9. LEARNING STEP/ACTIVITY No. 9. Identify the operational characteristics of the engine controls.
a. Engine Controls- The primary engine controls consist of the PCL and collective stick, with the controlling of the main fuel accomplished by the HMU. The HMU has three main inputs that can affect fuel flow metering, the PAS, the Load Demand Spindle (LDS), and the ECU.
(1)
Engine and Control System Schematic- The engine control system provides for fuel handling, computation, compressor bleed and variable geometry control, speed control, overspeed protection, and over temperature protection. The system also incorporates control features for torque matching of multiple engine installations. Shaft power absorber coordination is provided initially by a mechanical input signal to the control system, proportional to helicopter rotor collective pitch setting, and a final automatic trim of the power setting to precisely equal the rotor needs is provided electrically. The engine control system is self-contained and does not require external electrical power for any controlling functions. Design of the engine control system allows removal of major components from the engine separately from any input components and replacement without calibration or matching.
(a)
HMU Schematic Diagram- The HMU handles all main fuel metering tasks during normal operations. Three main inputs to the HMU, the PAS, LDS, and ECU, affect metered fuel flow, other HMU functions limit fuel flow in support of the main three.
(b)
Power Available Spindle- Fore and aft Power Control Lever movements are changed into rotary HMU inputs through a push-pull cable and the Power Available Spindle. This direct mechanical link between the PCL and HMU allows the pilot to set the engine power lever anywhere from OFF, to maximum (OFF-IDLE-FLY). The FLY setting equates to a max power setting, but during normal operations the engine governor (ECU) trims back the power level to maintain the Np and Nr at a constant 100%. A PAS cable shear will cause a loss of all mechanical PCL functions. The engine can be shut down by fuel cut off using the fuel selector lever.
(c)
Collective/Load Demand Spindle- Up/down collective movements are changed into rotary Hydromechanical inputs via a push-pull cable and the Load Demand Spindle. This direct mechanical link between the collective stick and HMU allows the pilot to reset the engine power level to match the main rotor load. The amount of power the collective may call upon through the LDS depends on the PCL position, which sets the maximum power limits. If the PCL is in the IDLE detent, collective movements will not reset the power any higher than idle. With the PCL in the FLY detent, collective movements can reset power levels anywhere from idle to maximum. A LDS cable shear (or collective bias tube shear pin failure) allows an internal spring within the LDS to reset the LDS to a position that equates to full up collective, ECU and/or PAS inputs adjust fuel flow to compensate for the loss of LDS function, attempting to maintain the %RPM and %RPM R at 100%.
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