A jet engine is different from a conventional piston engine, as thrust is the direct force output from the engines and is roughly proportional to the fuel flow. On modern commercial jets, the primary source of information regarding the engine’s power is the N1 and the EPR.

The EPR is the ratio between the low-pressure turbine exhaust pressure (the engine’s last stage) and the engine inlet pressure (the fan’s pressure). It is a very precise parameter, so very gentle adjustments on the thrust levers must be made to achieve the desired power setting. EPR indicator pressure sensors can either be of the electromechanical type (bellows) or systems that transform a mechanical movement into an electric signal (transducer).