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The ignition system components are part of a common cluster and are on the right side of the engine.
Ignition Unit
The ignition unit changes 28v dc power to high voltage pulsed current that goes to the ignitor plugs. The ignition unit has two channels: one for each igniter plug. Four bolts hold the ignition unit to the APU.
Ignition Leads
Ignition leads connect the ignition unit to the igniter plugs. The leads are insulated to protect against radio interference. There are connectors at each end of the ignition leads.
Igniter Plugs
Two igniter plugs supply the high energy spark for fuel/air ignition. One is on the right side and the other is on the bottom of the APU combustion section. The ignition lead connectors hold the plugs in the boss.
Functional Description
The APUC turns on the igniter plugs at 7 percent APU engine RPM during APU start. The APUC turns the plugs off at 50 percent RPM.
The APUC also turns the igniter plugs on if the APU engine RPM goes below 95 percent RPM during APU operation (speed droop).
Training Information Point
WARNING: DO NOT TOUCH THE IGNITION COMPONENTS UNTIL YOU RELEASE THE HIGH VOLTAGE FROM THE IGNITION UNIT. IF YOU DO NOT OBEY THIS PROCEDURE, INJURY TO PERSONS CAN OCCUR.
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APU IGNITION/STARTING -APU IGNITION SYSTEM -GENERAL DESCRIPTION

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777 AIRCRAFT MAINTENANCE MANUAL

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APU IGNITION/STARTING -APU ELECTRIC STARTER SYSTEM -GENERAL DESCRIPTION

General
The electric starter system has these components:
*
The electric starter motor

*
The electric starter clutch.

The electric starter system starts the APU when pneumatic power is not available or if the air turbine starter system has a failure. The APUC controls the electric starter system.
Physical Description
The electric starter motor is a 28v dc series-wound motor. Four bolts hold the motor to the gearbox adapter on the front of the APU. A ratchet on the electric starter motor shaft connects the starter motor to the electric starter clutch. The starter motor weighs 38 pounds (17 kg).
The electric starter clutch is a pawl and ratchet type mechanism which transmits the starter motor torque to the APU gearbox. The pawls turn with the APU and the ratchet wheel turns with the starter motor. Pawl springs hold the pawls against the ratchet wheel at low speeds. The pawls disengage by centrifugal force after the starter turns off. A single bolt attaches the clutch to the starter drive gear shaft. The clutch is inside the gearbox adapter.
Functional Description
During an electric start, the starter motor turns the ratchet wheel and the electric starter clutch through the three pawls. The clutch turns the APU engine through the APU gearbox.
The electric starter motor turns off at 49 percent engine RPM for low altitude starts and 55 percent engine RPM for starts above 22,000 feet (5800m). The pawls then move outward against the pawl springs by centrifugal force. This disengages the clutch from the starter motor.
During an air turbine start, the electric starter clutch turns with the APU engine. The electric starter motor and the ratchet wheel do not turn. The pawls move up the sloped edge of the ratchet teeth to permit rotation between the ratchet wheel and the clutch. At approximately 17 percent APU engine RPM, the pawls lift off of the ratchet by centrifugal force to disengage the clutch from the starter motor.
Training Information Point
A lifting lug on the top of the electric starter permits attachment of a fishpole hoist for removal and installation.
A hand crank access on the electric starter motor permits you to manually crank the engine through the starter motor.
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APU IGNITION/STARTING -APU ELECTRIC STARTER SYSTEM -GENERAL DESCRIPTION

You can try three APU starts in a 60 minute period with the electric starter motor.
A brush wear sensor inside the starter motor sends a signal to the APUC when the motor needs to be replaced. The APU CONTROL status message shows for this condition.
Both the electric starter motor and the electric starter clutch are LRUs.
The ATS clutch and electric starter clutch are not interchangeable.
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