General
The fuel metering unit (FMU) lets the correct amount of fuel for combustion flow to the combustor . It also has a shutoff valve to stop fuel flow to the combustor. The EEC controls the FMU.
Location
The FMU is on the forward, right face of the main gearbox. It attaches to the front of the fuel pump.
Physical Description
The mating surfaces of the fuel pump and FMU make the fuel connections between the two units. These connections include the fuel inlet port (fuel from the pump) and the bypass port (fuel to the pump). Servo fuel enters the FMU through an external supply line on the left side of the FMU. Metered fuel for the combustor leaves the FMU through a port on its front face.
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PW4000 SERIES ENGINES
777 AIRCRAFT MAINTENANCE MANUAL
ENGINE FUEL CONTROL -FMU
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PW4000 SERIES ENGINES
777 AIRCRAFT MAINTENANCE MANUAL
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ENGINE FUEL CONTROL -FMU -FUNCTIONAL DESCRIPTION
General
The FMU is a hydromechanical unit that controls fuel metering and fuel shutoff. The FMU has electrical connections that go to the:
*
EEC
*
ELMS
*
Fire switch.
Metered Fuel Flow Control
Fuel flows into the FMU from the fuel pump. A metering valve controls how much fuel goes to the engine (metered fuel). Fuel not used for combustion (bypass fuel) returns to the fuel pump through the pressure regulating valve.
A torque motor controls hydraulic (fuel) pressure to the metering valve to control its position. The EEC electrically controls the torque motor. The EEC gets fuel metering valve position feedback.
The pressure regulating valve keeps a constant differential pressure across the metering valve.
If the engine speed gets too high, the EEC energizes the overspeed solenoid. This moves the metering valve to a minimum flow position.
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Fuel Shutoff Control
The minimum pressure and shutoff valve starts and stops fuel flow to the combustor. It also makes sure there is sufficient fuel pressure available for the fuel injectors. If there is not sufficient fuel pressure available, the valve does not open. Fuel pressure and a spring keep the the valve closed.
A run command from the EEC energizes the start/run solenoid. This removes the hydraulic pressure that holds the minimum pressure and shutoff valve closed. Then metered fuel pressure opens the valve. If a cutoff solenoid is energized when the EEC sends a run command, the valve does not open. Once the valve is open, it stays open until a cutoff solenoid energizes.
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