*
Selected to a non-normal mode of control (related switch set to out).

The ASCPC logic that causes the CLOSED or OFF light, is different from that used to cause the related closed or off message. This means that the light and the related message do not always show at the same time.
The primary mode of the left ASCPC has the logic that calculates when to supply the indications related to the left and center isolation valves. The primary mode of the right ASCPC supplies the indications related to the right isolation valve and the APUSOV.

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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -ISOLATION VALVES AND APUSOV

Valve Closed/Off Indications

The ASCPCs usually use RVDT valve position information and non-normal condition information to calculate when to show the valve closed/off indications. If the RVDT circuit fails, the ASCPCs use non-normal condition information to calculate when to show the valve closed/off indications.
The applicable bleed isolation CLOSED light comes on if any of these non-normal conditions occur:
*
A bleed isolation switch is set to out

*
An isolation valve is latched closed

*
A valve is set to close as the ASCPCs try to find the location of a duct leak (wing, body)

*
An isolation valve fails closed and the RVDT circuit functions correctly

*
Source loss condition (L for L ISLN val, R for R ISLN val, L or R for C ISLN val).

The source loss condition logic lets the ASCPCs sense when there is not sufficient air pressure to the ADPs for landing gear retraction, immediately after takeoff.
There are two sets of conditions that cause the ASCPCs to set the left source loss condition to true. The first set is true if all of these conditions occur:
*
Landing gear down

*
Less than 180 seconds since takeoff

*
Left engine off.

EFFECTIVITY
BEJ ALL

The second set is true if all of these conditions occur:
*
Landing gear down

*
Less than 180 seconds since takeoff

*
Left duct pressure less than 20 psig and right duct pressure more than 20 psig.

The conditions that cause the right source loss condition to be true are equivalent to the conditions that cause the left source loss.
The APU bleed OFF light turns on if any of these non-normal conditions occur:
*
APU bleed switch set to out

*
APU shutoff valve latched closed

*
Left wing or body duct leak

*
APU shutoff valve failed close and the RVDT circuit functions correctly

*
APU fire switch pulled.

For each valve there are two sets of conditions that cause the related closed or off advisory message, BLEED ISLN CLOSED L, C, R or BLEED OFF APU. The first set causes the applicable message if all of these conditions occur:
*
Bleed isolation switch (L, R, C, APU) set to out

*
Related valve is in the closed position or has a failed RVDT circuit.

The second set causes the applicable message if all of these conditions occur:
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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -ISOLATION VALVES AND APUSOV

*
Related valve fails in the closed position

*
The RVDT circuit functions correctly.

Isolation Valve Open Indication Logic

The applicable advisory message, BLEED ISLN OPEN L,C,R shows if the RVDT circuit for the related valve operates correctly and the related valve fails in an open position.
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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -ISOLATION VALVES AND APUSOV

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AIR SUPPLY DISTRIBUTION -FUNCTIONAL DESCRIPTION -LEFT AND RIGHT ISOLATION VALVE CONTROL LOGIC

Control Logic

The ASCPCs have control logic that sets the left and right isolation valves to one of these configurations:
*
Open

*
Closed

*
Latched closed.

The following information tells about the control logic for the left isolation valve. The control logic used to set the right isolation valve is equivalent to the left isolation valve except where specified.
Valve Open Control Logic

There are three sets of conditions that cause the ASCPCs to open the left isolation valve. The first set of conditions is true if all of these conditions occur:
　
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