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acceleration (duration of the start).
The ignition system is fully automatic and controlled by the FADEC. Ignition occurs at 5% APU
RPM.
During APU operation in nonessential
mode (ground operation), ignition is terminated at 50%
RPM. During essential mode (in flight), ignition is terminated at 98% RPM.
Should flameout occur during operation, the ignition unit will automatically start through the “Auto
Relight” function of the FADEC.
Start System
Operation of the starter is automatically controlled by the FADEC, through the APU control switch.
Starter operation begins by selecting the START position on the APU control panel. At sea level,
starter cutout occurs at 46%. At altitude, starter cutout may be as high as 60% APU RPM to
ensure a positive start.
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 onspeed
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 nonessential
(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.
AUXILIARY POWER UNIT
Rev 3, Apr 25, 2005 Flight Crew Operating Manual
CSP 700−5000−6
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.
AUXILIARY POWER UNIT
Flight Crew Operating Manual Rev 3, Apr 25, 2005
CSP 700−5000−6
Volume 2
05−10−10
ECU COMPONENT CONTROL SCHEMATIC (CONT'D)
Pneumatic (Cont’d)
A Surge Control Valve (SCV) works to protect the APU compressor from a potential to Surge
(stall). The solenoid valve assembly (which allows pneumatic air to open the SCV) is energized
only when:
· The APU generator is providing electrical power and
· The APU LCV is CLOSED (APU not supplying any bleed air to the ECS) and
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