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to provide pneumatics for engine starting and air conditioning. Reverse airflow protection from the
main engines into the APU is provided by a one-way check valve and the interlock control logic of
the 10th-stage bleed air switch/lights (for additional information on the 10th-stage bleed air system,
see Chapter 18, Pneumatic System).
Components and Operation
The APU pneumatic control system consists of two main components, the load control valve (LCV)
and the surge control valve (SCV).
Bleed Air Supply
The APU delivers a portion of its compressor discharge air to the left side of the 10th-stage
manifold when the LCV is opened. This diversion takes pneumatic energy from the turbine, and
some of the cooling air from the APU’s combustion system. As with shaft loads, the rpm drops,
and the APU recovers full governed speed by commanding an increase in fuel flow. 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 located above and to the right of the APU turbine section. The valve is
controlled by the APU ECU when the APU LCV switch/light on the BLEED AIR panel is selected
to the open position. The LCV controls APU bleed air supply to the left side of the 10th-stage
manifold. The ECU ensures that system priority is given to electrical loads over the bleed air
demand, 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 legend of the APU LCV
switch/light. When the ECU cycles the LCV for EGT limit protection, the OPEN legend
extinguishes. If the APU LCV has failed open with the 10th stage switch/light interlock protection
activated, the APU LCV FAIL caution message is displayed on EICAS and the FAIL legend
appears on the LCV switch/light.
Surge Control Valve (SCV)
To prevent compressor surge, unused bleed air is vented through the SCV into the APU tailpipe,
whenever the LCV is commanded closed. Surge control valve operation is automatically
controlled by the electronic control unit.
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−20
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
PNEUMATIC SUPPLY (CONT'D)
Bleed Air Indication
APU bleed air indications are displayed on the EICAS below the APU gauges.
APU Pneumatic Supply
Figure 05−10−11
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−21
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
CONTROLS AND INDICATORS
The APU panel provides the control for the APU operation, and the EICAS provides a visual display
of APU indications. The APU fault panel supplements the EICAS for specific APU faults.
The APU panel has two switch/lights:
• PWR FUEL; Activates the ECU, which commands fuel valves and pump operation.
• START/STOP; Commands APU start and shutdown modes of operation.
APU monitoring is provided by RPM and EGT gauge readouts on the EICAS status page. These
indicators are activated when the PWR FUEL switch/light is pressed in and are removed from the
display 60 seconds after selecting the PWR FUEL switch/light out.
APU Panel and EICAS Indications
Figure 05−10−12
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−22
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
CONTROLS AND INDICATORS (CONT'D)
EICAS and Synoptic Page Color Coding
Figure 05−10−13
AUXILIARY POWER UNIT
Description APU 100E
Vol. 2 05−10−23
REV 76, Jul 17/09
CL−604 Operating Manual
PSP 604−6
EICAS MESSAGES
MESSAGE MEANING
AURAL WARNING
(IF ANY)
APU FIRE Fire detected in the APU enclosure. FIRE BELL
APU OVERSPEED APU has shut down due to an overspeed condition. “APU”
APU OVERTEMP APU has shut down due to an overtemperature
condition.
“APU”
APU LCV FAIL APU load control valve has failed in the open position with APU LCV interlock
activated.
APU SOV
APU fuel shutoff valve is open with the APU FIRE PUSH switch activated, or
the fuel shutoff valve position does not match the APU PWR FUEL switch
position, within 5 seconds.
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