The functions of the air system are to supply APU bleed air to the aircraft bleed-air system, as an alternative to the engine bleed systems and prevent an APU compressor surge during specified conditions of operation.

The air system supplies and releases compressed air from the APU power section. The full-authority digital engine-controller (FADEC) controls the operation of the air system. To do this, the FADEC uses input from many sources. This data and its 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 bleed management controller
• The position of the valves in the air conditioning and anti-ice systems

The APU bleed air system consists of a load control valve (LCV) and a surge control valve (SCV).

The APU can supply bleed air to the aircraft environmental control system (ECS) and for main engine starting (MES). APU air is not available for wing anti-ice (WAI) or cowl anti-ice (CAI) functions at any time.

On the ground, the APU can supply bleed air and electrical power loads simultaneously or individually. If required in flight, the APU can be started at altitudes up to 37,000 feet, and can supply bleed air for ECS and MES up to 30,000 feet.

A butterfly shutoff valve, called the load control valve (LCV), is installed on the bleed air output duct, which is connected to the aircraft pneumatic ducting. The APU FADEC modulates the LCV to control the APU bleed air output. FADEC will modulate the LCV towards closed to prevent the APU EGT trim limits to be exceeded.

A butterfly shutoff valve, called the surge control valve (SCV), is installed to protect the APU compressor from a potential to surge (STALL). The APU FADEC logic opens the valve at altitudes above 15,500 feet when providing electrical loads and not supplying bleed air.

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The function of the bleed-air supply system is to supply APU bleed air to:

• The engine starting system
• The air conditioning system

The full-authority digital engine-controller (FADEC) controls the operation of the bleed-air supply system. When the conditions are correct, the FADEC energizes the bleed-air supply system. When the bleed-air supply system operates, it supplies compressed air from the APU power section to the applicable aircraft system. This compressed air goes through the bleed air duct. The bleed-air supply system also prevents the flow of compressed air into the APU from the engines.

When the APU is prepared to supply a pneumatic load, the FADEC sends the applicable ready-to-load (RTL) signal. This RTL signal goes to the bleed management controller on ARINC. The bleed management controller and the FADEC work together to control the bleed systems. When the FADEC gets the applicable signals and the conditions are correct, the FADEC energizes the bleed-air supply system. This causes compressed air from the APU power section to flow into the bleed-air supply system.

The applicable signals for operation of the bleed-air supply system come from:

• The "APU BLEED" rotary switch on the bleed-air control panel
• The bleed management controller through ARINC

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

• The APU is at 99% speed plus 60 seconds for ground operation, or plus 2 seconds for flight operation
• The bleed-pressure regulating-and-shutoff valve (BPRSOV) for the left engine is closed
• The BPRSOV for the right engine is closed, or it is open but the cross-bleed valve is closed
• All of the anti-ice valves are closed
• At an altitude of usually no more than 30,000 ft (9,150 m), to supply bleed-air to the air conditioning system
• At an altitude of usually no more than 35,000 ft (10,650 m), to supply bleed-air to the engine starting system

Load Control Valve

The system employs a modulated butterfly shutoff valve called the load control valve (LCV) to control the APU bleed air output. The LCV is located in the bleed air output duct, which is connected to the aircraft pneumatic ducting. Control inputs for the LCV are initiated from a switch in the cockpit marked APU BLEED.

The LCV is an electrically-controlled, pneumatically-operated, usually-closed valve. The "position of the LCV" is the same as the "position of the butterfly plate of the LCV". When the LCV is open, the FADEC adjusts its position as necessary. These adjustments are related to the conditions and signals which follow: 

• The level of the APU bleed load
• The exhaust gas temperature (EGT) of the APU
• The signals from the bleed management controller 

The LCV is spring-loaded to the closed position.An external visual position indicator is provided on the valve body. An RVDT on the LCV body sends valve position signals to the FADEC.

Porting air into the valve-opening chamber controls operation of the LCV. Air pressure flows through the trailing probe and is sensed at the valve closing chamber and the supply nozzle. With no voltage at the torque motor, the supply nozzle is closed. Air pressure and spring force in the close chamber hold the valve closed.

To open the LCV, variable millivolt (mV) signals are sent from the FADEC to the torque motor to move a flapper in the LCV, allowing air to flow through the supply nozzle into the opening chamber. The flapper opens to let the inlet pressure into the opening chamber. The distance the flapper opens is in proportion to the current which the FADEC transmits to the torque motor. Thus the air pressure in the opening chamber is in proportion to the input current to the torque motor. The inlet pressure in the opening chamber moves against the opening chamber diaphragm. The balance of the resulting force controls LCV position. The force of the pressures that open and the force of the pressures that close balance each other. This lets the FADEC put the butterfly plate in a specified open position from fully-open to almost closed.

