Engine Fuel
System
Fuel is drawn from the wing tanks via the fuel shutoff valve. All fuel regulation and monitoring accomplished downstream of the airframe fuel shutoff valve is considered to be part of the engine fuel system.
Main Pump
The engine accessory gearbox drives the main pump which draws in fuel from the fuel hopper tank to maintain a pressurized feed to the high pressure fuel pump. A main pump failure triggers activation of the related standby pump which is inside the respective hopper tank. Main pump failure is indicated by the illumination of its respective amber MAIN PUMP light on the FUEL panel, a master caution chime and a flashing amber FUEL on the CWP.
Fuel Heater
Fuel filter icing is prevented by an oil-to-fuel heater that uses PGB oil as its heat source. Heating is regulated by a thermal sensor that controls the flow of oil through the heater. Low fuel temperature triggers illumination of the respective amber FUEL LOW TEMP light on the ENGINE panel, a master caution chime and a flashing amber ENGINE on the CWP.
Fuel Filter
Before it enters the high pressure pump, the fuel is filtered. If a blockage occurs it triggers illumination of the respective amber FUEL FILTER light on the FUEL panel and master caution alerting with flashing amber FUEL on the CWP. The filter is self-bypassing as necessary to meet engine feed requirements.
Hydro Mechanical Unit
The HMU controls all fuel flow to the engines in conjunction with the DECU. It's integral HP fuel pump is powered by the gas generator accessory gearbox. The HP pump develops the high pressure necessary for proper fuel nozzle operation and operation of the HMU actuator for the inlet guide vanes and start bleed valve. The HP pump inlet is pressurized by the main pump to prevent the cavitation or air bubbles in the fuel caused by pump operation within the HP pump itself. Fuel cavitation results in decreased lubrication and cooling and increased pump wear. Failure of the HP fuel pump will cause an immediate engine flameout. Fuel shutoff is controlled by the condition lever via a shutoff valve inside the HMU.
Overspeed Drain Valve
The fuel metered by the HMU is delivered to the oil cooler and then to the overspeed drain valve.
