The fuel control system supplies filtered and measured fuel for combustion. It also supplies pressurised fuel to operate the CVG system. See figure on following page.
The heart of the system is the electro-mechanical Fuel Pump and Metering Unit (FPMU) which is controlled by a FADEC. Pressurised and filtered fuel is fed to the servo operated main metering valve (MMV). The servo valve is biased towards the closed position so, in the absence of a torque motor signal from either FADEC, the fuel flow to the engine will be cut off. The FADEC controlled metered fuel flow is fed to the fuel nozzles via the pressure raising valve (PRV) which, by spring loading, ensures a minimum gear pump pressure at low engine speeds. Feedback is from a linear variable inductance transducer (LVIT).
A latching two position shut off valve (LSOV) is STOP controlled by two dual-coil torque motors. At 23% N2 in the engine start sequence the LSOV START solenoid is energised and fuel pressure from the LSOV to the PRV is removed so opening the PRV and allowing fuel to be supplied from the MMV to the fuel nozzles.
An air vent valve automatically vents entrapped air or fuel vapour during engine starting and motoring. A FADEC controlled solenoid valve is commanded to open at the initiation of the start sequence. The valve remains closed whenever the solenoid is not energised so
preventing fuel leakage from the system if the aeroplane booster pumps are turned on when the engine is not running.
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The CVG actuator assembly comprises a series of turnbuckles, a torque tube assembly and hydraulic actuator controlled by fuel pressure from the fuel pump. The hydraulic actuator rotates the torque tube which, in turn, by means of levers and turnbuckles, moves the variable vanes in unison. The fuel supply to the actuator is fed from the CVG control valve which is operated by a torque motor under FADEC control. The fuel pressure fed to the actuator will determine the setting the angle of the inlet guide vanes and the first five variable geometry compressor vanes. Feedback is from a liner variable differential transducer (LVDT) on the CVG actuator.
Lane Controller
The FADEC computer has a unit inside it that decides which of the two lanes or channels will control the fuel flow and other outputs.
Should the lane change element detect an error between the demand made by the thrust lever angle and the output signals from the engine, it will automatically switch lanes to disable the
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lane that is giving incorrect signals and allow the other lane to control fuel by enabling its command output drivers and disabling the drivers in the original lane.
This is effected by passing demand and engine feedback through a comparator unit and it is this unit that will signal the lane change element to switch lanes if a fault is detected.
FULL AUTHORITY FUEL CONTROLLER LANE CHANGE PROCESS
The system consists of several parts:
The Throttle Lever – throttle control unit transduces mechanical movement into electrical signal. It encloses the drive mechanism for potentiometers and resolvers which are protected within two cases.TLA operates in two of four quadrants:
The first quadrant serves for positive angles.
The fourth quadrant serves for negative angles.
The other two quadrants are not used.
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The pilot will have initial control of the engine through the throttle lever. Movement of the lever will be no different from the more ‘traditional’ form of control. The difference is that, instead of operating a rod or a cable, the throttle lever is connected to a variable resistor – normally a tri-lane potentiometer, which will send information to a computer attached to the engine. A tri-lane potentiometer is used in case of failure. The signal will be sent to a
‘majority wins’ circuit’ so that, if one of the potentiometers fails, the other two will still send valid information through to the computer(s).
FAFC. The Full Authority Fuel Controller (FAFC) receives the input signal from the throttle lever, The FAFC will process this information in one of its computers and compare the pilot’s requirement to existing engine parameters.
Once the FAFC decides that it is possible to feed more fuel into the burners it will then open up the Fuel Metering Unit (FMU) to permit this to happen. As soon as more fuel is fed into the combustor the FAFC will measure the effect this action has had upon the engine and take the appropriate action.
The action taken may be to increase the fuel feed until the selected rpm has been attained or it may be that one of the maximum operating parameters of the engine has been reached and the fuel flow will be trimmed to prevent exceeding that parameter. On many units the FAFC will also send a signal to other systems so that they can operate at their most efficient point – the airflow control system and the active clearance control systems are two of these.
Other interactions are, for example, with the thrust reverser to prevent it operating in flight, the ignition system linked with the start circuits and stall warning system as well as a manual selection from the flight deck and an inhibiting circuit which will prevent the igniters firing if the engine is wind-milling in the reverse direction of rotation on start select; if idle fuel flow will be revised depending on whether the aircraft is in flight or on the ground.
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