Originally Posted by
Mach Tuck
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.