Abstract: A method and apparatus for loading an aircraft fuel tank with sufficient fuel for a selected flight based on the measured energy content of the fuel. The density of the fuel loaded on the aircraft is measured and converted to a value for energy content based upon an observed relationship between energy content and density. In a second preferred embodiment, the energy content per unit volume of the loaded fuel is measured directly with a calorimeter. Both embodiments utilize learning to narrow the error band of the energy content density relationship and to recalibrate the calorimeter.