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Electrical Power and Distribution ................................................................................ 4-22
Start Control Schematic .............................................................................................. 4-22
FADEC Inputs and Outputs......................................................................................... 4-23
FADEC Schematic ...................................................................................................... 4-24
4-ii For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
EICAS Messages........................................................................................................ 4-25
EMS Circuit Protection................................................................................................ 4-27
For Training Purposes Only
Sept 04
4-1
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
INTRODUCTION
An Allied-Signal RE 220 gas turbine auxiliary power unit (APU) is installed in a fire
resistant enclosure in the tail cone of the airplane behind the rear pressure bulkhead.
The APU is installed outside the main engine rotor burst zone. This enhances airplane
high altitude operation.
The APU provides bleed air for cabin cooling and heating through the Environmental
Control System (ECS), main engine starting, and electrical power on the ground and in
flight.
Under high demand load conditions, the APU electrical load takes priority over
pneumatic load.
The main component of the APU electrical system is the Electronic Control Unit
(FADEC), also known as the Full Authority Digital Engine Control (FADEC). The
acronym FADEC will be used throughout this chapter in lieu of ECU. The APU
FADEC interfaces with EICAS and the Central Aircraft Information Maintenance
System (CAIMS), providing failure detection and isolation of faulty components.
Control of APU speed is automatic and monitoring of APU Exhaust Gas Temperature
(EGT), speed, fuel and oil pressure is through the FADEC. The FADEC also records
operating hours and start cycles.
The engine speed for normal continuous steady state is displayed as 100% rpm.
Overspeed protection is provided. The APU has automatic shutdown for abnormal
ground and in-flight conditions.
APU start and run is initiated by a single switch, located on the APU control panel in
the flight compartment. Operation of the APU electric and bleed system is performed
at the ELECTRICAL and BLEED/AIR CONDITIONING ANTI-ICE panels
respectively.
The APU is capable of operating up to 45,000 feet while supplying electrical power
with operating limitations. The APU starting envelope is up to 37,000 feet. For other
APU operating limitations, refer to AFM, Chapter 2, LIMITATIONS.
4-2 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
DESCRIPTION
APU LOCATION AND ACCESS
APU COMPARTMENT
The APU is mounted in the tail cone by a multi-axis suspension rod system securing it
in all axes.
The APU compartment is completely sealed from the atmosphere with the exception
of the inlet door and exhaust duct. The APU compartment is isolated from the rest of
the airplane for fire protection. The APU is designed for full rotor burst containment.
Cooling of the APU compartment is accomplished by an exhaust eductor system that
draws airflow across the APU engine and the oil cooler. The airflow through the inlet
splits in two sections. One section allows air to flow to the compressor, while the other
section allows the air to flow through the oil cooler to provide cooling for the APU oil.
The air flowing through the oil cooler then passes through the compartment to cool the
APU and its mounted components. This air is then exhausted overboard.
APU Inlet Door
APU Access Doors
APU Exhaust
Exhaust Discharge
Exhaust Baffle
APU Inlet Door
Inlet Duct
GX_04_001
For Training Purposes Only
Sept 04
4-3
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
Access to the APU for inspection, maintenance and servicing is provided by two cowl
doors located under the APU. These doors are to be kept closed during normal APU
operation. However, they may be opened for APU inspection while the APU is
running. In such case, to avoid an APU oil overtemperature, limit operation with the
doors opened to a maximum of:
• 10 minutes with the APU unloaded (no bleeds, no electrics), and
• 5 minutes with the APU loaded (bleed air on and a maximum electrical load of
5 kVA)
APU ENGINE
The APU engine consists of the following:
• Inlet Door - Variable position (0 to 47º). The door opening angle, controlled by the
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