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 2)  Second priority is pilots' electric trim (using the thumb switches) if only one autopilot FCC is engaged. Thumb switch electric trim is third prioirty if more than one FCC is engaged.
 3)  Third priority is autopilot automatic stabilizer trim control if one FCC is engaged. Automatic stabilizer trim is second priority if more than one FCC is engaged (Ref 22-22-00, Automatic Stabilizer Trim).
 4)  Fourth priority is Mach/speed trim (Ref 22-24-00, Mach Trim/Speed Stability).
 (b)  The two hydraulic stabilizer trim CUTOUT switches on the control stand operate shutoff valves on the stabilizer trim control modules.
 (2)  Stab Trim Operation
 (a)  The STCMs are operated from two inputs, mechanical and electrical. Mechanical input is with the manual trim levers, which move control arms on both STCMs. All electrical ARM and CONTROL input signals come from the stabilizer trim and elevator asymmetry limit modules (SAM). These signals are interrupted by column cutout and limit switches between each SAM and STCM.
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757 MAINTENANCE MANUAL
 (b)  
The SAM output signals are initiated by operating manual electric trim, automatic stabilizer trim, or within the SAM (Mach/speed trim). Inputs which initiate SAM outputs are: 1) Pilots' thumb operated switches - analog discrete 28 vdc

 for manual electric trim. 2) FCC - Analog discrete ARM and digital CONTROL signals for automatic stabilizer trim. 3) SPM and ADC - analog stabilizer position (SPM) and digital airspeed (ADC) for Mach/speed trim.

 (c)  
The STCMs provide the hydraulic pressure to the hydraulic motors on the ballscrew actuator assembly. Both motors operate with manual lever and thumb switch inputs. The maximum rate of stabilizer movement is 0.5 degrees per second.

 (d)  
The automatic stabilizer trim and Mach/speed trim control use only one hydraulic motor. Only one SAM is in command at any time as determined by the FCC engaged. The rate of stabilizer movement is 1/2 of that used for manual or manual electric control. Maximum rate is 0.25 degrees per second.

 (e)  
A hydraulic pressure signal from the elevator feel computer modulates the flow rate of fluid to the hydraulic motors via a rate valve in the STCM. This signal causes the stabilizer trim rate to vary as a function of airspeed. The trim rate produced by one STCM is between 0.1 and 0.25 degreees per second. Two STCMs produce a trim rate of 0.2 to 0.5 degrees per second.

 AA. Rudder Control System (Fig. 21)
 (1)  Rudder system control components are primarily in the vertical stabilizer. They consist of the following items:
 (a)  
Power Control Actuators (PCAs) (3)

 (b)  
Position Transmitter

 (c)  
Yaw Damper Servo (2) - Actuator

 (d)  
Rudder Ratio Changer - Servo and Assembly

 (e)  
Electric Trim Actuator and Trim Position Transmitter

 (f)  
Feel and Centering Device

 (g)  
Aft Quadrant

 (h)  
Autopilot Rollout Guidance Servos (3)

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MAINTENANCE MANUAL

RUD
AIL ELEV AIL
LOWER EICAS DISPLAY -POSITION INDICATOR (P1 & P3)
RIGHT YAW DAMPER MODULE
LEFT YAW DAMPER MODULE

RUDDER RATIO CHANGER MODULE
MANUAL RUDDER TRIM
RIGHT FLT CONTROL COMPUTER
CENTER FLT CONTROL COMPUTER
LEFT FLT CONTROL COMPUTER

POWER CONTROL
ACTUATOR (3 PLACES)

RUDDER
RUDDER POSITION
TRANSMITTER
CENTERING
BIAS SPRING

YAW DAMPER ACTUATORS
(2)
RATIO CHANGER FEEDBACK TRANSMITTER
RATIO CHANGER SERVO
ELECTRIC TRIM ACTUATOR & TRIM POSITION TRANSMITTER
FEEL & CENTERING UNIT
AUTOPILOT ROLLOUTGUIDANCE SERVOS (3)
RUDDER PEDALS
Rudder Control System
Figure 21
　
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