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OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-49-00
Page 5
October 11/02
Revision 8
Operation of the APU alternator is monitored by the same self-protective
functions as the engine-driven alternators. The APU alternator will trip off
for over and undervoltage, over and under frequency, polarity, overcurrent,
short circuit and feeder fault malfunctions. Like the engine-driven
alternators, the APU alternator has dual bearings. If the main bearing fails,
the APU ALT FAILED BRG light on the overhead annunciator panel will
illuminate accompanied by a CAS message. If the APU alternator exceeds
normal temperature limits (300¡F / 149¡C), a CAS message of APU ALT
HOT will be displayed.
B. Bleed Air System:
Bleed air may be extracted from the APU compressor for engine starting or
operating the air conditioning system while the airplane is on the ground
(the APU cannot be used for bleed air while the airplane is in ßight). After
starting the APU and 1) RPM has reached at least 95% for four (4)
seconds, and 2) EGT has stabilized for one (1) minute, the APU switchlight
on the BLEED AIR panel may be selected ON. (After six (6) seconds, verify
the ISOLATION valve is OPEN. Maximum APU EGT with APU BLEED AIR
selected ON is 680¡ C.) When the switch is selected, a signal is sent to the
ECU to open the load control valve, porting compressed air into the
pneumatic manifold. When air is bled from the APU compressor, less is
available to cool the turbine section of the APU. For this reason, the load
control valve does not open immediately, but takes one to one and a half
(1-1½) seconds to fully open, allowing the ECU to adjust fuel ßow to avoid
EGT spikes. On airplanes SN 1000-1310 having part 3 of ASC 390 and
production airplanes SN 1311 and subsequent, a venturi is installed
downstream of the load control valve to further moderate the initial air
extraction ßow and the rate of EGT rise.
To avoid reverse air ßow into the APU that would disrupt the path of cooling
air around the turbine, a check valve is installed in the pneumatic system
ducting that prevents the ßow of engine bleed air back to the APU. An
interlock is also incorporated that closes the load control valve if the APU
air bleed is open and engine air bleeds are open and engine power levers
are advanced from the idle position. In normal circumstances only one
bleed air source should be selected at a time (either APU or engine bleed).
The ECU maintains the APU at normal operating temperatures (665¡C and
100% RPM) during bleed air extraction by balancing fuel ßow with air bleed
demands. The ECU can add fuel ßow to maintain operating temperatures,
but can reduce EGT only by reducing the amount of air bled from the APU
(since RPM cannot by reduced), thereby increasing compressor discharge
air available to cool the turbine section. Operation of the load control valve
and ECU result in the following indications:
• Initial selection of APU ON for bleed air on the BLEED AIR panel will
result in an initial rise in APU EGT until the ECU balances fuel ßow
• The APU can deliver more bleed air pressure on cold days than on
hot days
• If APU EGT rises above normal, the ECU will start to close the load
control valve (if open) to cool the APU. If EGT reaches 732¡C, the
load control valve will be fully closed and no air will be available from
the APU
OPERATING MANUAL
2A-49-00 PRODUCTION AIRCRAFT SYSTEMS
Page 6
October 11/02
Revision 8
C. Fire Detection, Warning and Extinguishing System:
There are four Þre detectors positioned around the APU. Three of the
detectors are installed in the walls of the APU enclosure and one is
installed in the APU air inlet. The detector installed on the top of the turbine
section near the load control valve is set at a trip point of 600¡F, the other
three are set at 450¡F. The Þre detection and extinguishing system is
powered by 28V DC, and will operate from airplane batteries in order to
test the system prior to starting the APU. If the temperature within the APU
enclosure reaches a detector trip point, a ground is furnished to the
detection circuits and the following indications are displayed:
• Red APU FIRE light on the APU overhead panel
• Red APU FIRE light on the Standby Warning Lights Panel for SPZ-
8000 airplanes
• APU FIRE message on CAS
• Both red MASTER WARNING lights illuminate
• The APU FIRE checklist is displayed on the lower center portion of
the copilotÕs navigation display (DU 5) in MAP or COMP or PLAN
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湾流4操作手册 Gulfstream IV Operating Manual 2(144)
