(5)  
Pitch (Stabilizer) Trim

(a)  
Automatic pitch (stabilizer) trim positions the stabilizer

to compensate for airplane trim changes. The stabilizer trims remove sustained elevator displacements from neutral. Automatic stabilizer trim is disabled during control wheel steering or when the pitch channel is disengaged. The pitch channel is disengaged if the main electric trim switches on the pilots' control wheels are operated. The stabilizer trim on warning light illuminates whenever the main electric switches are operated. Elevator position signals from the elevator position transducer and stabilizer position from the neutral shift sensor are combined with the counter/converter output at summing point 10. The output from summing point 10 is amplified and fed to the automatic stabilizer trim potentiometers which vary the trim threshold as a function of stabilizer position. Output signals from the automatic stabilizer trim potentiometers are fed to summing point 11, past de-energized switch PS-10. (The output from the automatic trim potentiometers is the only input to summing point 11 when the pitch channel is engaged because of the action of energized switch PS-7 which shunts the other input to ground.) The output from summing point 10 is fed to the error detector which produces a train of pulses when its input signal exceeds the trim threshold. This train of pulses, which constitutes a trim command, is routed to the signal trim sensor which sends trim up or trim down signals to the trim logic on the pitch interlock card. The trim logic provides power to energize one of the trim servo relays and provides interlock voltage to prevent simultaneous trim-up and trim-down (hot short) commands. Output from the signal trim sensor is also routed to a pull-in/ drop-out switch in the amplifier. In its normal state, this switch grounds one end of a resistor to attenuate the input signal to the error detector. When a trim command is given, the pull-in/drop-out switch removes the ground from the resistor increasing the sensitivity of the error detector. Increased error detector sensitivity causes the stabilizer to be driven beyond the position corresponding to the trim threshold, eliminating hunting in the servo loop.

(b)  
When a trim-down command is given and the airplane is at cruising speed, power is applied through the autopilot trim cutout switch and limit switches to a nosedown relay in the stab trim servo. (See figure 17.) The nosedown relay applies 115 volts ac 3-phase power to the low speed windings of the servo and 28 volts dc power from pitch engage circuits to the clutch solenoid. This action energizes the clutch to drive the stabilizer toward a trimmed condition. A trim-up command follows a parallel path to the stab trim servo; however, the noseup relay is energized. The relays change the phase relationship of the 115 volts ac inputs to

the low or high speed windings thus reversing the output to the stabilizer. When the airplane is not in the cruising mode (flaps not up), 28 volts dc is supplied to the speed change relay which, when energized, routes power to the high speed windings of the servo.

(c)  
The stabilizer out of trim warning light, located on the center panel is controlled by the trim monitor circuit. The light will illuminate when an out-of-trim condition exists between the stabilizer and elevator for approximately 12 seconds.

(6)  
Control Wheel Steering (Pitch Rate Maneuvering)

500 
May 15/68  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-11-0 Page 57 

500 
22-11-0 Page 58  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Nov 15/68 

(a)  Force transducers (sensors) in the control column's elevator linkage provide an output proportional to force applied to the control column. When this output reaches a predetermined level, the deadzone amplifier and the control wheel steering (CWS) detent sensor portion of the deadzone amplifier provides a CWS out-of-detent (OD) interlock voltage. This interlock voltage deactivates the pitch computer motor clamp circuit and provides a path for the composite force transducer signal out of summing point 1. (See figure 15.) The summation performed at summing point 1 gives both control wheels equal authority. The force transducer signal is routed to summing point 2 through the deadzone amplifier, low pass filter amplifier, and l/q potentiometer in the air data computer. The low pass filter amplifier attenuates resonant frequencies from the deadzone amplifier output. The deadzone amplifier causes the pitch channel to ignore small nuisance signals from the force transducers. The 1/q potentiometer compensates for changes in airplane dynamics due to airspeed. The output from summing point 2 is applied to the unclamped motor amplifier in the pitch computer, causing the amplifier to drive the motor- tachometer generator, which drives the pitch CT through the low ratio gear train. Vertical gyro signal is applied to the pitch computer resulting in CT output which is routed to the valve amplifier to command a change in elevator position.
　
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