Thomas, I appreciate your critique! But let me explain some things where we differ.
First, FCOM is an official document. It provides datapoints on fuel burn at certain weights and altitudes. When I put these datapoints into a regression, it looks like x = ab + c, where x is fuel burn, a is a coefficient, b is weight and c is a constant. What I found is that a is around 1/38 and c can be set to 0 without a substantial increase in inaccuracy. The model gives R2 in excess of 0,99. In other words, there is a constant fuel burn to weight ratio, which is logical, given that L/D should be more or less constant for the optimal altitude and constant speed. I sufficiently trust these data: the hard work has been done by Boeing in providing the data points, my job was just to find a linear law behind them.
With Airbus it is more tricky, as I don't have access to a nice FCOM with data points. Those I collected from photos. I agree that these data are messier, where speed and temperature play a role (speed can be corrected, as 10 degrees above / below ISA lead to 3% extra or 3% less fuel burn respectively). But again, if one plots the data, there could found an simple law behind them. The deviation from the average of 1/38 to 1/39 is around 2,5%, not 5%, which is the difference between the extremes. I think the difference between specific airframes and engines within the same type can be even larger. This data driven approach is alternative to the engineering one you propose -- and of course I am interested in feedback on it, as my interest is in model validation and not in a discussion on which type is better (I know they are more or less equal, otherwise the market would kill one program over another
) By the way, both 77W and A346 burn the same for a given weight, circa 1/35 per hour. We know why 77W won -- it is lighter and has some more capacity (deck floor space).
Concerning the argument on a larger wing of A350, I am considering that it is heavier and has more wetted area. So the overall impact on the fuel burn in less clear cut, otherwise the industry would go to the designs with very long wings, which has not happened yet (yes I am aware about gate space issue too