ENGINE STARTING -FUNCTIONAL DESCRIPTION
The SAV logic does not let 28v dc power energize the SAV solenoid if the N2 is more than the maximum starter engagement limit. Below that limit, the EEC prevents crash engagements by slowly accelerating the starter to match the engine N2 speed. The EEC uses the starter speed sensor as an input to control the open/close rate of the ERU switch. This rate controls the open/close rate of the SAV solenoid and thus the average servo air pressure to the SAV. Different servo air pressures open the SAV to different positions. The SAV position controls the amount of air flow through the starter and thus its acceleration rate. When the starter engages, the EEC opens the SAV completely.
When the start/ignition selector moves to the NORM position, the engine start control relay de-energizes. This removes 28v dc power to the ERU.
AIMS
The AIMS controls the latching solenoid that keeps the start/ ignition selector in the START position. The solenoid cannot move the selector. It only holds it in the momentary START position.
The EEC sends a digital engine start signal to the AIMS. This signal is set to true when the engine start begins. When the engine start signal is true, the AIMS latches the selector by energizing the solenoid.
The EEC sets the engine start signal to false when any of these conditions occur:
*
The EEC aborts an autostart process
*
The engine gets to the starter cutout speed (approximately 41 percent N2)
When the engine start signal is false, the AIMS releases the selector by de-energizing the solenoid. The selector automatically moves to the NORM position when the solenoid de-energizes.
If the digital engine start signal is not valid or is not available, the AIMS uses the analog N2 signal from the permanent magnet alternator (PMA) to control the latching solenoid. Below 50 percent N2, the solenoid energizes. Above 50 percent N2, the solenoid de-energizes.
EFFECTIVITY BEJ ALL 80-00-00
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D633W101-BEJ Sep 05/2002
PW4000 SERIES ENGINES
777 AIRCRAFT MAINTENANCE MANUAL
ENGINE STARTING -FUNCTIONAL DESCRIPTION
EFFECTIVITY BEJ ALL 80-00-00
Page 21
D633W101-BEJ Sep 05/2002
ENGINE STARTING -FUNCTIONAL DESCRIPTION -START PROFILE
Normal Autostart
When you enable an autostart, the EEC automatically starts the engine. You enable an autostart by doing these steps:
*
Push the autostart switch to ON
*
Turn the start selector to START
*
Move the fuel control switch to RUN.
The EEC opens the starter air valve (SAV), which causes N2 to rotate.
At 15 to 28 percent N2, the EEC opens the minimum pressure and shutoff valve in the FMU and energizes one igniter.
When the engine speed gets to approximately 41 percent N2, the EEC closes the SAV and de-energizes the igniter. The EEC also sets the AIMS engine start signal to false. This causes the AIMS to de-energize the start/ignition selector latching solenoid. The selector returns to the NORM position.
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