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main engines into the APU is provided by a one-way check valve and the 10th-stage bleed air
switch/lights interlock control logic (Refer to Chapter 19, Pneumatic System, for additional
information on the 10th-stage interlock circuit).
The APU pneumatic control system consists of two main components: the load control valve (LCV)
and the surge control valve (SCV).
Components and Operation
Bleed Air Supply
The APU delivers a portion of its compressor discharge air to 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 operating limit, the ECU reduces the bleed air load by commanding the
LCV to cycle toward the closed position.
Load Control Valve
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.
Surge Control Valve
To prevent compressor surge during high altitude shaft operation in flight, unused bleed air is
vented through the SCV into the APU tailpipe. SCV operation is automatically controlled by
internal logic within the ECU. The valve’s open/closed schedule is based on pressure altitude
and APU inlet temperature. The surge control valve is only required during in-flight APU
operation, at altitudes above 15,000 feet.
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−20
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
PNEUMATIC SUPPLY (CONT'D)
Bleed Air Indication
APU bleed air indication is displayed on the EICAS page below the CABIN information section.
APU Pneumatic Supply
Figure 05−10−11
CONTROLS AND INDICATORS
The APU panel provides the control for the APU operation, and the EICAS page 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.
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−21
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
CONTROLS AND INDICATORS (CONT'D)
APU monitoring is provided by RPM and EGT digital readout on the EICAS page. The APU
indications are activated when the PWR FUEL switch/light is pressed in, and are removed
60 seconds after selecting the PWR FUEL switch/light out.
APU Panel and EICAS Indications
Figure 05−10−12
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−22
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
CONTROLS AND INDICATORS (CONT'D)
EICAS and Synoptic Page Color Coding
Figure 05−10−13
AUXILIARY POWER UNIT
Description
Vol. 2 05−10−23
Oct 05/06
CL−605 Flight Crew Operating Manual
PSP 605−6
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
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