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Questions and Answers
What is one of the primary functions of the electrical installation system in an APU engine?
What is one of the primary functions of the electrical installation system in an APU engine?
Which component does the APU wiring harness connect to for the electronic control unit?
Which component does the APU wiring harness connect to for the electronic control unit?
Which of the following sensors is associated with the APU's electrical components?
Which of the following sensors is associated with the APU's electrical components?
What role does the Electronic Control Unit (ECU) play in the APU electrical installation?
What role does the Electronic Control Unit (ECU) play in the APU electrical installation?
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Which of the following is NOT a component connected by the APU wiring harness?
Which of the following is NOT a component connected by the APU wiring harness?
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What happens when the APU N2 speed reaches 50%?
What happens when the APU N2 speed reaches 50%?
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How is the APU shut down using the APU STARTER switch?
How is the APU shut down using the APU STARTER switch?
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What effect does setting the APU MASTER switch to the OFF position have?
What effect does setting the APU MASTER switch to the OFF position have?
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What role does relay (518P) play in the shutdown process of the APU?
What role does relay (518P) play in the shutdown process of the APU?
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What is the purpose of the mounts used for the APU in its enclosure?
What is the purpose of the mounts used for the APU in its enclosure?
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What is the primary function of the APU GCU?
What is the primary function of the APU GCU?
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Where is the APU MASTER switch located?
Where is the APU MASTER switch located?
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What does the APU STARTER switch enable the flight crew to do?
What does the APU STARTER switch enable the flight crew to do?
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What component provides power to the APU driven starter-generator unit field winding during starter mode?
What component provides power to the APU driven starter-generator unit field winding during starter mode?
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What happens when the APU MASTER switch is set to the ON position?
What happens when the APU MASTER switch is set to the ON position?
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Which unit is responsible for controlling and monitoring the APU engine?
Which unit is responsible for controlling and monitoring the APU engine?
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What type of power does the APU MASTER switch receive to energize relays when set to ON?
What type of power does the APU MASTER switch receive to energize relays when set to ON?
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What is located in the aft equipment bay alongside the APU contactor box?
What is located in the aft equipment bay alongside the APU contactor box?
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Study Notes
APU Starting System
- The APU starting system uses switches on the cockpit overhead panel to initiate the sequence.
- The APU Electronic Control Unit (ECU) and Generator Control Unit (GCU) control the system.
Components
- Generator Control Unit (GCU): Mounted in the baggage compartment ceiling, controls the APU starter-generator unit in both starter and generator modes.
- Right DC Box: Located in the aft equipment bay, contains relays, contactors, fuses, and circuit breakers for controlling APU starting, power generation, and distribution.
- APU Contactor Box: Also found in the aft equipment bay, houses relays, contactors, fuses, and circuit breakers for APU starting, power generation, and distribution.
- APU MASTER Switch (513P): On the cockpit overhead panel, enables APU operation when set to ON.
- APU STARTER Switch (514P): Another cockpit-mounted switch, allows the flight crew to start and stop the APU engine.
- Starter-Generator Unit: Mounted on the APU accessory gearbox, provides starting and power generation functions.
- Field Resistor (274P): Located near the GCU in the baggage compartment, supplies battery power to the starter-generator unit's field winding during starter mode.
- Electronic Control Unit (ECU): Mounted on the right side of the APU enclosure, controls and monitors the APU engine.
Starting the APU
- The APU requires fuel, ignition, and the starting subsystem to initiate.
- Setting the APU MASTER switch to ON (513P):
- Energizes relays (520P, 522P, 629P)
- Supplies power to the APU STARTER switch (514P)
- Provides power to the APU ON input of the ECU (602P) and relay (518P).
- Energizes relay (520P) supplying battery bus power to the start relay (7000S).
Electrical Installation System
- Provides a path for electrical power to operate APU units/components.
- Facilitates communication between the ECU and APU subsystems.
- Forms the electrical interface between the aircraft, APU, and ECU.
- The ECU connects via aircraft wiring to the APU/aircraft interface connection on the APU enclosure.
Electrical Components
- The APU wiring harness connects to the ECU interface connection on the APU enclosure (P1).
- The harness also connects to:
- Oil level sensor
- Low oil pressure switch
- Fuel solenoid control valve
- Start counter
- Starter / generator
- Fuel control unit
- Ignition exciter
- Speed sensor
- Oil temperature sensor
- Inlet temperature sensor
- Exhaust gas temperature sensor
APU Turbine Speed and Operation
- The ECU (602P) manages fuel and ignition as the APU turbine gains speed.
- When APU N2 speed reaches 50%:
- The ECU drops the STARTER signal, causing relay (7000S) to open.
- The START SW signal is dropped to the APU GCU (250P).
- The GCU removes the START CONTACTOR signal, de-energizing the APU start relays (No. 1 and No. 2).
- The APU contactor (265P) opens, disconnecting the battery from the field winding and armature.
- The GCU can then control the starter-generator (260P) to produce electrical power.
APU Shut-down
-
Setting the APU STARTER switch (514P) to the STOP position:
- Removes the latching voltage to relay (518P), causing it to de-energize.
- The START/STOP signal is dropped to the ECU (602P).
- The ECU performs an orderly shutdown of the APU engine.
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Setting the APU MASTER switch (513P) to the OFF position:
- Removes power to the APU STARTER switch (514P), ECU (602P) input, and relays (520P, 522P, 629P).
- Relay (522P) opens, applying voltage to close the APU fuel shut-off valve (33Q).
Powerplant Unit Mounting
- The APU is mounted inside of a fire-resistant enclosure.
- Mounts support and isolate the APU from vibrations.
- The forward end is supported by adapters, brackets, and isolators.
- The aft end is supported by a mount adapter and an isolator.
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Description
This quiz covers the APU starting system, including the roles of the Electronic Control Unit (ECU) and Generator Control Unit (GCU). It details key components like the APU MASTER Switch, APU STARTER Switch, and various contactor boxes located in the aircraft. Test your knowledge of how these components interact to initiate the APU sequence.
