ESDU 94039 gives simple formulae for the effect of small changes in thrust, power (for propeller-driven aircraft), weight and drag (assuming a parabolic drag law) at constant airspeed and, when divided by the acceleration factor, when airspeed is changing. Rather more complex formulae are also given for the effect of changes in airspeed and relative density. All the equations assume that there are no consequential changes in the other independent variables. Such formulae are useful in the adjustment of measured results to other conditions and in assessing changes required to achieve a specific performance level.