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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 nozzle (tailpipe) directs exhaust gases from the APU, into the exhaust duct
located below the right engine. The nozzle is smaller in diameter than the exhaust duct and
therefore acts as an ejector, drawing air from inside the enclosure and mixing it with the exhaust
gases. This method cools the exhaust gases and also establishes a ventilation flow in the
enclosure, by drawing cooling air via the spring-loaded flapper door that is installed on the
opposite side of the APU enclosure.
Heat Shield
The exhaust gas heat shield is installed on the aft side of the fuselage, below the right engine
pylon. The heat shield is used to minimize the exhaust gas impact on the aircraft skin.
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 and a digital readout that is
displayed in degrees Celsius.
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−6
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
AIR INTAKE AND EXHAUST (CONT'D)
Air Intake, Exhaust and Indication
Figure 05−10−4
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−7
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION
Description
The APU fuel system provides fuel for APU start, acceleration, and governed speed control.
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 normal fuel supply to the APU is from the right main tank. A backup fuel supply from the left
engine fuel feed manifold is provided in case negative-G conditions are encountered.
Components and Operation
APU Fuel Pump
The DC battery bus powers the APU fuel pump, which draws fuel from the right main tank. The
pump provides fuel to the fuel control unit through the feed line and the APU fuel shutoff valve.
A portion of the fuel from the APU fuel pump is directed to the APU fuel control unit for
combustion, while the remainder is routed through the fuel/oil heat exchanger to provide cooling
of the APU generator adapter oil. Fuel that has passed through the fuel/oil heat exchanger is
returned to the right main tank.
Selection of the APU PWR FUEL switch/light activates the APU fuel pump. This selection also
opens the APU fuel shutoff valve and negative-G shutoff valve. Insufficient APU fuel pump
output pressure is indicated by an APU PUMP caution message and illumination of the PUMP
FAIL switch/light legend on the APU panel.
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/light position within 5 seconds, an
APU SOV caution 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 close. Successful closing of the valves is indicated by an APU SOV
CLSD advisory message and illumination of the green SOV CLOSED switch/light legend on the
APU panel. If the APU fuel shutoff valve fails to close after APU FIRE PUSH switch/light
selection, an APU SOV caution message is displayed.
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−8
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
APU Fuel Pump and Shutoff Valve
Figure 05−10−5
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−9
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
APU FUEL DISTRIBUTION (CONT'D)
APU Negative-G Shutoff Valve
The APU negative-G shutoff valve opens and closes when the PWR FUEL switch/light is
pressed. If the valve fails to close when the PWR FUEL switch/light is pressed out, the APU NEG
G SOV caution message is displayed.
If a negative-G condition exists or the APU fuel pump fails, a one-way check valve, that
operates based on positive differential pressure, allows fuel from the negative-G shutoff valve to
feed the APU. An additional one-way check valve prevents this alternate fuel from entering the
　
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