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· Atmosphere pressure of 7 psia is sensed (at and above 16,375 during ISA conditions)
The exaust of the APU SCV is located on the left side of the fuselage, aft of the engine pylon. The
operation of the SCV is transparent to the pilot.
AUXILIARY POWER UNIT
Rev 3, Apr 25, 2005 Flight Crew Operating Manual
CSP 700−5000−6
Volume 2
05−10−11
APU SYSTEM OPERATING MODES
The APU system operating modes are as follows:
ECU Powerup
The ECU enters a powerup
mode when the airplane BATTERY MASTER switch is selected to
ON. It does a test of its circuitry to verify that it is capable of executing the APU control
requirements for operation.
Start Mode
Upon receiving an active start signal and successful completion of the prestart
test sequence, the
FADEC will initiate an APU start. During the start mode, the FADEC activates the fuel request
signals, positions the APU door, activates and/or deactivates the starter and ignition control
systems. It also controls fuel flow as a function of timed acceleration and EGT schedule as the
APU accelerates to 100 percent.
Onspeed
Mode
When the APU reaches 95 percent speed, the FADEC transitions to the Onspeed
Mode to control
governed speed and load control activation.
Cooldown Mode
With the APU operating in the Onspeed
Mode, the FADEC transitions to the Cooldown Mode
when the APU switch is selected OFF. Upon entering this mode, the load control valve is closed
and the ready to load signal is removed. During cooldown, the APU speed is lowered to 70% RPM
for 60 seconds below 20,000 feet or remains at 100% RPM for the 60 second cooldown above
20,000 feet.
Shutdown Mode
Upon completion of the cooldown period or in the event of a protective shutdown, the FADEC
transitions to the Shutdown Mode and controls the APU shutdown sequence.
APU SYSTEM OPERATING SEQUENCE
The following events occur when the APU is selected from RUN to START position:
APU Starting
The APU can operate with battery power only. The APU is started automatically when the START
switch on the APU control panel is selected and held for one second. The RUN position
commands the APU fuel SOV and right DC auxiliary pump to energize. The START position is
spring loaded to RUN from the start position. When the door is correctly positioned, the FADEC
engages the starter. When the speed reaches 5%, fuel and ignition are activated. At 50% speed,
the starter disengages. For ground starts, ignition is deactivated at 50%. In flight, ignition is
deactivated at 98%.
APU Door Positioning
The APU door is positioned automatically as a function of WeightOnWheels
(WOW), Mach
number and APU speed. When RUN is selected on the APU control panel and the airplane is on
ground, the APU door is commanded to the full open position. When the airplane is in flight, the
FADEC schedules the door position.
AUXILIARY POWER UNIT
Flight Crew Operating Manual Rev 3, Apr 25, 2005
CSP 700−5000−6
Volume 2
05−10−12
APU SYSTEM OPERATING SEQUENCE (CONT'D)
APU Loading
The FADEC controls the ready to load signals, one for electrical loading and one for pneumatic
loading. Electrical loading is activated 2 seconds after the APU speed reaches 99%. If the APU
drops below 95%, electrical loading is not available (on ground and in flight).
On ground, pneumatic loading is available 60 seconds after the APU reaches 99% if the APU
prestart
EGT was < 149°C. If the APU prestart
EGT was ³ 149°C, pneumatic loading is available
on the ground 2 seconds after the APU reaches 99%.
In flight, pneumatic loading is available 2 seconds after the APU reaches 99%, regardless of the
prestart
EGT. Bleed air extraction is limited to 30,000 feet.
The APU LCV is opened upon receiving a bleed air request from the airplane bleed management
control system or the APU BLEED switch on the BLEED/AIR COND/ANTIICE
panel is selected.
Open and closed positions of the control valve are determined by the status of weightonwheels,
main engine starting and environmental control system signals. The bleed and load management
systems are responsible to ensure that the bleed air request is activated only when another source
of bleed air is not commanded. Example: L engine bleed valve open and/or the R engine bleed
valve open and crossbleed valve opened (control valve remains closed).
The normal position for the APU BLEED switch is AUTO. The AUTO position automatically
integrates control of the APU LCV. The manual ON position will command the LCV to open as
selected, but only if alloed by the FEDEC (to garantee APU selfpreservation).
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