As rpm increases, mass flow, temperature, and efficiency  also increase. Therefore, much more thrust is produced per  increment of throttle movement near the top of the range  than near the bottom. One thing that seems different to the piston pilot transitioning  into jet-powered airplanes is the rather large amount of  thrust lever movement between the flight idle position and  full power as compared to the small amount of movement  of the throttle in the piston engine. For instance, an inch of  throttle movement on a piston may be worth 400 horsepower  wherever the throttle may be. On a jet, an inch of thrust lever  movement at a low rpm may be worth only 200 pounds of  thrust, but at a high rpm that same inch of movement might  amount to closer to 2,000 pounds of thrust. Because of this,  in a situation where significantly more thrust is needed and  the jet engine is at low rpm, it does not do much good to  merely “inch the thrust lever forward.” Substantial thrust lever  movement is in order. This is not to say that rough or abrupt  thrust lever action is standard operating procedure. If the  power setting is already high, it may take only a small amount  of movement. However, there are two characteristics of the  jet engine that work against the normal habits of the piston- engine pilot. One is the variation of thrust with rpm, and the  other is the relatively slow acceleration of the jet engine. Fan speed (N$_{1}$) is the primary indication of thrust on most  turbofan engines. Fuel flow provides a secondary thrust  indication, and cross-checking for proper fuel flow can help  in spotting a faulty N$_{1}$ gauge. Turbofans also have a gas  generator turbine tachometer (N$_{2}$). They are used mainly for  engine starting and some system functions. In setting power, it is usually the primary power reference  (EPR or N$_{1}$) that is most critical and is the gauge that first  limits the forward movement of the thrust levers. However,  there are occasions where the limits of either rpm or  temperature can be exceeded. The rule is: movement of the  thrust levers must be stopped and power set at whichever the  limits of EPR, rpm, or temperature is reached first. Whereas piston engines normally operate in the range of  40 percent to 70 percent of available rpm, jets operate most  efficiently in the 85 percent to 100 percent range, with a  flight idle rpm of 50 percent to 60 percent. The range from  90 percent to 100 percent in jets may produce as much thrust  as the total available at 70 percent.  [Figure 15-6] In a piston-engine, propeller-driven airplane, thrust is  proportional to rpm, manifold pressure, and propeller blade  angle, with manifold pressure being the most dominant  factor. At a constant rpm, thrust is proportional to throttle  lever position. In a jet engine, however, thrust is quite  disproportional to thrust lever position. This is an important  difference that the pilot transitioning into jet-powered  airplanes must become accustomed to. On a jet engine, thrust is proportional to rpm (mass flow) and  temperature (fuel/air ratio). These are matched and a further  variation of thrust results from the compressor efficiency at  varying rpm. The jet engine is most efficient at high rpm,  where the engine is designed to be operated most of the time.  15-5 Figure 15-6.  Variation of thrust with rpm. Figure 15-5.  EPR gauge. Variation of Thrust with RPM Thrust To Thrust Lever Relationship Percent maximum thrust Percent maximum RPM 100 90 80 70 60 50 40 30 20 10 0 Variation of Thrust with RPM (constant altitude and velocity) 0 10 20 30 40 50 60 70 80 90 100 TAT 13° C 2.00 2.00 2.00 2.00 25.8 25.8 25.8 25.8 EPR CAB.LIGHT OFF FUEL VALVE 1 OP FUEL VALVE 2 OP FUEL VALVE 3 OP FUEL VALVE 4 OP ELEC GEN 1 ON ELEC GEN 2 ON ELEC GEN 3 ON ELEC GEN 4 ON HYDR SYS 1 HYDR SYS 2 HYDR SYS 3 HYDR SYS 4 TRIM 0.0 339 339 339 339 EGT TOTAL FUEL  335.0 LBS × 1000 N1