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AUXILIARY POWER UNIT
TABLE OF CONTENTS
AUXILIARY POWER UNIT ................................................................ 1
General ....................................................................................... 1
APU Start and Shutdown ............................................................. 1
Aft Accessory Compartment Overheat Detection......................... 2
APU Fuel Supply ......................................................................... 2
APU Inlet Doors........................................................................... 2

Continental
Rev. 12/01/00 #29 Flight Manual
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AUXILIARY POWER UNIT

General
The GTCP 85-98 DHF auxiliary power unit (APU) is a gas turbine engine installed to supply pneumatic requirements for cabin air conditioning and engine starting as well as electrical power for normal airplane systems operation while on the ground. The APU is operable in flight to supply an alternate source of electrical power in the event that one or both of the engine generators are unavailable.
The APU is installed in the unpressurized, fireproof, enclosed area of the lower fuselage, aft of the pressure bulkhead. Start, operation, and fire controls are located on the APU control panel located on the overhead panel in the flight compartment. An exterior APU ground control panel, located on the lower fuselage aft of the APU compartment, is used for emergency shutdown and fire agent discharge. A fire detector system, having the same characteristics as the main engine fire detector system, is installed on the APU.
APU Start and Shutdown
All APU starts, both in flight and on the ground, receive DC power from the airplane battery. A start abort function precludes depletion of the airplane battery if the start is not completed within a prescribed time due to malfunction.
Bleed air, for starting the main engines and operating the air conditioning system, is controlled by the APU AIR switch. The switch, when in ON, opens a load control valve and provides APU bleed air to the airplane pneumatic system. When in OFF the load control valve closes, thus removing APU bleed air from the pneumatic system.  A check valve prevents reverse airflow from the engines from re-entering the APU.
Circuits are installed to provide an automatic 60-second warm-up period when starting the APU. The APU MASTER switch, located on the overhead panel, is used for APU start and normal shutdown. When moved to OFF, the APU air is immediately terminated. The APU then executes a 60-second time delay, which allows the APU to cool down, thus minimizing thermal shock. The 60-second cool-down period is bypassed if the cockpit fire control (FIRE CONT) switch is moved to OFF & AGENT ARM.

Continental
Rev. 12/01/00 #29 Flight Manual
Aft Accessory Compartment Overheat Detection
Overheat sensors, located in the aft accessory compartment, detect high compartment temperatures resulting from a broken or disconnected pneumatic duct. The red TAIL COMP TEMP HIGH warning light on the overhead annunciator panel and the glareshield MASTER WARNING lights will illuminate.
APU Fuel Supply
Fuel is normally supplied to the APU from the right main tank. The DC start pump or any operating right main or center tank boost pump will provide fuel to the APU. Fuel can also be supplied to the APU from a left main tank boost pump when the FUEL X FEED lever is in the ON position (crossfeed valve open).
Once the APU has started, whenever possible, at least one main boost pump should be on to provide fuel to the APU.  Operating the APU with the center boost pumps on and main boost pumps off should be avoided if center tank quantity is low (below 800 pounds), otherwise air may be ingested from the center tank into the APU fuel line.  Air in the APU fule line can cause APU flameout and can prevent the APU from starting.  Output pressure from the DC start pump is much lower than pressure from the AC powered boost pumps.
The APU fuel control automatically regulates fuel flow to maintain APU RPM. Pneumatic bleed extraction is limited by a bleed load control valve controlled by a pneumatic thermostat. The APU generator shaft horsepower requirements have priority over the pneumatic bleed air extraction.
APU Inlet Doors
The APU doors, two non-ram (forward and aft) and one ram (center), located on the aft underside of the fuselage, provide intake air for the APU.
An APU DOORS control switch, located on the overhead instrument panel, provides for manual or automatic selection of the inlet doors position. The switch is positioned to AUTO for all normal start and operating conditions. During normal operation, the ram air door opens automatically at the beginning of the start cycle. The ram air door will close and the non-ram air doors open when APU RPM reaches 95%. Door sequence provides ram assist for windmill starting with the switch in AUTO position. It is not necessary to manually select RAM position for windmill starts.
　
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