The rotary variable differential transducer (RVDT) feedback signal is used by the FADEC to monitor valve operation, and to compare FADEC command versus valve position. These signals are voltages that are continuous, linear and in proportion to the position of the butterfly plate. The FADEC compares the voltage signal value from the RVDT to the voltage value the FADEC calculates. (The FADEC uses inputs on conditions from the aircraft and the APU to calculate the voltage value that will give the necessary butterfly-plate position.) The FADEC then adjusts the current to the torque motor to move the butterfly plate until the two voltage values are the same. Thus the FADEC can accurately adjust the LCV to be in the correct position for the related conditions. If there is disagreement between the FADEC commanded position of the LCV and the RVDT position, a caution (amber) APU BLEED SYS FAIL CAS message is displayed.

The LCV valve body connects to the bleed air duct with a coupling and two E-seal gaskets which seal the bleed-air check valve. The other end of the LCV valve body connects to the bleed diffuser with a clamp. This clamp connects to small struts which help to hold the weight of the LCV.

If excessive air is being bled from the APU and the maximum EGT shutdown limit is exceeded, the FADEC commands a protective shutdown of the APU. This occurs if the APU is operating in the"nonessential" (ground) mode.

If the APU is operating in the "essential" (flight)mode under the same conditions, the APU is not shut down, but a red APU OVERTEMP warning message is displayed on the primary EICAS page.

This provides the pilot with the option of continued APU operation after closing the LCV or shutting down the APU if not required for flight.

Bleed Diffuser

The bleed diffuser is the tee duct that connects the APU power section to the LCV and the surge valve. Clamps attach the LCV and the surge valve to the bleed diffuser. The bleed diffuser attaches to the power section with bolts. There is a seal between the bleed diffuser and the power section.

Bleed-Air Check Valve

The bleed-air check valve is circular, with two semicircular flappers which open in only one direction. It installs between the load control valve and the bleed air duct. It has an arrow which gives an indication of the permitted airflow direction. The coupling holds the bleed-air check valve, with two E-seal gaskets, between the load control valve and the bleed air duct.

When the LCV opens, the flow of compressed air from the APU opens the bleed-air check valve. This compressed air flows through the bleed-air check valve and into the bleed air duct. When compressed air flows from the aircraft to the APU, this airflow closes the bleed-air check valve. This gives protection to the LCV and the APU power section.

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The function of the compressor control system is to release some compressed air from the APU, when necessary to prevent a compressor surge.

A surge is possible when some conditions occur. As the altitude increases, there is less oxygen that is available to burn. The APU must then do more work to supply the same quantity of energy. This increases the temperature in the combustion chamber, which increases the pressure downstream of the impeller (which is also known as a "compressor"). If this downstream pressure becomes too high, it can cause a compressor surge (the impeller continues to turn, but it does not compress more air). But a surge will not occur if this downstream pressure decreases sufficiently.

The compressor control system operates only when specified conditions occur. To control this system, the full-authority digital engine-controller (FADEC) uses inputs on the ambient, APU and aircraft conditions from:

• The APU engine-controls system
• The bleed management controller

Compressed air from the APU power section is always available to the compressor control system. When necessary, the FADEC causes the compressor control system to release some of this compressed air. This decreases the pressure which is downstream from the impeller and prevents a compressor surge. The released air goes through the surge duct and then out of the aircraft.

The FADEC does this during APU operation when these specified conditions occur: 

• When the APU is at more than 70% speed
• When the ambient pressure (P2) is less than 7.85 psia (54.2 kPa), which is usually at a flight altitude of more than 16,500 ft (5,030 m)
• When the load control valve is closed 

The compressor control system does not operate when the aircraft is on the ground.

Surge Control Valve

The surge control valve (SCV) protects the APU compressor from a potential to surge (stall) when providing electrical loads at altitudes above 15,500 feet, and not supplying bleed air.

The valve is a 2-inch, two-position, butterfly-type shutoff valve operated pneumatically and controlled by an electric solenoid valve. It receives its inputs from the surge control logic of the APU FADEC.

The SCV is mounted on the APU bleed air diffuser by means of a "V" band clamp. When the SCV is open, a portion of the compressed air is discharged overboard near the tailpipe through a separate port.

The surge control logic in the FADEC is active only in the on-speed mode. The SCV is only open when the APU is supplying electrical power, but not pneumatic power, in flight when above altitudes of 15,500 feet. When a pneumatic load request is received by the FADEC, the SCV closes. To open the surge valve, the FADEC energizes the solenoid valve assembly.

