APU SYSTEM MANAGEMENT

APU SYSTEM MANAGEMENT

1. keeps the APU rotation speed constant to obtain a constant AC generator frequency output, protects the power unit from over temperature, avoids load compressor surge, controls safe APU start and eases the APU maintenance

   • power-up state,
     • When the APU MASTER SWitch is set to ON, the APU fuel Low Pressure (LP) valve opens and the APU fuel pump logic is energized. The Electronic Control Box (ECB) controls the opening of the air intake flap and initiates a power-up test:
       • - test of On Board Replaceable Module - test of speed circuit, oil level, discrete and analog outputs,
   • watch state
     • After completion of the power-up state, the ECB automatically enters the watch state
       • Upon receipt of the start command, provided that there are no prohibitive conditions and rotation speed is less than or equal to 7% rpm, the ECB enters the start preparation state.
   • starting state
     • The starting state begins when the APU START P/BSW is pressed and continues until APU AVAILable light activation.
       • When the START P/BSW has been pressed in, the ECB initiates the start sequence and energizes the backup start contactor. The receipt of a stop signal at any time during the starting state leads to a shutdown without any cool down period.
         • BACKUP START CONTACTOR SUPPLIED + 1.5 S The components activated by the ECB are: - the gearbox de-oiling valve, - the ignition unit, - the main start contactor.
           • ROTATION SPEED N > 3% RPM The components activated by the ECB are: - the 3-way solenoid valve for the High Pressure (HP) fuel ON/OFF function, - the fuel servovalve for the fuel metering function.
             • ROTATION SPEED N > 5% RPM During starting, fuel flow is controlled under the following programs: - begin manifold fill algorithm, - open loop fuel schedule.
               • DELTA EGT > 10°C The initiated acceleration is controlled to 100% rpm. ROTATION SPEED N > 55% RPM The ECB de-energizes: - the ignition unit, - the gearbox de-oiling valve, - the main start contactor.
                 • ROTATION SPEED N > 55% RPM + 5 S The backup start contactor is de-energized.
                   • ROTATION SPEED N > 95% RPM The ECB functions are: - surge control and IGV control loops activation, - APU AVAIL signal activation, - start in progress output de-activation, - steady state speed control loop activation, - enter run state.
   • steady state
     • In steady state the ECB controls the speed, the load compressor operation, and monitors the Exhaust Gas Temperature (EGT).
       • SPEED CONTROL
         • This function keeps the rotation speed of the APU rotating assembly constant, whatever the load applied. This is achieved by metering the fuel flow, which varies with the air inlet pressure and temperature.
       • EGT CONTROL
         • This function avoids APU over temperature by controlling the Inlet Guide Vane (IGV) position to closed. Thus, the pneumatic load is first reduced to give priority to the electrical load. This is achieved by controlling the position of the IGVs to the closed position as a function of the EGT as long as the over temperature condition is present.
       • LOAD COMPRESSOR CONTROL
         • This function opens the Inlet Guide Vanes (IGVs) according to the bleed air demand from the A/C, ECS or MES. Protection against load compressor surge is achieved by controlling and reading the position of the bleed control valve, according to the inlet temperature, and differential and discharge pressure signals of the load compressor (DELTA P/P ratio). If the ECB detects a surge condition, the bleed control valve is positioned to discharge air to the exhaust.n.
   • cool down state
     • When the ECB is powered, it controls the APU starting and running phases. If an abnormal parameter is detected, it initiates an immediate shutdown without time delay, even if the APU bleed air system is used.
       • The ECB protective shutdown parameters are:
         • parameters are: - over speed, - over temperature - Low Oil Pressure (LOP), - High Oil Temperature (HOT), - start period timer, - sensor failure, - air intake flap, - no flame, - reverse flow, - no acceleration, - loss of DC power, - ECB failure, - generator HOT - loss of speed sensing, - IGV shutdown, - oil filter clogged.
   • shutdown state
     • The ECB initiates an emergency shutdown when either the APU FIRE P/B, which is located in the cockpit, is released out, or when the APU SHUT OFF pushbutton, which is located on the external power receptacle panel, is pressed. The ECB receives automatically an emergency shutdown signal when an APU FIRE is detected on ground by the APU Fire Detection Unit. The APU shuts down immediately without time delay, even if the APU bleed air system is used.