(6)  
Cranks

 (a)  The internal crank roller is clamped by the detent pistons when the autopilot is engaged. The crank moves with the actuator piston. Motion of the internal crank moves the output position LVDT. One end of the output crank is directly connected to the internal crank at a common pivot point. The other end of the output crank is connected to the elevator linkage.

 (7)  
Mechanical Control Sequence

 (a)  Initially the actuator piston is fixed by the centering springs. With the autopilot not engaged (armed), SV1 is open, SV2 is closed, detent pistons are disengaged and the internal crank is free to move within the piston cavity. The output position LVDT provides internal crank position to the FCC for autopilot synchronization to elevator position. The FCC commands through the EHSV cause the actuator piston to follow the internal crank so that the internal crank remains centered within the piston cavity. The actuator piston position LVDT nulls the command signal to stop the actuator piston.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL

 (8)  
Autopilot Control Sequence

 (a)  With the autopilot engaged, SV1 and SV2 are open, the detent pistons are pressurized and the internal crank is clamped on the center of the actuator piston. When the EHSV receives a command from the FCC, hydraulic pressure is ported to one end of the actuator piston. The detent pistons carry the internal crank with the actuator piston to its commanded position. The output crank moves the linkage to the elevator power control actuators while the output position LVDT sends position information back to the FCC to null the command signal and stop the elevator.

 (9)  
Camout

 (a)  Camout occurs when the output crank position (surface position) does not correspond to the actuator piston position (servo position) as sensed by differing LVDT outputs. Causes of camout are mechanical jamming of output linkage, pilot override of an engage autopilot channel, or mechanical override of one autopilot channel by the remaining two during multi-channel operation.
 D.  Feel and Centering Unit Assembly (Fig. 5)
 (1)  
The neutral shift transducers provide an analog input to each FCC. The input is proportional to the elevator position which varies with stabilizer position. The three transducers are installed on the feel centering assembly aft of the horizontal stabilizer in fuselage Section 48.

 (2)  
The stud on each transducer housing is mounted to a bearing in the centering unit housing assembly with a nut and washer. The adjustable rod end of each transducer is attached to the centering unit crank assembly. The left and right transducer rod ends are each attached to the crank assembly with a bolt and nut. The center transducer attaches to the bottom side of the crank assembly with a countersunk screw. The rod ends are adjusted during transducer installation to set the null voltage of each unit. Each transducers electrical cable is clamped to the centering unit housing assembly.

 (3)  
The primary winding of each neutral shift transducer uses 400 Hz 26 vac from its associated FCC. The secondary winding output to the FCC is in phase with the primary winding when the transducer is in the retracted position. The secondary output phase is reverse of the primary when the transducer is in the extended position. The output is nulled with the centering unit assembly crank is in a neutral position.

 3.  Operation
_________
 A.  Functional Description
 (1)  Mode Control Panel - Pitch Controls (Fig. 6)
 (a)  
Flight Director ON/OFF switches

 1)  Two F/D toggle switches control the left and right flight director dislays.

 (b)  
Command Engage switches

 1)  GUN 051; three CMD ENGAGE paddle switches (L, C, R) provide inputs for command engage logic to dedicated FCCs.
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 22-12-00
 ALL  ú ú 11 Page 14 ú Jun 20/97
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
　
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