Air flows through the solenoid valve assembly to the valve actuator piston. When the solenoid valve is deenergized, the solenoid valve ball blocks air pressure to the actuator, and the actuator spring closes the valve. The opening chamber is vented overboard through the solenoid valve closing rate orifice, and the surge control valve is held closed by spring force.

When the solenoid valve is energized, the solenoid valve ball retracts, blocking the closing rate orifice. Pressure will flow through the rate control orifice into the opening chamber. As pressure increases in the opening chamber, the valve begins to open. The minimum opening pressure is 8 psig.

The rate control orifice controls the SCV opening rate. The rate control orifice puts a limit on the rate at which the surge valve can open when there is a low inlet pressure. This prevents the release of too much compressed air from the power section. Too much air released too quickly can cause the APU to do one or more of these: 

• Have a flame-out
• Have a speed adjustment problem
• Become too hot

The closing rate orifice controls the closing rate of the SCV; the valve will close 2.0 seconds after the solenoid valve is deenergized.

The surge valve does not open when the aircraft is on the ground. In flight, the surge valve closes when the load control valve (LCV) opens. The surge valve is then closed at all times when the LCV is open.

The valve body of the surge valve connects to the bleed diffuser with a clamp. This clamp connects to small struts which help to hold the weight of the surge valve. The other end of the valve body connects to the surge duct with a coupling and an E-seal gasket.

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An APU BLEED switch is located on the overhead BLEED/AIR/AIR COND/ANTI-ICE panel. The switch has three positions: OFF, AUTO and ON.

The LCV operation in response to selections of this switch is dependent on electrical interlocks controlled by the FADEC and the Integrated Air Management System (IAMS).

Associated CAS messages are:

• APU BLEED ON – Status message (white) is displayed when the LCV is open with the APU BLEED switch selected to ON
• APU BLEED OFF – Status message (white) is displayed when the LCV is closed with the APU BLEED switch selected to OFF

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Ground Operation

The APU load control valve operation is initiated through the APU BLEED switch located on the cockpit overhead panel.

Normally the APU BLEED switch is left in the AUTO position. This supplies the pneumatic system with bleed air controlled by the FADEC.

The FADEC sends a ready to load (RTL) signal to the LCV, under the following conditions:

• On the ground, the FADEC sends a command to the LCV to open (via the RTL signal) at the end of a start
  

  • When the rpm reaches 99% for 60 seconds,if the EGT at the beginning of the start sequence was <149 °C, or
  • When the rpm reaches 99%, after a 2-seconddelay, if the EGT at the time of the start sequence was >149 °C
    
    

• In flight, if the LCV receives an RTL signal, it will open at 99% rpm after 2 seconds, no matter what the EGT was prior to the APU start sequence

When the APU BLEED switch is selected to the ON position, the cross-bleed valve in the pneumatic system is automatically opened, provided the X BLEED switch is not in the CLSD position.

With the cross-bleed valve open, APU bleed air can supply the pneumatic requirements of the air conditioning and main engine starting systems.

The following explains the action of the APU BLEED AIR switch, with both engines running:

If the X BLEED switch is in the AUTO position, selecting the APU BLEED switch to the ON position will cause both the L & R engine pressure-regulating SOVs (PRSOVs) to close.

If the X BLEED switch is in the CLSD position and the APU BLEED switch ON, only the left engine PRSOV will close.

In-Flight Operation

The in-flight availability of APU bleed air is interlocked with the main engine bleed supply and the positions of the bleed switches.

The APU is capable of supplying pneumatic power for engine starting in flight. With the APU BLEED switch in AUTO, and both ENG BLEED switches selected to OFF, the APU LCV automatically opens 2 seconds after the APU reaches 99%. This ensures that pneumatic air is available immediately for engine start attempts.

The APU has no function in the start sequence if a running engine is used for start assist of the other engine since an engine bleed remains selected ON, and the LCV is interlocked to close. The ENGINE BLEED switch of the running engine must be switched off, if APU bleed air is to be used for MES in flight of the other engine.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
MPE2	SURGE VALVE	ZONE(S) 320	49-51-01 [ GX ] [ GXRS ] [ G5000 ]
-	BLEED DIFFUSER	ZONE(S) 320	49-52-01 [ GX ] [ GXRS ] [ G5000 ]
MPE1	LOAD CONTROL VALVE	ZONE(S) 320	49-52-05 [ GX ] [ GXRS ] [ G5000 ]
-	BLEED AIR CHECK VALVE	ZONE(S) 320	49-52-09 [ GX ] [ GXRS ] [ G5000 ]

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