(4)  
The flow control tees assure even distribution of hydraulic fluid to each half of the thrust reverser.

 (5)  
A linear variable differential transformer (LVDT) for the reverser position feedback which provides reverser position to the EEC which commands the interlock actuator for the thrust lever to ensure proper sequencing of engine power with thrust reverser position.

 C.  
To operate the thrust reverser, the airplane must be on the ground to close the air/ground relays and the forward thrust lever must be at the idle position. The reverse thrust lever is lifted closing the reverser control switch which completes the electrical circuit that opens the isolation valve, admitting hydraulic fluid to power the thrust reverser.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
////////////////////////A / PW4000 SERIES /747-400 / ENGINES/MAINTENANCE MANUAL ////////////////////////
D.  A mechanical interlock on the thrust levers prevents the reverse thrust lever from moving more than 8.5 degrees except when the forward thrust lever is within 2 degrees of idle. The interlock actuator will not allow movement of the reverse thrust levers to the high power, reverse thrust position until the two reverser sleeves have extended to about 60% of their full deployment. When the thrust reverser sleeves are at about 60% deployment, the EEC completes the electrical circuit to the interlock relay. The interlock relay will energize, which will extend the interlock actuator. When the interlock actuator extends, the reverse thrust levers can move freely to the high power, reverse thrust position. The interlock actuator uses approximately one second to extend or retract. When stowing the thrust reverser sleeves, the electrical power will be removed from the interlock relay when the sleeves have retracted to their 20% deployed position. When the electrical power is removed from the interlock relay, the interlock actuator will retract. This will prevent movement of the reverse thrust levers to the high power, reverse thrust position.
 E.  The thrust reversers are retracted by returning the reverse thrust levers to the stow position, thus de-energizing the directional control valve to the retract mode. Although the control switch for the thrust reverser in the thrust lever is also returned to the off (stow) position, proximity sensors for the auto restow on each of the reverser's translating sleeves provide target far condition to the proximity switch electronic unit (PSEU) which holds the hydraulic isolation valve open until both halves are stowed. The auto-restow circuit is automatically de-energized 5 seconds thereafter.
 F.  An air/ground relay operated from the tilt switch of the landing gear prevents commanded reverser operation in flight by maintaining an open circuit to the isolation valve while the airplane is airborne.
 2.  Isolation Valve (Fig. 1)
_______________
 A.  The purpose of the isolation valve is to provide hydraulic pressure, on command, to the thrust reverser system. The isolation valve consists of a 2-stage solenoid operated valve (arming spool and pilot valve), check valve, bypass valve, and pressure switch. The isolation valve is also used to isolate the thrust reverser system from any inadvertent cockpit actuation during ground maintenance or during ferry flights after a failed thrust reverser. A manual lockout is used for this purpose.
 B.  The isolation valve is a hydraulic servo valve with an arming spool actuated by a solenoid operated pilot valve. With the solenoid de-energized, the arming spool is biased to the closed position where the PRESS port is blocked and the CYLINDER port is connected to RETURN port.
 C.  When the solenoid is energized, the pilot valve is opened and fluid is ported to one end of the arming spool, shifting it to the open position. The PRESS port is connected to the CYLINDER port. Return flow to the hydraulic power system is ported continuously from the CONT port to the RETURN port, independent of solenoid operation. A check valve is placed in this return path to prevent pressure surges from propagating back into the reverser's return system.
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