27-51-00
 ALL  ú ú 01 Page 8 ú Oct 10/97
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 2)  The control valve consists of a ported housing and a spool. Ports on the housing connect to system pressure, return, and to both sides of the hydraulic motor. When the valve is in neutral position, fluid flow is blocked. When the flap control linkage is moved by the flap actuator, the valve spool is repositioned, permitting fluid to flow to the extend or retract side of the hydraulic motor. As the flaps approach the selected position the follow-up feedback mechanism positions the valve spool back to the neutral position, stopping the hydraulic motor.
 (c)  Priority and Flow Control Valve
 1)  The priority and flow control valve contains both a priority valve and a flow limiter. The priority valve permits fluid flow to the hydraulic motors only when hydraulic pressure exceeds 2000 |300 psi (14 |2 megapascal). This makes sure that maximum fluid flow is available to the landing gear.
 2)  The flow limiter is an orifice which limits maximum speed of the hydraulic motor. The flow is adjusted differently on the two power packages so that inboard and outboard flaps extend at the same rate.
 (d)  Bypass Valve
 1)  The bypass valve is a two-position valve operated by a 28 volt dc motor. The function of the valve is to disable the hydraulic motor, permitting alternate operation of the flaps or system shutoff due to flap asymmetry or other failure. The valve is actuated when the ALTN FLAPS ARM switch is pressed or when the FCU's are operating in primary electric mode. The valve or motor can be removed from the airplane without removing the control module or disconnecting fittings from it.
 2)  The bypass valve is normally open, porting hydraulic fluid to the flap drive motor. When the valve is closed, fluid flow to the drive motor is blocked and the drive motor ports are interconnected. The interconnect prevents a hydraulic lock while the flaps are being driven by the alternate motors. A manual override lever is provided on the valve for ground operations.
 (3)  
RVDT Position Transmitter

 (a)  
An RVDT position transmitter (Ref 27-58-00) is installed on each power package flap control assembly and is geared to the output of the flap power unit. The RVDT supplies a power package position signal to the FCU's. The RVDT may be removed without removal or disassembly of the flap control assembly.

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 27-51-00
 ALL  ú ú 01 Page 9 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (4)  
Alternate Limit Switch Pack

 (a)  
An alternate flaps limit switch pack is mounted on the flap control assembly. The pack contains two limit switches, one for flaps full up and one for flaps 25-units detent. One of the switches opens at flaps full up and the other opens at full alternate extension (25-unit detent) to cause the alternate electric system to disengage power to the electric motors. The limit switch pack may be removed without removal or dissasembly of the flap control assembly.

 (5)  
Follow-Up Mechanism

 (a)  
The flap control assembly housing contains a follow-up feedback mechanism which consists of a cam driven from the flap power unit output and a set of levers linked to the power package input crank. The follow-up mechanism repositions the control valve to shut off hydraulic power to the hydraulic motor when selected flap position is reached.

 C.  
Hydraulic Motor

 (1)  
The hydraulic motor for each power package is a reversible axial piston motor which operates at a nominal supply system pressure of 3000 psi (20 megapascal). The motors are mounted on the flap power unit (gearbox).

 D.  
Alternate Electric Motor

 (1)  
The alternate electric motor mounted on each power package provides an independent backup for the trailing edge flap system in the event of a failure in the primary hydraulic drive system. The motor assembly consists of an 115 vac motor, a clutch and torque limiter unit and reduction gearing. The motor is controlled by the ALTN FLAPS ARM and ALTN FLAPS EXT/RET switches on the pilots' center instrument panel or by a signal from the FCU's.

 (2)  
The alternate electric motor is coupled through a double planetary gear train and the combination solenoid operated disconnect clutch/mechanical torque limiter to the main flap drive gear train. During hydraulic flap operation, the motor clutch allows the flap power unit gearing to back drive the second stage of the planetary reduction gearing without driving the electric motor. The internal torque limiter protects against a jammed flap system.
　
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