The ducts are along the wing front spar. In the fuselage, the ducts go from the ECS bays in the wing-to-body area to APU firewall.
Physical Description

The large main distribution ducts are seven inches (18cm) in diameter. The smaller ducts are of different sizes from four inches (10cm) to three eights of an inch (1cm).
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AIR SUPPLY DISTRIBUTION -DUCT

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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -ISOLATION VALVES AND APUSOV

General

The left, center and right bleed isolation (L,C,R ISLN) switches and the APU bleed switch have two positions: in (AUTO) and out. The isolation switches have a CLOSED light. The APU bleed switch has an OFF light.
The AUTO position lets the ASCPCs set the position of the valves. The out position commands the ASCPCs to close the related valve. The CLOSED and OFF lights give indication for non-normal conditions.
These are the usual positions of the valves when the switches are set to AUTO and both engine air supply systems are on:
*
Left isolation valve -open

*
Center isolation valve -closed

*
Right isolation valve -open

*
APUSOV -closed.

The digital (primary) mode of both ASCPCs control the valves at the same time. The valves have a solenoid with two coils, one for each controller. A RVDT gives valve position information directly to one of the two ASCPCs. The other ASCPC monitors valve position through the ARINC 629 buses.
If the digital mode for both ASCPCs fail, the valves go to a default position. With duct pressure available, the valves open. With no duct pressure, a spring in the actuator closes the valves.
EFFECTIVITY
BEJ ALL

The left controller supplies BITE for the solenoid coils and RVDTs that connects to it. The right controller supplies BITE for the solenoid coils and RVDTs that connects to it.
The left ASCPC gets 28v dc for control and operation of the valves from the section 2 battery bus (not shown). The right ASCPC gets 28v dc for control and operation of the valves from the first officer’s flight instrument bus (not shown).
See the engine air supply section for more information on ASCPC power (SECTION 36-11).
See the DC generation section for more information about DC power (SECTION 24-30).
Valve Functional Description
The internal parts for the left isolation valve is shown, the other isolation valves and the APUSOV have the same parts.
Air pressure to operate the valve comes from one of the two sides of the valve. The side of the valve that has the higher pressure supplies the air that goes through the shuttle valve to the switcher valve. The ASCPCs use the coils in the solenoid to control switcher valve position.
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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -ISOLATION VALVES AND APUSOV

To open the valve, the ASCPCs remove power from the solenoid coils. The switcher valve spring moves to the position that lets air pressure go to the actuator. Pressure more than 10 psi opens the valve.
The RVDT sends a valve position signal to the ASCPC.

To close the valve, the ASCPCs energize the solenoid coils. The switcher valve moves to block pressure from the shuttle valve and releases pressure from the actuator. The spring in the actuator closes the valve.
The ASCPCs have control logic that sets the valves to one of these configurations:
*
Open

*
Closed

*
Latched closed.

See the pages in this section about isolation valve and APUSOV control logic for more information on valve configurations.
Indications

These are the flight deck effects that have a relation to the position of the isolation valves and the APUSOV:
*
Left, center, right bleed isolation CLOSED lights

*
APU bleed OFF light

*
Advisory message, BLEED ISLN CLOSED L,C,R

*
Advisory message, BLEED ISLN OPEN L,C,R (not shown)

*
Advisory message, BLEED OFF APU.

A status message, (not shown) also shows if a valve is not in its set position.
The CLOSED light lets the flight crew know that the valve is closed due to a non-normal condition of the valve or the pneumatic system. The OFF light lets the flight crew know that APU bleed air is not available due to a non-normal condition of the valve or the pneumatic system.
　
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