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Components and Operation
Air Intake
The air intake is an s-shaped duct with an inlet screen and a water trap (with associated
drain line). The intake is attached to the top of the APU enclosure.
Exhaust
The APU exhaust gases are discharged overboard through an exhaust duct located
below the right engine. The exhaust duct is equipped with a muffler assembly which
also acts as an ejector, drawing cooling air through the APU enclosure by means of a
spring-loaded flapper door installed on the opposite side of the APU enclosure.
EGT Indications
APU exhaust gas temperature is indicated on the EICAS status page using an
indicating arc that expands and contracts as APU operating limits change.
AUXILIARY POWER UNIT
For Training Purposes Only
Sept 04
5-7
P I LOT T R A I N I N G GU I D E
APU Schematic (Air Intake and Exhaust)
Figure 5-4
ECU
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
P604_05_010
APU
Inlet
Air
Fuel
Control
Unit
Thermo-
Couple
Exhaust
LEGEND
Electrical Signals
AUXILIARY POWER UNIT
5-8 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
APU Fuel Distribution System
Description
The APU fuel system provides fuel for start, acceleration, and governed speed control
of the APU. During start, the fuel system provides the correct amount of fuel to
support combustion and for smooth acceleration of the engine to full power. Once full
power is reached, fuel flow is modulated as necessary to meet the demands of varying
pneumatic and electrical loads, while maintaining a constant speed.
The APU is protected from overtemperature by the electronic control unit.
The normal fuel supply to the APU is from the right main tank. A backup fuel supply
from the left engine motive flow system is provided in case negative-G conditions are
encountered.
The APU fuel feed system incorporates an APU fuel pump, an APU fuel shutoff valve,
a negative-G shutoff valve, one-way check valves, and a fuel control unit.
Components and Operation
APU Fuel Pump
The APU fuel pump is powered by the DC Battery Bus, and draws fuel from the right
main tank. The pump provides fuel to the APU fuel control unit through the feed line
and the APU fuel shutoff valve.
Selection of the APU PWR FUEL switch/light activates the APU fuel pump and opens
the APU fuel shutoff valve and negative-G shutoff valve. Insufficient pump output
pressure is indicated by illumination of the PUMP FAIL indicator in the PWR FUEL
switch/light and the APU PUMP caution EICAS message.
A portion of the fuel from the APU fuel pump reaches the APU fuel control unit for
combustion, while the remainder is directed through the fuel/oil heat exchanger to
provide cooling of the APU generator adapter oil. Fuel that has passed through the
heat exchanger is returned to the right main tank.
APU Fuel Shutoff Valve
The APU fuel shutoff valve opens when the PWR FUEL switch/light is pressed in, and
closes when it is pressed out. If the valve does not match the switch position within 5
seconds, the APU SOV caution EICAS message is displayed.
When the APU FIRE PUSH switch/light is pressed in, the APU fuel shutoff valve (and
the APU negative-G shutoff valve) will close. Successful closing of the valves is
indicated by illumination of the green SOV CLOSED switch/light indicator, and the
APU SOV CLOSED advisory EICAS message. If the APU fuel shutoff valve fails to
AUXILIARY POWER UNIT
For Training Purposes Only
Sept 04
5-9
P I LOT T R A I N I N G GU I D E
close after APU FIRE PUSH switch/light selection, the APU SOV caution EICAS
message will be displayed.
APU Negative-G Shutoff Valve
The APU negative-G shutoff valve opens when the PWR FUEL switch/light is pressed
in, and closes when it is pressed out. The APU negative-G shutoff valve allows fuel
from the left engine feed line (by motive flow or electric boost pump operation) to
automatically supply the APU fuel control unit during negative-G conditions or in the
case of APU fuel pump failure. Positive pressure is required for fuel to pass through
the differential pressure check valve. A one-way check valve prevents this fuel from
entering the fuel-oil heat exchanger.
APU FIRE PUSH switch/light operation commands the APU negative-G shutoff valve
closed (as described in the previous section). If the valve fails to close, the APU NEG
G SOV caution EICAS message is displayed. In this condition, the left engine's motive
　
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