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APU IGNITION/STARTING -FUNCTIONAL DESCRIPTION

General
To start the APU, turn the APU switch to START and then release it to ON. This signals the APUC through the ELMS to start the APU.
Pneumatic Start
The APUC gets pneumatic system pressure signals through ARINC 629. If pressure is available, the APUC energizes the solenoid valve on the ATSCV. This opens the starter control valve and pressure regulated air flows to the air starter and turns the APU engine.
At approximately seven percent engine RPM, the APUC energizes the ignition unit. The ignition unit supplies the ignition power to energize the igniter plugs. The ignition turns off at approximately 50 percent engine RPM.
At approximately 55 percent engine RPM, the APUC removes power from the ATSCV solenoid valve to shut off the air turbine starter. The APU continues to increase the speed of rotation to its operating RPM (100 percent).
Electric Start
If pneumatic pressure is not available, the APUC energizes the APU crank contact relay. This supplies 28v dc power from the APU battery to the electric starter which turns the APU. At about 49 percent engine RPM, the APUC removes power from the electric starter and the APU continues to increase the speed of rotation to its operating RPM.
The ignition system operates the same during both types of starts.
Autostart
The APU will automatically start for these conditions:
*
Airplane is in the air

*
Power removed from the left and right transfer buses.

The ELMS contains the APU autostart logic and sends the start signal to the APUC.
To shutdown the APU after an autostart, you must first move the APU selector to ON and then back to OFF.
The ram air turbine will automatically extend during an APU autostart.
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APU IGNITION/STARTING -FUNCTIONAL DESCRIPTION

Start Failure
These conditions cause an APU start failure:
*
No acceleration (no RPM increase after ignition or start command failure)

*
No ignition (no EGT increase above 7 percent RPM).

These are the indications of a start failure:
*
APU SHUTDOWN advisory message

*
APU status message

*
APU fault light.

Training Information Point
If you lift the airplane on jacks, the APU may do an autostart. To prevent this, you need to pull some circuit breakers.
See the applicable section in part two of the airplane maintenance manual for the full procedure.
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APU IGNITION/STARTING -FUNCTIONAL DESCRIPTION

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777 AIRCRAFT MAINTENANCE MANUAL

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APU AIR -INTRODUCTION

Purpose
These subsystems make up the APU Air System:
*
APU cooling air system

*
APU air supply system

*
APU surge bleed system.

The APU air system does these functions:
*
Cools the APU compartment and the APU engine oil

*
Supplies compressed air to the airplane pneumatic system

*
Releases excess load compressor output (surge control).

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777 AIRCRAFT MAINTENANCE MANUAL

APU AIR -INTRODUCTION

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APU AIR -APU COOLING AIR SYSTEM -GENERAL DESCRIPTION

General
The APU Cooling Air System cools the APU compartment and the APU engine oil. There are no moving parts in this system.
Compartment Cooling
A cooling eductor behind the APU engine turbine uses high speed exhaust flow to form low pressure suction. This suction pulls a continuous cooling air flow from the compartment through the oil cooler and out through the exhaust duct.
See the APU and generator lubrication section for more information about the oil cooler (SECTION 49-27).
Outside air goes into the APU compartment through an opening in the back of the air inlet duct.
Training Information Point
There is a small negative pressure in the APU compartment when the APU is running. APU vent doors on the APU compartment doors permit you to equalize pressure before you open the doors.
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