Loss of aircraft mass is assumed to be a consequence of fuel consumption only. The rate of mass loss is assumed to be directly proportional to engine thrust. Expressed mathematically, the mass model becomes,
where is a constant whose nominal value is 4x10-6 slugs/pounds-second, and is engine thrust. The instantaneous aircraft mass, , is obtained by integrating Equation 6-1 over time with the initial condition,
where is the mass of the fully loaded aircraft.