777 AIRCRAFT MAINTENANCE MANUAL
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APU IGNITION/STARTING -AIR TURBINE STARTER SYSTEM -GENERAL DESCRIPTION
General
These are the air turbine starter system components:
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Air turbine starter
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Air turbine starter clutch
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Air turbine starter control valve (ATSCV).
The air turbine starter system starts the APU when pneumatic pressure is available.
A magnetic chip collector on the air turbine starter collects metallic particles in the oil.
Physical Description
The air turbine starter is an axial flow turbine. The starter attaches with a V-band clamp to the gearbox adapter which is on the APU gearbox. A ratchet wheel on the air turbine starter shaft connects the starter to the clutch. The APU lubrication system supplies the air turbine starter with oil for cooling and lubrication. The air turbine starter weighs 9.5 pounds (4.3 kg).
The air turbine starter clutch is a pawl and ratchet type mechanism which transmits the starter motor torque to the APU gearbox. Pawl springs hold the pawl in the ratchet wheel at low speeds. The pawls disengage by centrifugal force after the ATSCV closes and the starter speed decreases. A single bolt attaches the clutch to the starter drive gear shaft. The clutch is inside the gearbox adapter.
EFFECTIVITY
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The ATSCV is a pneumatically actuated, solenoid-controlled butterfly valve. It is spring-loaded closed. The ATSCV is above the air turbine starter. The ATSCV weighs 6.5 lbs (2.9 kg).
Functional Description
The air turbine starter clutch is functionally equivalent to the electric starter clutch. During an air turbine start, the ATSCV opens to permit air from the pneumatic system to go to the air turbine starter. The air turbine starter turns the ratchet wheel. The ratchet wheel turns the air turbine starter clutch through the three pawls. The clutch turns the APU engine through the APU gearbox.
At 55 percent APU engine RPM, the ATSCV closes to stop the air supply to the air turbine starter. The pawls then move outward against the pawl springs by centrifugal force. This disengages the clutch from the air turbine starter motor.
During an electric start, the air turbine starter clutch turns with the APU engine. The air turbine starter 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 wheel by centrifugal force to disengage the clutch from the air turbine starter.
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APU IGNITION/STARTING -AIR TURBINE STARTER SYSTEM -GENERAL DESCRIPTION
Training Information Point
The air turbine starter, the air turbine starter clutch, and the ATSCV are LRUs.
The air turbine starter clutch and the electric starter clutch are not interchangeable.
There is no limit to the number of APU starts you may try with the air turbine starter.
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APU IGNITION/STARTING -AIR TURBINE STARTER SYSTEM -GENERAL DESCRIPTION
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APU IGNITION/STARTING -AIR TURBINE STARTER SYSTEM -AIR TURBINE STARTER CONTROL VALVE
Purpose
The ATSCV controls pneumatic pressure to the air turbine starter during APU start.
Physical Description
The ATSCV is a pneumatically-actuated, solenoid-controlled, butterfly valve. It is spring-loaded closed. The ATSCV is above the air turbine starter.
Functional Description
To start the APU air turbine starter operation, the APUC opens the solenoid valve. This permits bleed air to flow to the reference pressure regulator. The reference pressure regulator controls air pressure to chamber A of the spring and pressure balanced butterfly valve. This opens the butterfly valve. Duct pressure downstream of the butterfly valve pressurizes chamber B to balance the butterfly valve. Air output of the starter control valve is 18 to 22 psig.
An RVDT sends butterfly valve position to the APUC for feedback.
Training Information Point
A visual position indicator shows butterfly valve position.
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APU IGNITION/STARTING -AIR TURBINE STARTER SYSTEM -AIR TURBINE STARTER CONTROL VALVE
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777 AIRCRAFT MAINTENANCE MANUAL
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