The document presents the Brequet Range Equation, which is used to calculate an aircraft's range. It defines range as the integral of velocity over time from takeoff to landing. It then derives an equation for the rate of change of an aircraft's weight due to fuel burn. Finally, it shows that assuming the lift-to-drag ratio is constant, this results in the Brequet Range Equation, which calculates range as a function of true airspeed, lift-to-drag ratio, specific fuel consumption, and initial and final weights.