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ECU (Electrical Control Unit): The ECU is an electronic engine governor that reduces pilot load by maintaining a constant power turbine speed of 100%. Due to the power turbine speed mechanical link to the main rotor speed, this yields a constant main rotor rpm of 100%. The ECU also provides Turbine Gas Temperature limiting, (limits the TGT to 843 oC), load sharing (which keeps the torques of both engines matched within 5%), and power turbine overspeed protection (limits the power turbine speed to 106 ±1%). The ECU also processes power turbine speed and torque signals for the cockpit and feeds data to the history recorder.
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POU (Pressurizing and Overspeed Unit): The Pressurizing and Overspeed Unit (POU) is a fuel-controlling unit with four functions. The POU sequences metered fuel between the start and main fuel nozzles/injectors. The POU also purges the fuel nozzles with P3 air, preventing the fuel nozzles from clogging. An overspeed solenoid in the POU reduces the fuel flow to Ng, an ECU circuit activates the solenoid when the Np speed reaches 106 ±1%, once activated the solenoid actuates a valve inside the POU to reduce the fuel flow. This reduces the Ng output acting upon the Np. The reduction in fuel flow will continue to cycle until the cause of the overspeed is removed or %RPM is reduced manually.
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VG (Variable Geometry): The HMU mechanically positions Variable Geometry (VG) linkage that actuates four items: the IGV, 1st and 2nd stage variable stator vanes, and the Anti-Ice and Start Bleed Valve (AISBV) for engine bleed functions. The pilot controls the AISBV for engine anti-ice functions.
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IGV (Inlet Guide Vane): The Inlet Guide Vanes (IGV) precedes the first stage of axial compression. The IGV's work together with the 1st and 2nd stage variable stator vanes, which precede the 2nd and 3rd stages of axial compression. IGV's and variable vanes (stage 1 and 2) are positioned by the HMU, and operate in accordance with the gas generator speed and engine inlet air temperature. Use of the IGV's and the variable stators facilitates rapid stall-free accelerations, optimizing fuel consumption at partial power conditions.
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Q (Power Turbine Torque): Power Turbine Torque (Q) is an engineering term used to express engine torque, #1 represents the engine being discussed, and #2 represents the other engine. Both engines function independently, with one exception. The torque signals are relayed between each others ECU's for load sharing functions.
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T4.5 (Power Turbine Inlet Temperature): The Power Turbine Inlet Temperature (T4.5) is the temperature at the engineering stage 4.5 and Turbine Gas Temperature (TGT) is identical. Seven turbine gas temperature probes are positioned between the Ng and Np, and measure the temperature of combustion gases produced by the Ng that are about to drive the Np. The maximum temperature at station 4.5 of the power turbine is 843o C, which is monitored/limited by the ECU.
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