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For some conditions when pack air flow is too low, the CTCs send bleed bias signals to the ASCPCs.
ASCPC Functions
The ASCPCs set the flow schedule for the CTCs, give backup pack flow control, and control the engine bleed air system.
The ASCPCs get information about bleed air users and about airplane operating conditions from the left and right system ARINC 629 buses. The ASCPCs use the information to set the pack air flow schedule and send the flow schedule data to the CTCs. The left ASCPC is primary for this function and the right ASCPC is the backup.
The ASCPCs cause the pack flow control valves to close for some conditions. The ASCPC is redundant with the CTC for these conditions.
The ASCPCs are also able to keep the pack outlet temperature in limits by controlling the pack air flow. This is called backup pack flow control. Backup pack flow control is a degraded mode of operation that maintains cabin pressurization but does not give accurate temperature control. The packs operate in this mode if the CTCs do not have electrical power.
If an ASCPC gets a bleed bias signal from the CTC, the ASCPC adjusts the engine bleed air system. This decreases the engine bleed air temperature or increases the HPSOV regulation set point.
See the pack flow control section for more information about the ASCPC functions that have a relation to the air conditioning pack flow control (SECTION 21-51).
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AIR CONDITIONING -COOLING -GENERAL DESCRIPTION -CONTROL

Electrical Power
The CTCs get electrical power from the 28v dc main buses and the 115v ac transfer buses. The ASCPC digital circuits get 28v dc power from the section 2 battery bus, the captain’s flight instrument bus and the first officer’s flight instrument bus. The left ASCPC can operate on battery power. The right ASCPC can operate on standby power from the captain’s and first officer’s flight instrument buses. The CTCs can not operate with only battery or standby power.
Economy Cooling Mode
For some conditions when pack air flow is too low, the CTC adjusts the pack temperature control components to help increase flow. An ARINC 629 signal from the CTC to ELMS puts the pack into the economy cooling mode.
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AIR CONDITIONING -COOLING -GENERAL DESCRIPTION -CONTROL

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Interfaces
These systems have an interface with the CTC on the Systems ARINC 629 buses:
*
Overhead panel ARINC 629 system (OPAS): air conditioning panel pack controls and indications

*
Power panels: control of the economy cooling valve

*
Proximity sensor electronics unit (PSEU): air ground

*
Environmental control system miscellaneous cards
(ECSMCs): recirculation fan overheat switches

*
Air supply cabin pressure controller (ASCPC): flow schedules, stall recovery

*
Auxiliary power unit controller (APUC): APU status, pack demand

*
Airplane information management system (AIMS) cabinet: primary display system (PDS), central maintenance computing system (CMCS)

*
Air Data Inertial Reference Unit (ADIRU): Airplane Altitude.

The CTC supplies analog control signals to these components:
*
Flow control and shutoff valves

*
Ram air inlet door actuator

*
Low limit valve

*
Second stage turbine bypass valve.

These components supply analog data to the CTC:
*
Pack flow sensor

*
Flow control valve proximity switches

*
Primary heat exchanger inlet temperature sensor

*
Ram air inlet RVDT

*
Low limit valve RVDT

*
Second stage turbine bypass valve RVDT

*
Primary heat exchanger outlet temperature sensor

*
Compressor discharge temperature sensors 1 and 2

*
Secondary heat exchanger outlet temperature sensor

*
Turbine inlet temperature sensor

*
Condenser inlet temperature sensors 1 and 2

*
Pack discharge temperature sensor

*
Mix manifold temperature sensors 1 and 2.

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777 AIRCRAFT MAINTENANCE MANUAL

AIR CONDITIONING -COOLING -INTERFACES

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AIR CONDITIONING -COOLING -OPERATION
　
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