Originally Posted by
CL300
The indication is weight, but through the engines nozzles goes volume. Therefore as long as the tanks are full (volume) irrespective of the weight being shown, you will get the range.
You cannot escape the fact that the calorific value of fuel is directly proportional to its
mass, not its volume. A pound of fuel generates n units of heat (thrust) regardless of its S.G. and temperature but the volume of that pound varies with both these factors.
It follows that the total heat generated by a tankfull of 0.79 S.G. fuel is only 97.5% of that of the same tankfull of 0.81 S.G. fuel. Engine performance is unaffected; fuel may well be metered by volume but the lower the S.G. the more volume the FADEC demands to achieve the required thrust (2.5% more, using this example) and that reduces range by an equivalent amount.