qantas would be paying 120% of the ticket price for fuel compared to airlines who fly similar distances where the FE map doesn't stretch the distances so much. They'd have gone out of business decades ago!
Don't you imagine that if this was true that they'd be shouting from the rooftops to find science to explain this bizarre phenomenon? They really don't seem to be doing that.
You need to come up with a better argument - this one is an utter joke.
Hmmm - now here is an interesting thought: Knowing that 40% of the ticket price is fuel cost - I wonder what would happen if we graphed flight distance against air fare? If the world is round and the flight distance that the airlines claim is correct - I bet we'd see a graph which is a nice curve. If the world is flat then there should be a wide spread between fuel cost (and therefore ticket price) between overland and overwater routes - because overwater routes are the ones with the largest distortions compared to the airlines claimed distances.
Oh - wait...here's that graph:
![](https://i0.wp.com/www.rome2rio.com/wp-content/uploads/2013/03/distancegraph-scaled1000.png)
Seems that the cost per mile is pretty linear. With the discrepency between FET an airline-provided distances getting larger for longer routes - you'd expect a very non-linear curve here.
Honestly Tom...you really imagine you can win this debate? I don't think so.