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 A.  The solenoids, which are electrically operated, single coil, pneumatic type solenoid valves, supply pneumatic servo pressure to actuate the HPC secondary flow control valves.
 B.  There are two solenoids (designated right and left) for controlling the right and left HPC secondary flow control valves respectively.
 C.  The right solenoid is assembled to a common valve housing with the IDG air/oil heat exchanger valve override solenoid, and is positioned on the right side of the common valve housing. The right HPC secondary flow control valve solenoid is installed on the intermediate case aft face at the 1 o'clock position.
 D.  The left solenoid is assembled to a common valve housing with the TVBCA valve solenoid, and is installed on the right side of the common valve housing. The left HPC secondary flow control valve solenoid is installed on the intermediate case aft face at the 10 o'clock position.
 E.  The solenoids are fail-safe to interrupt pneumatic servo pressure to the HPC secondary flow control valves which causes the valves to open.
 5.  Operation
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 A.  Functional Description
 (1)  During climb, with increasing altitude and decreasing high pressure rotor speed (N2), the EEC sends aircraft power (28 volts dc) to both solenoids separately, energizing them. The energized solenoids permit 15th-stage air to flow through in-line filters to the valves, closing them.
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(2)  
When the valves are closed, 9th-stage air is no longer bled to the HPC inner cavity, which increases the engine efficiency. The resulting decreased low pressure turbine/high pressure turbine (LPT/HPT) cooling airflow is not detrimental due to some air still leaking through the 9th-stage stator vane knife-edge seals into the inner cavity. In addition, less cooling airflow is necessary at lower power settings. The reduced cooling airflow increases HPC drum or rotor temperature, allowing drum growth to tighten the HPC blade tip and knife-edge air seal clearances, thus increasing HPC efficiency.

 (3)  
If engine speed is increased, while the altitude is above the valve-closed point, the solenoid interrupts servo pressure to the valves. When servo pressure is lost, the valves spring open and duct 9th-stage air from the HPC external bleed annulus to the inner cavity through hollow 9th-stage stator vanes. This supplies cooling airflow to the HPC and LPT areas, which decreases the HPC internal drum temperature, and seal pressurization air for No. 1.5 bearing forward carbon seals.

 (4)  
The EEC receives a position feedback signal from both valves. If the EEC detects a valve is not opened when it should be, it will hold the other valve open at all times during engine operation.

 (5)  
If HPC secondary flow control valve solenoid fails, an ENG-# HPC SEC FLOW SOL WIRING FAIL (CH-X) is stored in the central maintenance computer system (CMCS).

 NOTE: "ENG-#" denotes engine 1, 2, 3, or 4; "CH-X" denotes channel
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 A or channnel B.

 (6)  If HPC secondary flow control valve(s) fails, an ENG-# HPC SEC FLOW CONTROL VALVE/WIRING FAIL (CH-X) is stored in the central maintenance computer system (CMCS).
 NOTE: "ENG-#" denotes engine 1, 2, 3, or 4; "CH-X" denotes channel
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 A or channnel B.

 B. Control
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / PW4000 SERIES /747-400 / ENGINES/MAINTENANCE MANUAL ////////////////////////
(1) All operations are automatic and no operator action is necessary.

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