B. The captain or first officer may override the force limiter by applying sufficient force on the control wheel to drive through the limiting mechanism. The autopilot continues to drive the AP actuator throughout the control wheel steering regime after the control wheel is returned to less than 25 or 17 degrees. Manual control is used if the control wheel is turned greater than 25 or 17 degrees.
C. The force limiter is deactivated when the autopilot is disengaged, regardless of control wheel position.
9. Autopilot Actuator
A. Four autopilot actuators are installed, two in the main wheel well area for the aileron axis and two in the aft fuselage for the elevator axis. One set, aileron and elevator, is controlled by the A autopilot system and the other set by the b autopilot system. The units are mechanically linked to aileron and elevator power control units (PCU’s) which drive the flight control surfaces.
B. Autopilot system electrical signals operate valves which modulate hydraulic pressure to displace a hydraulic piston and provide a rotary output to the respective PCU. Control and position signals are provided by the following components which are installed on each actuator: engage solenoids, transfer valve, linear variable displacement transducer (LVDT), and pressure regulator.
C. Engage Solenoids
(1) Two engage solenoids are on each autopilot module. Each solenoid is an electrically operated valve (28 volts dc) which, when energized, applies hydraulic pressure within the module. Engage solenoid –1 provides hydraulic pressure to the transfer valve and to engage solenoid –2. Engage solenoid –2 provides hydraulic pressure to the detent mechanism. Both solenoids are energized at AP engagement; however, solenoid –2 is delayed slightly from solenoid –1. The solenoids are attached to the module with four bolts. Electrical pins mate with wiring within the module when the units are installed. Hydraulic pressure is ported into the units through ports which align when the solenoids are installed. The solenoids are line replaceable units.
D. Transfer Valve
(1) The transfer valve distributes hydraulic fluid to the actuator piston through two ports. With the AP engaged and no command signal output (null), the hydraulic flow is equal between output ports. An autopilot signal (vdc) torques the transfer valve to provide an unbalanced hydraulic flow through the control ports which in turn caused the actuator piston to displace. Electrical connections and hydraulic fluid is supplied to the transfer valve similar to the solenoid valves. The transfer valve is installed with four bolts. The transfer valve is a line replaceable unit.
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Nov 15/79 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 22-11-01 Page 11
E. Linear Variable Displacement Transducer (LVDT)
(1) The linear variable displacement transducer provides positional information for the actuator piston. The LVDT is excited by 26 volts ac and provides an ac output signal in proportion to piston position. The LVDT is not line replaceable since it requires mechanical adjustment for nulling to match the actuator.
F. Pressure Regulator
(1) The pressure regulator is in line with the hydraulic passages between solenoid -2 and the detent piston (which locks the actuator piston to the output crank). The regulator bypasses hydraulic fluid to limit the output force (autopilot authority) of the actuator when the unit is backdriven or stalled.
10. Position and Neutral Shift Sensors
A. The aileron position sensor, elevator position sensor, neutral shift sensor and rudder position sensors (when installed) are all identical and interchangeable. They are three-wire synchros with 26 volts ac supplied to the excitation winding. The body of the sensor and the knurled shaft have alignment marks so that the sensor may be properly installed for a null signal condition.
11. Automatic Stabilizer Trim Potentiometer
A. The automatic stabilizer trim potentiometer is in housing similar in appearance to the position sensors but of larger size. The unit incorporates dual variable attenuation networks each consisting of a potentiometer, isolation transformer, and various resistors. The common potentiometer shaft is mechanically coupled to a splined output shaft by an internal anti-backlash coupler. Index marks are provided on the housing and the knurled shaft for proper alignment during installation.
B. The stabilizer trim potentiometer receives trim input signals from the elevator neutral shift and position sensor. Stabilizer position attenuates trim input signals as a function of stabilizer potentiometer shaft position. The shaft is mechanically coupled to and driven by the stabilizer.
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