(5)  
The solenoids direct flow according to the commanded trim direction to position corresponding directional selector valves; the arming valve and the control valve. When energized, each solenoid releases pressure from a balance cylinder at one end of its corresponding valve. Pressure exerted via the opposite solenoid upon the other end of the valve drives the valve to the end of its travel in the selected direction. With both the up and down solenoids de-energized, the valve is pressure balanced and spring-loaded to the center neutral position. With both the arming and control valves positioned to command trim in the same direction, fluid is directed through outlet ports to the trim drive mechanism. No flow can take place if the valves are commanded in opposite directions or if only one valve is directionally positioned.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 (6)  
With trim selected, pressure is applied sequentially to the hydraulic brake and to the nose up or nose down port on the hydraulic motor, which drives the trim drive mechanism. Pressure is also applied simultaneously to close a pressure switch. Hydraulic power is sustained until the STAB TRIM switch returns to neutral, terminating the command signal. The arming and control valve ports are designed so that pressure is always applied to release the trim drive mechanism hydraulic brake before driving the motor. Conversely, pressure to the motor is always cut off before brake pressure so that the brake is not applied while the motor is powered.

 (7)  
A pushbutton on the front of each STCM operates a manual brake bypass valve for maintenance checks. When the button is pressed, the valve releases the pressure in the hydraulic brake and motor, applying the brake.

 (8)  
When the hydraulic output from each STCM is delivered to the hydraulic brake and motor, the hydraulic brake releases and the hydraulic motor rotates in the selected direction. Each motor drives the trim drive mechanism through the differential. When the STCM cut off the hydraulic pressure, the motors stop rotating and the hydraulic brakes are applied by spring pressure.

 (9)  
When stationary, the stabilizer is prevented from creeping by the primary and secondary brakes. The primary brake is applied by the thrust exerted on the ballscrew by stabilizer air loads. Downward thrust compresses the lower brake disk of the primary brake between the flange and the lower ratchet. As the ballscrew is right-hand threaded, downward thrust attempts to turn it clockwise (viewed from the drive end). Two diametrically opposed pawls prevent the lower ratchet from turning clockwise, constraining the screw. Upward thrust exerted on the screw compresses the upper disk and applies anticlockwise torque which is similarly resisted by the upper ratchet. Actuator drive to trim the stabilizer in a direction opposed to airload thrust is unresisted, because the ratchet and compressed brake disk rotate freely with the flanged screw shaft. Drive in a direction with the thrust is opposed by primary brake. The hydraulic motors, assisted by the thrust exerted by the airload, overcome brake friction to trim the stabilizer in this direction. Secondary braking is provided through the hydraulic brakes and by the control valves which hydraulically lock each motor when no trim signal exists.

 B.
Control

 (1)  
General

 (a)  
This section provides instructions for stabilizer control in the two manual operating modes; manual-electric.

 (b)  
For information concerning control in the auto flight control modes, refer to 22-14-00.

 (2)  
Manual-Electric Operation

 (a)  
Provide electrical power (Ref 24-22-00/201).

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 WARNING: AILERONS, ELEVATORS, RUDDERS, TRAILING EDGE FLAPS AND
_______ SPOILERS ARE FULLY POWERED. PERSONNEL AND EQUIPMENT MUST BE CLEAR OF ALL CONTROL SURFACES BEFORE PROVIDING HYDRAULIC POWER. ELEVATORS WILL MOVE RAPIDLY TOWARDS NEUTRAL. POWERED CONTROL SURFACES CAN CAUSE INJURY AND DAMAGE.
　
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