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The starter is capable of an immediate restart on roll down when APU RPM is at or below 7% RPM.
Speed Indication
Two speed sensors provide indicated speed for on-speed control and overspeed APU protection.
During operation, the FADEC monitors the input and should either sensor fail, there will be an APU FAULT advisory message displayed on EICAS.
A failure of either sensor will not cause an APU protective shut down to occur.
However, a failure of either sensor will not cause an APU protective shutdown both on the ground and in the air.
Temperature Indication
The EGT system consists of a single temperature sensing unit with two (2) probes. The probes provide redundant signals to the FADEC for fuel schedule trim, turbine temperature monitoring and Load Control Valve (LCV) modulation. The APU is protected from overtemperature during acceleration by protective features incorporated in the FADEC.
Loss of one probe will not affect APU operation.
In the non-essential (ground) mode, failure of both probes will cause the FADEC to shut down the APU and inhibit start.
In the essential (in flight) mode, failure of both probes will not cause a shut down. Instead, the FADEC programs a preset signal to allow pneumatic loading and normal operation of electrical power.
Hourmeter/Cycle Counter
An hourmeter is powered on at 95% RPM and records the running hours of the APU. It is deactivated when APU shutdown is initiated.
Rev 3, Apr 25, 2005 Flight Crew Operating Manual Volume 2
AUXILIARY POWER UNIT
ECU COMPONENT CONTROL SCHEMATIC
BLEED/AIR CONDITIONING
ANTI-PANEL
APU
RUN
OFF START L ENG BLEED XBLEED R ENG BLEED AUTO AUTO AUTO
OFF ON CLSD OPEN OFF ON
FADEC
APU CONTROL PANEL LOAD CONTROL VALVE APU BLEED AUTO OFF ON
SPEED SENSOR INLET LCV SCV SURGE CONTROL VALVE APU BLEED SWITCH
GEARBOX THERMOCOUPLE
IGNITION
PRIMARY FUEL SECONDARY FUEL MANIFOLD MANIFOLD
FUEL
CONTROL
UNIT FLOW DIVIDER
Control System
The APU electrical control system consists of two major sections: FADEC and electrical accessories. The FADEC is designed to execute precise control of the APU. Programming within the FADEC controls the APU through all modes and operating conditions. The FADEC monitors EGT, rpm, oil pressure, oil temperature and provides output signals for information display on EICAS. The electrical accessories are used, in conjunction with the FADEC, to perform sensing/control functions required for safe reliably starting and continuous monitoring of the APU.
Pneumatic
The pneumatic bleed load system consists of the LCV, connected to the bleed ducting and is normally modulated by the FADEC. The LCV can either be controlled automatically by the bleed management control system (AUTO selection) or manually by the APU BLEED switch (ON selection) on the BLEED/AIR COND/ANTI-ICE panel. The LCV will not open if:
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