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EFFECTIVITY BEJ ALL
AUXILIARY POWER SYSTEM -OPERATION
APU Maintenance Switch
The APU maintenance switch (P61) in the TEST position supplies power to the APUC when the APU control selector is OFF. This permits the APU data to show on the status and maintenance page formats.
Training Information Point
After an emergency shutdown from the APU fire shutdown switch (P40), push the fire and overheat test switch to reset the start system.
After an APU fire switch (P5) shutdown, push the fire switch back in to reset the system.
The APU uses 675 lb (306 kg) of fuel for each hour of APU operation after the first hour.
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777 AIRCRAFT MAINTENANCE MANUAL
AUXILIARY POWER SYSTEM -OPERATION
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AUXILIARY POWER SYSTEM -STARTING
General
The APU can be started at any altitude.
The APU selector makes inputs to the P310 standby power management panel in the ELMS. The ELMS sends signals to the APUC.
The APUC controls the APU fuel, ignition, and starter systems during APU start.
APU Selector -ON
When you move the APU selector to ON, the APU shutoff valve and the APU air inlet door open. The APU dc fuel pump also turns on if ac power is not available. If ac power is available, fuel system boost pumps supply fuel to the APU and the APU dc pump stays OFF.
When you move the APU selector to START, hold the APU selector at the START position for one second and manually move the APU selector to the ON position, the APUC then selects the air turbine starter system or the electric starter system.
The APUC selects the air turbine starter system if pneumatic pressure is available. The APUC selects the electric starter system if pneumatic pressure is not available or if the air turbine start fails.
APU engine RPM must be less than 12 percent RPM and the air inlet door must be fully open before the start sequence starts.
As the APU speeds up, the pressurizing shutoff valve in the fuel cluster opens at 7 percent RPM. The igniter plugs energize immediately after the pressurizing shutoff valve opens.
During a low altitude electric start (below than 22,000 feet, (6700 meters)), the starter motor turns off at approximately 49 percent RPM. For a high altitude electric start (above 22,000 feet, 6700 meters), the starter motor turns off at 55 percent RPM.
During a pneumatic start, the ATSCV closes at 55 percent RPM.
The APUC turns off the igniter plugs at 50 percent RPM.
The APU continues to speed-up. Two seconds after the APU reaches 95 percent RPM, pneumatic and electric loading become available.
The APU speeds up to its normal operating speed of 100 percent (39,044) RPM.
Training Information Point
During an APU start, if the APU stops its acceleration for more than seven seconds, the APUC shuts down the APU.
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777 AIRCRAFT MAINTENANCE MANUAL
AUXILIARY POWER SYSTEM -STARTING
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AUXILIARY POWER SYSTEM -NORMAL SHUTDOWN
General
The APUC controls the APU shutdown sequence. APU shutdown is not immediate. The APU goes through a cool-down period before shutdown.
There are different cooldown schedules for when the airplane is above 14,000 feet (4265 m) and below 14,000 feet (4265 m).
APU Selector to OFF
When you turn the APU control selector to OFF, the APUC sends a signal to the ASCPCs to close the APU shutoff valve. The APUC also closes the IGVs and opens the SCV to pneumatically unload the APU.
Thirteen seconds after you move the selector to OFF, the APUC sends a signal to the APU generator control unit (GCU) to unload the APU generator.
Above 14,000 Feet Cooldown
When the airplane is above 14,000 feet (4265 m), the APU cools down at 100 percent RPM for 105 seconds after you move the selector to OFF.
Below 14,000 Feet Cooldown
When the airplane is below 14,000 feet (4265 m), the APU stays at 100 percent RPM for 15 seconds. The APU then starts a 90 second deceleration to 70 percent RPM.
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