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flow will continue to supply fuel to the APU. It may be necessary to shut down the left
engine in order to shut off the fuel supply to the APU if the APU FIRE PUSH switch/
light was selected due to an APU fire condition.
The APU may be only operated for a short time using the
alternate fuel source through the APU negative-G shutoff valve
because of the danger of generator adapter oil overheating.
Refer to your Minimum Equipment List for operating time limits.
APU Fuel Control Unit (FCU)
The APU fuel control unit is driven by the accessory gearbox and meters fuel to the
combustor under command of the electronic control unit (ECU). The APU fuel control
unit is equipped with an internal fuel shutoff solenoid, located between the fuel control
unit and the combustor fuel nozzles, which opens and closes when commanded by the
ECU. In addition to controlling fuel shutoff, the ECU regulates fuel flow to ensure
proper operating speed and temperatures throughout the operating envelope of the
APU.
During normal or automatic APU shutdown, an overspeed signal is generated to close
the fuel control unit shutoff solenoid. Closing the solenoid shutoff valve interrupts the
supply of fuel, causing the APU to shut down.
NOTE
AUXILIARY POWER UNIT
5-10 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
APU Schematic (Fuel Distribution, Start and Ignition)
Figure 5-5
APU
START/ STOP
START
AVAIL
PWR FUEL
PUMP
FAIL
SOV
CLOSED
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_011
APU
Inlet
Air
Start
Motor
Fuel
Control
Unit
FCU
Shutoff
Valve
Combustor
Thermo-
Couple
RPM
Sensor
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
Motive
Flow
Line
ECU
Note: Left motive 1 flow line can be fed by L Eng fuel boost pump, R Eng fuel boost pump, L Eng driven pump
1
AUXILIARY POWER UNIT
For Training Purposes Only
Sept 04
5-11
P I LOT T R A I N I N G GU I D E
Start and Ignition
Description
The starting and ignition systems operate automatically during the APU start
sequence.
Components and Operation
APU Battery
The 24-volt, 43-ampere-hour APU battery is located below the APU enclosure in the
aft equipment bay. The APU battery provides the cranking power required to turn the
APU starter. In the event of APU battery rundown, external DC power may be
connected to the aircraft to supply the APU Battery Direct Bus, allowing APU start.
Electronic Control Unit (ECU)
The ECU monitors APU sensors and switches, and sets the appropriate fuel and
temperature schedules. In addition, the ECU relays APU operating data to the flight
crew through the data concentrator units (to EICAS) and to the APU fault indication
panel located on the left aft fuselage (next to the ground air adapter).
The ECU senses changes in shaft or bleed load and adjusts the fuel supply to maintain
a constant rpm at 100%. If EGT operating limits are exceeded, the ECU will reduce the
bleed load by cycling the load control valve closed.
The ECU is powered by the Battery Bus when the APU PWR FUEL switch/light is
pressed in.
APU Starter
The APU starter motor is DC powered by the APU Battery Direct Bus. It transfers
cranking power to the APU drive shaft through the accessory gearbox. The white
START indicator in the top half of the START/STOP switch/light illuminates
whenever the starter motor is engaged.
　
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