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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.
AUXILIARY POWER UNIT
REV 41, Jul 08, 2004 Flight Crew Operating Manual
CSP 700−6
Volume 2
05−10−9
ECU COMPONENT CONTROL SCHEMATIC
GF0510_007
GEARBOX
INLET
SPEED
SENSOR
THERMOCOUPLE
FLOW DIVIDER
PRIMARY FUEL SECONDARY FUEL
MANIFOLD MANIFOLD
IGNITION
LOAD
CONTROL
VALVE
SURGE
CONTROL
VALVE
FUEL
CONTROL
UNIT
LCV
SCV
APU CONTROL PANEL
BLEED/AIR CONDITIONING
APU BLEED SWITCH
ANTIPANEL
FADEC
XBLEED
AUTO
CLSD OPEN
L ENG BLEED
AUTO
OFF ON
R ENG BLEED
AUTO
OFF ON
APU BLEED
AUTO
OFF ON
APU
RUN
OFF START
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/ANTIICE
panel. The LCV will not open if:
· Manually selected ON and antiice
is active.
· The left engine pressure regulating valve is open.
· The right engine pressure regulating valve and the crossbleed valve are open.
A Surge Control Valve (SCV) controls the APU surge potential when operating at altitudes above
15,500 ft. The system gives priority to AC generator loads in the event of a combined
generator/pneumatic power overload condition. The SCV closes in flight for main engine starting
or PACK operation.
AUXILIARY POWER UNIT
Flight Crew Operating Manual REV 41, Jul 08, 2004
CSP 700−6
Volume 2
05−10−10
APU SYSTEM OPERATING MODES
The APU system operating modes are as follows:
ECU Powerup
The ECU enters a powerup
mode when the airplane BATTERY MASTER switch is selected to
ON. It does a test of its circuitry to verify that it is capable of executing the APU control
requirements for operation.
Start Mode
Upon receiving an active start signal and successful completion of the prestart
test sequence, the
FADEC will initiate an APU start. During the start mode, the FADEC activates the fuel request
signals, positions the APU door, activates and/or deactivates the starter and ignition control
systems. It also controls fuel flow as a function of timed acceleration and EGT schedule as the
APU accelerates to 100 percent.
Onspeed
Mode
When the APU reaches 95 percent speed, the FADEC transitions to the Onspeed
Mode to control
governed speed and load control activation.
Cooldown Mode
With the APU operating in the Onspeed
Mode, the FADEC transitions to the Cooldown Mode
when the APU switch is selected OFF. Upon entering this mode, the load control valve is closed
and the ready to load signal is removed. During cooldown, the APU speed is lowered to 70 percent
RPM for 60 seconds below 20,000 feet or remains at 100 percent RPM for the 60 second
cooldown above 20,000 feet.
Shut down Mode
Upon completion of the cooldown period or in the event of a protective shut down, the FADEC
transitions to the Shut down Mode and controls the APU shut down sequence.
APU SYSTEM OPERATING SEQUENCE
The following events occur when the APU is selected from RUN to START position:
APU Starting
The APU can operate with battery power only. The APU is started automatically when the START
switch on the APU control panel is selected and held for one second. The RUN position
commands the APU fuel SOV and right DC auxiliary pump to energize. The START position is
spring loaded to RUN from the start position. When the door is correctly positioned, the FADEC
engages the starter. When the speed reaches 5 percent, fuel and ignition are activated. At 50
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