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pneumatic energy from the turbine and some of the cooling air from the combustion
system. As with shaft loads, the rpm drops, and the ECU commands an increase in fuel
flow to recover full governed speed. The loss of pneumatic energy is replaced with
thermal energy (fuel), and EGT increases.
Two variables influence EGT under bleed load conditions:
• Aircraft demand for bleed air (air conditioning packs or engine starting)
• Ambient air temperature and air density
If the EGT reaches its limit, the ECU will reduce the bleed air load by cycling the LCV
toward the closed position.
Load Control Valve (LCV)
The APU LCV is controlled from the flight deck by selecting the APU LCV OPEN
switch/light on the BLEED AIR control panel. It controls APU bleed air supply to the
left side of the 10th-stage manifold. The ECU ensures that priority is given to
electrical loads by cycling the LCV closed (reducing bleed load) when the exhaust gas
temperature approaches operating limits.
Load control valve operation is indicated by illumination of the OPEN indicator in the
APU LCV switch/light. When the ECU cycles the LCV for EGT limit protection, the
OPEN indicator will extinguish. If the APU LCV has failed open with the 10th stage
switch/light interlock protection activated, the APU LCV FAIL caution EICAS
message is displayed.
AUXILIARY POWER UNIT
For Training Purposes Only
Sept 04
5-15
P I LOT T R A I N I N G GU I D E
Surge Control Valve (SCV)
To prevent compressor surge, unused bleed air is vented through the SCV into the
APU exhaust muffler whenever the LCV is commanded closed. Surge control valve
operation is automatically controlled by the electronic control unit.
Bleed Air Indication
Left and right 10th-stage bleed air indications are displayed on the EICAS status page
below the APU gauges.
APU Schematic (Pneumatic Supply)
Figure 5-6
ECU
P604_05_014
Bleed
Air to
10th-Stage
Manifold
APU
Inlet
Air
Surge
Control
Valve
Thermo-
Couple
APU Load
Control
Valve
Exhaust
LEGEND
Electrical Signals
Electrical Commands
DUCT
MON
10TH STAGE
APU LCV
FAIL
OPEN
ISOL
OPEN
L
DUCT
FAIL
CLOSED
R
DUCT
FAIL
CLOSED
BLEED AIR
Panel
CABIN TEMP
BLEED PRESS
FUEL TEMP
ENGINE
BULK
OXY
C ALT
RATE
P
47
23
0
36 35
1820
100
0
0.0
APU
100 323
RPM EGT
15
AUXILIARY POWER UNIT
5-16 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
APU System Schematic
Figure 5-7
APU
START/ STOP
START
AVAIL
PWR FUEL
PUMP
FAIL
SOV
CLOSED
ECU
APU BATT
DIR BUS
BATT
BUS
To ECU
APU FAULT
Panel
APU Panel
HIGH TEMP
LOW PRESS
HIGH TEMP
LOW PRESS
CABIN TEMP
BLEED PRESS
FUEL TEMP
ENGINE
BULK
OXY
C ALT
RATE
P
47
23
0
36 35
1820
100
0
0.0
APU
100 323
RPM EGT
15
Fire
Shutdown
Relay
OVER
SPEED
OVER
CURRENT
EGT
HIGH
PRESS. LOW TEMP. HIGH
PRESS. LOW TEMP. HIGH
IND. RESET APU STOP
APU FAULT
OIL
GEN. ADAPTER OIL
P604_05_007
Bleed
Air
APU
Inlet
Air
Start
Motor
Fuel
Control
Unit
Fuel/Oil
Heat Exchanger
FCU
Shutoff
Valve
Gearbox
Oil Tank
Cooling
Fins
Surge
Control
Valve
Generator
Adapter
Oil Tank
Generator
Adapter
30 kVA
Generator
Hour
Meter
Combustor
Thermo-
Couple
APU Load
Control
Valve
RPM
Sensor
Accessory
Gearbox
Exhaust
APU
FUEL
PUMP
To Left
Engine
Back to
Right Main
Tank
Right Main
Tank
APU
Negative-G
SOV
APU
Fuel
SOV
LEGEND
Fuel Line
Pressurized Fuel Line
Electrical Signals
Electrical Commands
Electrical Power
DUCT
MON
10TH STAGE
APU LCV
FAIL
OPEN
ISOL
OPEN
L
DUCT
FAIL
CLOSED
R
DUCT
FAIL
CLOSED
BLEED AIR
Panel
From
Left
　
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