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64.26 Turbine Engine Indicating and Instrumentation
64.26.2 Describe the following types of turbine engine indicators and instrumentation including their function and basic principles of operation:
(a) engine rpm
The engine RPM indicator shows the rotational speed of the engine’s rotating assemblies, such as the compressor, gas-producer turbine, or power turbine.
Depending on engine design, RPM may be indicated as:
N₁ or Nᵍ — gas-producer speed; and
N₂ or Nₚ — power-turbine speed.
RPM is commonly displayed as a percentage of the maximum governed or rated speed.
The signal may be produced by a tachometer generator or electronic speed sensor.
(b) engine pressure ratio
Engine Pressure Ratio (EPR) is the ratio of the total pressure at the turbine discharge to the total pressure at the compressor inlet.
It can be expressed as:
EPR = Turbine discharge pressure ÷ Compressor inlet pressure
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