The functions of the air system are to:

• Supply the APU bleed air to the aircraft bleed-air system as an alternative to the engine bleed systems
• Prevent an APU compressor surge during specified conditions of operation.

The air system supplies and releases the compressed air from the APU power section. The electronic control unit (ECU) controls the operation of the air system. The ECU uses the input from many sources to do this function. The input data and sources include:

• The properties of APU operation
• The level of the bleed or shaft load which the APU supplies
• The altitude (ambient pressure)
• The weight-on-wheels (WOW) indication
• The air conditioning/bleed control panel indications
• The position of the valves in the air conditioning and anti-ice systems

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The function of the compressor control system is to prevent compressor surge in the APU. If the gas pressure downstream of the turbine increases sufficiently it can cause a compressor surge. When this occurs the compressor continues to turn but it does not compress more air. A compressor surge will not occur if this downstream pressure is decreased sufficiently by the compressor control system.

The compressor control system has interfaces with Electronic Control Unit.

Surge Valve

The surge valve vents excess pressurized air overboard through the exhaust duct. The valve connects to the load and surge control-valve adapter with a clamp. The other end of the valve connects to the surge duct with a coupling. The surge valve is attached to a bracket on the turbine housing with a clamp around the middle of the valve body.

The ECU controls the operation of the surge valve, which is a solenoid-controlled, pneumatically-operated, normally-closed valve. To open the solenoid valve poppet, the ECU energizes the solenoid valve. The poppet closes when the surge valve solenoid valve is de-energized.

Load and Surge Control-Valve Adapter

The load and surge control-valve adapter connects the APU compressor to the load control valve and the surge valve. It is attached to the APU compressor with a coupling.

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The function of the bleed-air supply system is to supply APU bleed air for engine starts and air conditioning. When the conditions are correct, the APU electronic control unit (ECU) energizes the bleed-air supply system. The system supplies compressed air from the APU power section to the applicable aircraft system through the bleed air duct.

Load Control Valve

The load control valve (LCV) is connected with a coupling to the compressor outlet of the APU. The other end of the LCV is connected to the aircraft bleed-air duct with a coupling and a gasket. The valve is controlled electrically and the valve mechanism operates a butterfly valve pneumatically. When the LCV is open, the ECU adjusts the butterfly plate position to supply the correct quantity of bleed air. These adjustments are related to the conditions and signals which follow:

• The quantity of the bleed air load which is necessary
• The APU exhaust-gas temperature.

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The bleed-air duct is located downstream from the LCV in the tail-cone. The bleed-air ducting system for the APU supplies air from the load control valve (LCV) to the aircraft bleed-air ducting. The APU bleed-air duct is made of two parts, a flexible bellows duct that connects the LCV to the short bleed-air duct and allows APU movement during operation, and a short bleed-air duct that goes through the forward firewall to the aircraft bleed-air ducting.

A surge control valve duct is installed aft of the surge control valve that points air to the exhaust eductor which then releases the surge air overboard through the APU exhaust muffler.

The check valve is installed between the short bleed-air duct and the aircraft bleed-air ducting. It prevents the backflow of main engine bleed air into the APU when the APU or APU bleed air is not in operation. The bleed-air check valve has a notch to align it correctly with the short bleed-air duct during installation.

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Surge Valve

The APU electronic control unit (ECU) controls the operation of the compressor control system. It calculates when the gas pressure makes it necessary to open the surge valve to release some of the compressed air from the power section. The ECU does this when the APU operates during the specified conditions that follow:

• The APU is at more than 95% of its maximum speed
• The load control valve is closed
• The weight-on-wheels signal is in the flight position
• The aircraft is on the ground, and the ambient temperature is less than 20 °F (-6.7 °C) or more than 85 °F (29.4 °C).

To open the surge valve, the ECU energizes the solenoid valve. When the solenoid valve opens, the high pressure inlet air goes from the APU compressor through the load and surge control-valve adapter, through the opening orifice and into the surge valve opening chamber. The inlet air applies a force on the diaphragm which moves against the force of the closing spring. When the force to open is more than the force to close, the valve opens and releases excess pressure.

The released air goes through the surge valve and the surge duct, then into the APU exhaust eductor. The surge valve closes when the ECU de-energizes the solenoid valve. The solenoid valve poppet then seals the opening chamber against the inlet air. The air pressure in the opening chamber releases to ambient pressure through the closing orifice. The pressure decrease in the closing chamber lets the closing spring close the surge valve. The surge valve is closed at all times when the load control valve is open.

Load Control Valve

The ECU controls the operation of the LCV. When the APU is prepared to supply a pneumatic load, the ECU sends the applicable signal to the aircraft system. The aircraft system and the ECU operate together to control the bleed air supply. When the conditions are correct, the pilot pushes the APU bleed pushbutton on the AIR COND/BLEED control panel. The ECU then energizes the bleed-air supply system. This causes compressed air from the APU power section to flow into the bleed-air supply system. A check valve prevents the flow of bleed air from the aircraft engines into the APU.

For the ECU to energize the bleed-air supply system, the correct conditions include:

• The APU is at more than 95% of its maximum speed
• The maximum altitude of the aircraft is not more than 20,000 ft (6,098 m).

When the ECU energizes the torque motor, the LCV flapper opens to let the inlet pressure into the opening chamber. The distance the flapper opens is in proportion to the current which the ECU transmits to the torque motor. Thus the air pressure in the opening chamber is in proportion to the torque motor input current. The inlet pressure in the opening chamber moves against the opening chamber diaphragm and against the closing chamber diaphragm at the same time. The pressures that open and the pressures that close balance each other. This lets the ECU put the butterfly plate in a specified open position, from fully open to almost closed.

When the torque motor is de-energized, the LCV is in the closed position. When the LCV closes, the inlet pressure goes from the LCV through the downstream sense port to the closing chamber and to the torque motor. A flapper keeps the inlet pressure out of the opening chamber. The forces of the closing spring and the closing chamber pressure hold the butterfly plate in the fully closed position.

The APU BLEED ALT LIMIT caution message will show when the APU bleed air is commanded above 20,000 ft or when the LCV has moved due to an excessive EGT.

The APU FAULT advisory message will show when the LCV has failed.

The EICAS messages that follow are related to the bleed air supply system:

EICAS MESSAGE(S)	LEVEL (COLOR)
APU BLEED ALT LIMIT	CAUTION (amber)
APU FAULT	ADVISORY (cyan)

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
V8	LOAD CONTROL VALVE	ZONE(S) 320	49-52-01

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