*
APU fault indication
*
APU starter control.
APUC Outputs to the APU
The APUC sends signals to control these APU components:
*
IGVs
*
SCV
*
Fuel metering valve
*
Fuel shutoff solenoid
*
Ignition unit
*
Air turbine starter control valve
*
Data memory module.
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APU CONTROL SYSTEM -APU CONTROLLER -OUTPUTS
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APU CONTROL SYSTEM -APU CONTROLLER -FAULT MONITORING
General * Generator filter bypass valve/switch
* Fuel filter differential pressure switch
The APUC does tests of APU components and the internal * Fuel metering valve *
functions of itself. The APUC tests some components Fuel shutoff solenoid valve
* Igniter (2) continuously and others only when the APU is in a specific * Ignition unit mode (power-up, power-down, shutdown).
* Data memory module.
The APUC memory holds 256 faults. When the memory is full, new faults record over the oldest faults.
APU Faults
The APUC monitors these APU components:
*
EGT thermocouple (2)
*
Speed sensor (2)
*
Air inlet temperature sensor
*
Fuel temperature sensor
*
Oil temperature sensor
*
Total air pressure sensor
*
Inlet air pressure sensor
*
Differential pressure sensor
*
Oil pressure sensor
*
IGV actuator
*
Surge control valve actuator
*
ATSCV
*
Oil quantity/temperature probe
*
Low oil pressure switch
*
Electric starter (brush wear)
*
Electric starter contactor
*
APU oil heater assembly
*
Oil filter bypass valve/switch
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APU CONTROL SYSTEM -APU CONTROLLER -FAULT MONITORING
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APU CONTROL SYSTEM -SPEED SENSORS
Purpose
Two speed sensors measure APU shaft speed and send this data to the APUC for control and display functions.
Location
The speed sensors are on each side of the compressor section.
Functional Description
The speed sensors are variable reluctance motion sensors. Each speed sensor sends a signal with a frequency in proportion to engine RPM. The APUC selects the higher of the two signals for control and display functions.
Control
The APUC uses engine speed to control these functions:
*
Ignition system control
*
Starter system control
*
Fuel system control
*
Engine speed control
*
Overspeed automatic shutdown protection
*
No-acceleration automatic shutdown protection.
Indications
APU engine RPM shows on the status display and the maintenance page.
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777 AIRCRAFT MAINTENANCE MANUAL
APU CONTROL SYSTEM -SPEED SENSORS
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APU CONTROL SYSTEM -INLET TEMPERATURE SENSOR
Inlet Temperature Sensor
The inlet temperature sensor supplies inlet air temperature data (T2). The APUC uses this data for these functions:
*
Fuel control
*
IGV control
*
SCV control
*
Turbine inlet temperature calculation.
The inlet temperature sensor is an RTD (resistance temperature detector) type sensor.
This sensor is on the left side of the APU.
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APU CONTROL SYSTEM -INLET TEMPERATURE SENSOR
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777 AIRCRAFT MAINTENANCE MANUAL
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APU INDICATING SYSTEM -EGT INDICATING SYSTEM
Purpose
The APU EGT indicating system supplies APU exhaust gas temperature data for flight deck indication and APU control.
Component Locations
Four chromel/alumel thermocouple probes, grouped in two rakes, are on the exhaust section 90 degrees apart. The thermocouple housing is stainless steel. Each probe has two temperature sensing junctions.
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