2.9 Bleed System -Normal Operation
During an engine start, both 2.9 bleed valves are held open by a spring. This bleeds 12th stage air to let the HPC turn easier. At approximately 2 percent below idle N2, the EEC sends signals to both solenoids to close the bleed valves. The airflow through the HPC increases.

2.9 Bleed System -Non-normal Operation
If the EEC makes a decision that a surge is about to occur, it commands the 2.9 stability bleed valve to open. The pneumatic relay valve quickly vents the left bleed valve so it can open. This bleeds 12th stage air to the turbine exhaust.
The 2.9 stability bleed valve also opens if there is a rapid deceleration of the engine.
The loss of PS3 servo air or the failure of the solenoid signal also cause a bleed valve to open.
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AIR -COMPRESSOR BLEED CONTROL -FUNCTIONAL DESCRIPTION

Indication
The EEC sends the position of the 2.5 bleed valve actuator piston and fault data to the EDIU. The EDIU sends the data to the AIMS.
The EPCS maintenance page shows actuator position as percent of full extension. 100 percent shows the 2.5 bleed valve is full open and zero percent shows the valve is full closed.
There is no indication of 2.9 bleed valve position.
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AIR -COMPRESSOR BLEED CONTROL -FUNCTIONAL DESCRIPTION

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PW4000 SERIES ENGINES

777 AIRCRAFT MAINTENANCE MANUAL

THIS PAGE IS INTENTIONALLY LEFT BLANK

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AIR -THRUST BALANCE CONTROL -GENERAL DESCRIPTION

Purpose
The thrust balance control system controls 8th stage air to the LPT thrust balance annulus.
General Description
The thrust balance control system operates automatically. The EEC controls the system as a function of this information:
*
Altitude

*
N2

*
HPT 2nd stage internal cavity seal temperature.

The turbine cooling air thermocouple probe measures this temperature.
The EEC sends a signal to a solenoid in the TVBC air shutoff valve solenoid. The solenoid controls the PS3 air that closes the thrust balance air shutoff valve.
The EEC sends TVBC air valve solenoid fault data to the AIMS.
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AIR -THRUST BALANCE CONTROL -GENERAL DESCRIPTION

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AIR -THRUST BALANCE CONTROL -THRUST BALANCE AIR SHUTOFF VALVE
　
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