The equilibrium operalton of a simple turbojet engine is analyzed, using logarithmic differential methods to infer performance changes about a design point. A simple method assuming constant specific heats and ignoring mass addition effects, is compared with a more rigorous analysis where both these effects are considered; it is shown that the computation of fuel flow, al least, should follow the rigorous analysis. The application of the analysis is illustrated by considering the performance response of a turbojet engine as the air flow is reduced. T he effects of simple mass flow change and simple compressor efficiency change are considered separately, and superposition to determine the combined result is illustrated. Extensive changes of cyele parameters (CPR and TIT), and component efficiencies (ñc and ñt), are shown to have only small changes on engine thrust and sfc response; somewhat stronger effects are associated with changing the engine flight condition.