I guess they will work out something with the regulator. The longest continuous duty I’ve done was just under 28hrs last year under an alleviation.
I would need to check the maths, but I would not be so sure about that. Yes, fuel consumption is highest at takeoff, and is a direct function of takeoff weight, but the equation is definitely not 1:1. Also the 20 hr sector needs to be compared to two 10.5 hr to allow for another approach/landing & takeoff. But the big savings is doing away with the interim airport landing and handling fees which is a considerable portion of total cost. On the revenue side I'm not so sure how it will work out. As a rule of thumb, an airline can charge a premium for a non-stop service (or rather competitors with a connection need to offer a discount), however on such a long route where the time saving is marginal, I don't know to what extent will this work. I for one would rather pay less and spend a night in a Bangkok hotel to break it into two shorter legs. The key is how much of the premium traffic will they manage to capture to fill F/C. I have taken QF first, and the experience was quite disappointing in comparison with what the competition has to offer.
It’s non-linear and by the time you get out to the 20th hour of flying, you have had to carry the fuel to do that for 19hrs, burning a significant amount to do that. For a given L/D ratio, there is a point eventually where adding more fuel doesn’t measurably increase the range, similar to the rocket equation’s relationship to velocity. It’s an informed guess, but I’d expect to use at least 40% more fuel on a 20hr ULH sector than two 10.5hr ones with the same payload; at ~USD1,200/mt that’s going to be significant. Also, you might save in cycle-denominated engineering costs but you’re going to work the engines harder, especially taking off with likely very little or no derate.