TM 1-1520-238-10 Change 9 2-25 remain continuously illuminated unless the nose gearbox fairings overheat +121 °C (250  °F) or under heat 96 °C (205 °F) or if the engine inlet section receives anti-ice air at less than 150 °F. If any of these three conditions occur, the advisory lights extinguish and the ENG ANTI ICE fail lights illuminate. M01-143 Figure  2-17. Pilot/Engine Anti-Ice Panel 2.26  ENGINE FUEL CONTROL SYSTEM. The engine has a conventional fuel control system: PWR lever position and the degree of collective pitch basically establish  the  power  output  demands  placed  on  the  en- gines. Engine power is trimmed automatically through in- teraction of the engine HMU and the ECU or DECU . The ECU/DECU of each engine exchanges torque signals  with  the  opposite  engine  to  achieve  automatic load-sharing between engines. 2.26.1  Fuel Boost Pump. A low-pressure suction fuel boost pump is installed on the front face of the engine ac- cessory gearbox. It ensures that the airframe fuel supply system is under negative pressure, thus reducing the dan- ger of fire in case of fuel system damage. If the FUEL PSI ENG 1 or FUEL PSI ENG 2 segment on the pilot caution/ warning panel illuminates at idle speed and above, it could indicate a leak or restriction in the helicopter fuel system or a failed engine boost pump. 2.26.2  Fuel  Filter. A  fuel  filter  is  located  between  the fuel boost pump and the high-pressure pump in the HMU. If this filter becomes clogged and impedes the passage of fuel, a bypass valve permits fuel to bypass the filter. The differential  pressure  initiating  bypass  actuates  the  fuel- pressure  bypass  sensor,  thus  causing  the  FUEL  BYP ENG 1 or FUEL BYP ENG 2 segment on the pilot caution/ warning panel to illuminate (fig 2-44). An impending filter bypass button on the filter housing pops out when filter element differential pressure indicates impending bypass. 2.26.3  Hydromechanical Unit (HMU). The HMU pro- vides metered fuel to the combustor to  control the gas generator (N_G) speed. The HMU contains a high pressure fuel pump to supply fuel to the metering section. The HMU responds  to  mechanical  inputs  from  the  crewmembers through the power available spindle (PAS) and the load demand  spindle  (LDS).  The  PAS  is  mechanically  con- nected to the pilot PWR levers while the LDS is connected to a bellcrank attached to the collective servo. The HMU regulates  fuel  flow  and  controls  positioning  of  the  inlet guide vanes, variable compressor stage 1 and 2 vanes as well as the anti-ice and start bleed valve in response to engine  inlet  air  temperature,  compressor  discharge  air pressure, N_G speed, PAS and LDS positioning, and the ECU or DECU . The torque motor feedback sig- nals from the HMU to the ECU/DECU are provided by the linear  variable  displacement  transducer  (LVDT)  to  com- plete the control activated within the HMU at 100 – 112% N_G speed. The HMU uses signals from the ECU/DECU to interpret fuel requirements and to vary fuel flow for auto- matic power control. The HMU will additionally provide N_G overspeed protection in the event the gas generator ex- ceeds 108 – 112% N_G. The reaction of the HMU to an N_G overspeed  is  the  same  as  for  an  N_p  overspeed.  Over- speed protection protects the gas generator turbine from destructive  overspeeds.  When  an  N_G  overspeed  is sensed, fuel is directed to the MIN pressure valve of the HMU which causes it to close and shut off fuel to the en- gine. 2.26.4  Overspeed and Drain Valve (ODV). The ODV responds to a signal from the ECU/DECU. Under normal operation, fuel is routed from the HMU via the oil cooler and through the ODV to the combustor. When an over- speed condition is sensed, a signal from the ECU or DECU closes a solenoid in the ODV, thus routing fuel back into the HMU. All residual fuel is drained overboard. Fuel  flow  to  the  fuel  manifold  ceases,  and  the  engine flames out.