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 ALL  ú ú 01 Page 9 ú Jul 15/96
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737-300/400/500MAINTENANCE MANUAL
 B.  Actuation of the control columns in either direction will rotate the centering cam and force the roller up the face of the cam. A change in stabilizer position with the control columns held fixed, will rotate the feel and centering unit body and force the roller up the face of the cam. Either action places a load on the control system and artificially provides feel at the control columns. The resulting load is removed by rotating the control columns forward or aft to run the roller back into the cam detent. A change in stabilizer position with the control columns free to move will allow the roller to remain in the cam detent and cause the control columns to move to the new neutral position.
 10. Elevator Power Control Unit (Fig. 5)___________________________
 A.  The elevator power control units provide actuation of the elevators in response to manual inputs from the control columns. The control units are mounted vertically in the empennage above the aft elevator control quadrant torque tube (Fig. 1). The two control units operate independently from separate hydraulic systems, the left unit from hydraulic system A pressure and the right unit from hydraulic system B pressure. The upper mounting terminals attach to lugs on the output torque tube, which is directly linked by pushrods to each elevator. Failure of either hydraulic system will render one control unit inoperable. The remaining unit will then drive both elevators through the full range of travel.
 B.  Each elevator power control unit incorporates a main actuator, a gust damping bypass valve, filters, and a dual concentric main control valve operated by a dual input crank. The main control valve comprises a primary slide acting within a secondary slide. The secondary slide will permit control should the primary slide become jammed in an open position. An exterior input rod provides actuation of the main control valve from the aft elevator quadrant torque tube.
 C.  Forward or aft movement of the control columns rotates the input torque tube and applies an input to the power control unit input crank. As the input crank displaces the main control valve slide, hydraulic pressure is directed through the valve to move the main actuator piston. Movement of the input crank in the opposite direction displaces the main control valve slide to route hydrualic pressure to the other side of the main actuator piston. The actuator piston rod end is attached to structure, thereby causing the power control unit body to extend or retract. Movement of the power control unit will return the input crank to neutral and close the main control valve.
 D.  Two autopilot actuators drive the elevator input torque tube, which then provides an input to the power control unit input cranks. For autopilot equipment, refer to Chapter 22, Autoflight.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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737-300/400/500MAINTENANCE MANUAL
MAIN ACTUATOR
GUST DAMPING
BYPASS VALVE

PRESSURE PORT
FLOW RESTRICTOR 1
MAIN CONTROL VALVE
1 737 SERVICE LETTER 27-071-B RETURN PORT CALLS FOR REPLACEMENT OF FLOW RESTRICTOR (FIXED RESISTOR).
Elevator Power Control Unit Schematic Figure 5
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737-300/400/500MAINTENANCE MANUAL
 11. Elevator Tab Control Mechanism (Fig. 6)______________________________
 A.  The elevator tab control linkage provides tab-assisted elevator movement when the elevators are operated. There are two tab control linkage mechanisms, one for each elevator, consisting of a pushrod and crank mechanism. The mechanism is located inside the horizontal stabilizer aft of the stabilizer rear spar. The mechanism is attached at the forward end to the inside upper surface structure of the stabilizer and at the aft end to the elevator front spar. The forces are transmitted by the bellcranks and pushrods to the elevator tab.
　
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