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In the fixed shaft constant-speed engine, the engine rpm may be varied within a narrow range of 96 percent to 100 percent. During ground operation, the rpm may be reduced to 70 percent. In flight, the engine operates at a constant speed that is maintained by the governing section of the propeller. Power changes are made by increasing fuel flow and propeller blade angle rather than engine speed. An increase in fuel flow causes an increase in temperature and a corresponding increase in energy available to the turbine. The turbine absorbs more energy and transmits it to the propeller in the form of torque. The increased torque forces the propeller blade angle to be increased to maintain the constant speed. Turbine temperature is a very important factor to be considered in power production. It is directly related to fuel flow and thus to the power produced. It must be limited because of strength and durability of the material in the combustion and turbine section. The control system schedules fuel flow to produce specific temperatures and to limit those temperatures so that the temperature tolerances of the combustion and turbine sections are not exceeded. The engine is designed to operate for its entire life at 100 percent. All of its components, such as compressors and turbines, are most efficient when operated at or near the rpm design point. 

Powerplant (engine and propeller) control is achieved by means of a power lever and a condition lever for each engine. [Figure 14-3]  There is no mixture control and/or rpm lever as found on piston-engine airplanes. 

On the fixed shaft constant-speed turboprop engine, the power lever is advanced or retarded to increase or decrease 

forward thrust. The power lever is also used to provide reverse thrust. The condition lever sets the desired engine rpm within narrow range between that appropriate for ground operations and flight. 

Powerplant instrumentation in a fixed shaft turboprop engine typically consists of the following basic indicators. [Figure 14-4] 

• Torque or horsepower 

• Interturbine temperature (ITT) 

• Fuel flow 

•  RPM 

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