H82388
Autopilot and Yaw Damper Circuit  542 
22-11-0  Figure 1A  Feb 01/76 
Page 14 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  Stabilizer Trim Servo
(1)  
The stabilizer trim servo is located on the stabilizer jackscrew gearbox assembly in the tail cone. The trim servo consists of a nose-up relay, a nose-down relay, a speed change relay, a two-speed three-phase ac motor, a gearbox assembly with an electromagnetic clutch and an output shaft. Stabilizer trim control signals (trim up or trim down) are generated in the pitch control channel and are sent to the appropriate relay. Actuation of a relay supplies 115-volt ac, three-phase power to drive the servomotor in the desired direction. An additional set of contacts of the relays complete 28 volts dc from the pitch channel to the electromagnetic clutch solenoid.

(2)  
Trim servomotor power is controlled by the autopilot stabilizer trim circuit in the pitch channel. Power to the clutch solenoid is supplied from the pitch control channel through the autopilot trim cutout switch on the control stand. When the autopilot trim cutout switch is placed in the CUTOUT position, power is removed from the motor and clutch solenoid. The stabilizer trim limit switches also remove power from the clutch solenoid. Protection against a hot short (simultaneous trim-up and trim-down command) is provided in the pitch channel.

E.  Position and Trim Sensors
(1)  
The position and trim sensors consist of the linear position transducers, the automatic stabilizer trim potentiometer and the elevator neutral shift sensor. The linear position transducers furnish elevator and aileron position feedback to the pitch and roll control channels. The automatic stabilizer trim potentiometer and the elevator neutral shift sensor furnish signals to the stabilizer trim circuit in the pitch control channel.

(2)  
There are identical linear position transducers, one integrally contained in each B system elevator and aileron power control unit. The transducers are variable reluctance transformers which are excited by 26 volts ac. The magnitude of the output signal varies directly with the length of the input stroke and the phase of the signal changes with the direction of the stroke from the center null position.

(3)  
The automatic stabilizer trim potentiometer provides variable control of the threshold of the automatic pitch trim. Trim potentiometer provides variable control of the threshold of the automatic pitch trim. The potentiometer is mounted on the elevator and rudder feel computer unit and is driven by the stabilizer. The trim potentiometer consists of a potentiometer, an input isolation transformer, two fixed resistors, and a fixed calibration resistor. The input is fed from the pitch control channel with signals from the elevator position transducer and the elevator neutral shift sensor.

(4)  
The elevator neutral shift sensor is a three wire synchro using 26-volt ac excitation. The sensor is mounted next to and is driven by the elevator feel and centering unit to determine the neutral shift of the elevator as a function of stabilizer position or mach trim.

545 
May 01/75  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-0 Page 14A/14B 

F.  Autopilot Transfer and Solenoid Valves
(1)  
One autopilot solenoid valve is mounted on B elevator and aileron control units and may be installed on A control units. They are solenoid-actuated valves which are energized when the autopilot axes are engaged; they complete hydraulic power to the transfer valves in the power control units.

(2)  
A transfer valve is also mounted on B elevator and aileron power control units and may be installed on A control units. They convert the autopilot electrical signals into hydraulic flow. The unit consists of a torque motor which moves either a flapper valve or a jet pipe assembly which regulates the hydraulic flow to the power control unit output piston.

G.  Autopilot Accessory Box
(1)  
The accessory box is mounted on the E1-3 shelf of the equipment racks in the electrical and electronic compartment.

(2)  
The autopilot accessory box contains the necessary components to interconnect the autopilot and yaw damper system. It contains the yaw damper engage relay, the glide slope engage relay, the OMEGA steering and system selection relays, loading resistors for the navigation and ILS receivers, the autopilot warning lights flashers, time delay gate module and the circuit interrupters for the yaw damper, auto pilot engage and mode switching circuits. The circuit interrupters ensure the engage and NAV mode holding coils drop out whenever the system select switch or NAV mode select switch is operated, thereby allowing the interlock circuits to be satisfied before the coils can be energized again.
　
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