APU AIR -APU COOLING AIR SYSTEM -GENERAL DESCRIPTION

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APU AIR -APU AIR SUPPLY SYSTEM -INTRODUCTION

Purpose
The APU air supply system supplies compressed air to the airplane pneumatic system for:
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Main engine start

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Air conditioning and pressurization

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Air-driven hydraulic pump operation

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Other pneumatic system functions.

Components
Air supply components include:
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Inlet guide vanes (IGVs)

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Inlet guide vane actuator

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APU check valve.

Control
The APUC sends signals to the IGV actuator to control the IGV angle position. This controls the output of the air supply system.
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APU AIR -APU AIR SUPPLY SYSTEM -INTRODUCTION

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APU AIR -APU AIR SUPPLY SYSTEM -INLET GUIDE VANES

Purpose
The inlet guide vanes control the air flow to the load compressor and thus control APU air output.
Physical Description
Twenty eight inlet guide vanes (IGVs) are inside the APU around the load compressor inlet.
Functional Description
The guide vanes rotate between 76.2 degrees (fully closed) and -10 degrees (fully open). This angle is in relation to the airflow.
Sector gears connect each IGV to a ring gear. The IGV actuator connects to the ring gear by the tie rod.
Training Information Point
The IGV position shows on the APU maintenance page.
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APU AIR -APU AIR SUPPLY SYSTEM -INLET GUIDE VANES

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APU AIR -APU AIR SUPPLY SYSTEM -IGV ACTUATOR AND CHECK VALVE

Inlet Guide Vanes Actuator
The IGV actuator uses high pressure fuel to operate the inlet guide vanes.
The APUC controls a torque motor on a two-stage servo-valve to move the actuator. An internal LVDT sends actuator position to the APUC for position feedback.
The IGV actuator connects to the IGVs through a tie rod, ring gear, and sector gears (not shown).
The IGV actuator is on the right side of the APU below the pneumatic cluster.
APU Check Valve
The APU check valve prevents pneumatic system flow from turning the APU load compressor in reverse.
The valve is in the pneumatic duct between the load compressor and the air turbine starter control valve (ATSCV).
The check valve is spring-loaded closed and opened by airflow from the load compressor.
Training Information Point
A cover plate supplies access to a captive bolt that connects the IGV actuator to the tie rod. You loosen this captive bolt when you remove the IGV actuator.
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APU AIR -APU AIR SUPPLY SYSTEM -IGV ACTUATOR AND CHECK VALVE

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777 AIRCRAFT MAINTENANCE MANUAL

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APU AIR -APU AIR SUPPLY SYSTEM -FUNCTIONAL DESCRIPTION

General
The APUC contains pneumatic mode selection logic and IGV control logic to control the inlet guide vanes. The APUC monitors pneumatic system demands and selects a pneumatic mode. The IGV control logic sends signals to the electro-hydraulic servo valve (EHSV) to change the IGV position. A linear variable differential transformer (LVDT) sends actuator position feedback signals to the APUC.
The APUC also calculates a turbine inlet temperature (T4). If T4 is too high, the APUC starts to close the IGVs to reduce the pneumatic loading. This permits electrical power output of the APU to have the highest priority.
Pneumatic Modes
The APUC uses these inputs to select a pneumatic mode:
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APU shutoff valve position

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Main engine start

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ADP operation

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A/C pack operation

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Air inlet pressure (P2).

These are the pneumatic modes:
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Idle

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Duct Pressurization

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Main Engine Start

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Air Driven Pump (ADP)

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ECS

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ADP/ECS.

The APUC selects the idle mode when the APU shutoff valve is closed and there is no pneumatic system demand. The IGVs are closed in this mode. The APUC also selects this mode any time the airplane is above 22,000 feet (6700 meters).
　
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