Specific Air Range & Specific Endurance 

Key cruise parameters are Range (distance flown) and Endurance (time of flight). It is important to be able to relate these to the amount of fuel burned; in particular, it may be desirable to maximize the range or the endurance for a given quantity of fuel.

Specific Air Range (SAR) = distance flown per unit fuel burned

Specific Endurance (SE) = time of flight per unit fuel burned

Drag, power, functions of weight, which reduces during cruise due to fuel burn SAR and SE are therefore point performance parameters
Need to integrate SAR and SE over cruise to determine range and endurance