B.  Each elevator power control unit (Fig. 5) incorporates a main actuator, a gust damping bypass valve, a thermostatic flow control valve (replaced by plug on later units), filters, and a dual concentric main control valve oprated 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 hydraulic 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.  The right power control unit is equipped to receive input from the autopilot. The left power control unit is not equipped to receive input from the autopilot. For autopilot equipment on the right power control unit, refer to Chapter 22, Autoflight. For the right power control unit, the following applies:
(1)  Engaging the autopilot actuates a solenoid to open the autopilot shutoff valve. The shutoff valve directs hydraulic pressure to the autopilot engage mechanism and the autopilot control valve. Actuating the engage mechanism holds the main control valve closed by locking the manual input crank. The engage mechanism is spring loaded to allow manual override of the autopilot system. A control column or feel control unit input, equal to 23-pounds control column force, will allow the input crank to override the engage mechanism spring pressure. Hydraulic pressure at the autopilot control valve, displaces the spring-loaded control valve slide and applies pressure to the transfer valve. The transfer valve slide is displaced by position signals from the pitch channel of the autopilot. Hydraulic pressure is thus directed through the control valve to extend or retract the main actuator piston. As the transfer valve slide moves in the extend direction, the retract port is opened to return. Extension of the main actuator piston displaces hydraulic fluid from the retract side of the piston to return. Disengaging the autopilot de-energizes the solenoid and closes the shutoff valve. Hydraulic bleed at the transfer valve depressurizes the autopilot system and allows the control valve to close. The control valve prevents loss of pressure through the transfer valve when the power control package is operating with the autopilot disengaged.
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27-31-0 Page 10  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 15/77 

G08665
545  Elevator Power Control Unit Schematic 
Feb 01/75  Figure 5 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  27-31-0 Page 11 

11.  Elevator Tab Lock Mechanism
A.  The elevator tab lock linkage provides tab-assisted elevator movement when the elevators are operated in the manual mode and provides lockout of the tab in faired position when the elevators are operated in the power mode. (See figure 6). There are two tab lock linkage mechanisms, one for each elevator, consisting of a pushrod bellcrank mechanism and lock actuator. The mechanism is located inside the horizontal stabilizer elevator balance bays aft of the stabilizer rear spar. The mechanism is attached at the forward end to the inside lower surface structure of the stabilizer and is hinged at the aft end to the elevator front spar. The forces are transmitted by the bellcranks and pushrods to the elevator tab.
B.  During power mode operation, the lock actuator is extended by hydraulic power to position the tab lock mechanism against the spring force so that the centerline of the tab pushrods forward attachments are in alignment with the elevator hinge centerline. This causes the elevator tabs to remain faired with the elevator as the elevator and tab pushrods rotate about a common hinge centerline when the elevators are deflected. During manual mode operation, hydraulic power is removed from the tab lock actuator and the springs on the lock linkage mechanism compress to reposition the bellcranks and the pushrods with respect to the elevator hinge centerline so that elevator movement will cause tab deflection.
12. Operation
A.  The elevators may be operated manually by means of mechanical linkage, or hydraulically using system A and system B hydraulic pressure. (See figure 7.) Forward or aft movement of the control columns transmits motion through two sets of control cables to the aft control quadrants. Motion is then transmitted through a pair of input rods and cranks to the power control units. This mechanical linkage from the control columns is utilized during either manual or hydraulic operation. During manual operation, the power control unit input cranks contact stops and use the power control units to mechanically drive the linkage to the elevators. Elevator neutral position relative to stabilizer is determined by stabilizer attitude. The range of elevator neutral shift varies with mach trim actuator position. With the stabilizer at 3 units of trim, the elevator neutral position is 4 degrees down from faired. As the stabilizer leading edge moves down the elevator neutral position moves up. When the stabilizer moves to 13 units of trim, the elevator neutral is at zero degrees, and with the stabilizer at 17 units of trim, the elevator neutral is 2-3/4 degrees up. Elevator maximum travel with the stabilizer at zero units is 20 degrees up and 22 degrees down.
　
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