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Functional Description
During an engine start, the variable vanes move to the closed position. This lets minimum airflow through the HPC. At approximately 35 percent N2, the variable vanes start to open. The vanes continue to open as N2 speed increases to let more airflow through the HPC.
If the EEC makes a decision that a surge is about to occur, it starts to close the variable vanes.
A failure of the EEC torque motor signal causes the variable stator vane actuator to fully open the variable vanes.
Two LVDTs monitor the position of the actuator. Each LVDT sends data to an EEC channel. The EEC sends the position of the actuator and fault data to the EDIU. The EDIU sends the data to the AIMS.
Indication
The EPCS maintenance page one shows the actuator position of each engine as a percent of full extension. 100 percent shows the variable vanes are fully open and 0 percent shows the vanes are fully closed.
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AIR -COMPRESSOR STATOR VANE CONTROL -FUNCTIONAL DESCRIPTION

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PW4000 SERIES ENGINES

777 AIRCRAFT MAINTENANCE MANUAL

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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details. 

AIR -COMPRESSOR BLEED CONTROL -GENERAL DESCRIPTION

Purpose
The compressor bleed control system modulates the 2.5 bleed valve and controls the two 2.9 (start/stability) bleed valves. This makes the stability of the high pressure compressor better during starting, transient, and reverse thrust operation.
General Description
During engine operation, the 2.5 bleed system and 2.9 bleed system operate automatically.
The EEC controls the 2.5 bleed system and 2.9 bleed system as a function of this information:
*
Thrust lever resolver angle (TRA)

*
LPC rotor speed (N1)

*
HPC rotor speed (N2)

*
Mach number

*
Inlet total temperature (T2)

*
Altitude.

2.5 Bleed System
Servo fuel changes the 2.5 bleed valve actuator position to move the 2.5 bleed valve. The 2.5 bleed valve is an internal ring that opens and closes bleed ports in the fan case.
The EDIU sends 2.5 bleed valve actuator position and fault data to the AIMS.

2.9 Bleed System
The EEC uses a fixed schedule to control the 2.9 bleed system. The schedule makes sure there is a sufficient stability limit for engine operation.
During engine start, both valves bleed 12th stage air. If an engine surge occurs, the pneumatic relay valve can quickly bleed PS3 air to open the 2.9 stability bleed valve. This lets 12th stage air bleed to the turbine exhaust.
　
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本文鏈接地址：777 AMM 飛機維護手冊 發動機引氣 Engine Air PW 777 AIRCRAFT MAINTENANCE MANUAL(17)