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A
737-300/400/500MAINTENANCE MANUAL
 HORIZONTAL STABILIZER TRIM CONTROL SYSTEM -
___________________________________________ DESCRIPTION AND OPERATION
_________________________
 1.  General (Fig. 1 and 2)_______
 A.  The horizontal stabilizer trim control system provides longitudinal trim of the airplane by varying the angle of attack of the horizontal stabilizer. The horizontal stabilizer is moved through 17 degrees of travel by means of a jackscrew with ball nut. The primary stabilizer trim actuator and a cable drum on the jackscrew gearbox provide for trim control from three separate control systems. The normal control is an electrical system which actuates the jackscrew through the primary stabilizer trim actuator. Autopilot control is provided by the PSTA. A manual control system drives through cables to the cable drum on the jackscrew gearbox. The manual system remains engaged at all times and is therefore back-driven by the PSTA during normal operation. Manual system operation will disengage the PSTA if this system becomes jammed. A continuous indication of stabilizer position is provided by trim position indicators adjacent to trim wheels on the control stand. The indicators are positioned by the manual system. A takeoff warning system indicates any unsafe stabilizer position for takeoff (AMM 31-26-00/001). The control column actuated cutout switch located under the cockpit floor, stops electric trimming of the stabilizer when opposed by the motion of the elevator control.

Horizontal Stabilizer Trim Control System Component Location
Figure 1 (Sheet 1)

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A 737-300/400/500MAINTENANCE MANUAL
STABILIZER TRIM CONTROL SWITCH CONTROL COLUMN
COLUMN ACTUATED
CUTOUT SWITCH
OVERRIDE
STABILIZER TRIMSTABILIZER CONTROL SWITCH TRIM WHEEL
STABILIZER POSITION INDICATOR
ELEVATOR TORQUE TUBE
IDLER
SPROCKET
CRANK COLUMN ACTUATED CUTOUT SWITCH
CHAIN ASSEMBLY
INDICATOR FLEXIBLE SHAFT AFT SUPPORT LINK
STB CABLE 1
STA CABLE 1
FORWARD 
FORWARD  CABLE DRUM 
SUPPORT 
LINK 
FORWARD TRIM 
MECHANISM 
SPROCKET 
FORWARD TRIM  1  APPROXIMATLEY 4-1/2 WRAPS 
MECHANISM  OF STB CABLE FROM TOP OF 
TURNBUCKLE  GEAR HOUSING  DRUM AND 28-1/2 WRAPS OF 
STA CABLE FROM BOTTOM OF 
DRUM WITH STABILIZER 
POSITIONED AT FULL AIRPLANE 
NOSEDOWN 

POSITION INDICATOR AND MANUAL CONTROL MECHANISM
A

Horizontal Stabilizer Trim Control System Component Location
Figure 1 (Sheet 2)

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A

737-300/400/500
MAINTENANCE MANUAL

SAFETY ROD 
JACKSCREW 
BALLNUT 
STABILIZER 
PRIMARY  STRUCTURE 
STABILIZER 
TRIM ACTUATOR 
GREASE FITTING 
UPPER GIMBAL 
LOWER STOP 
GEARBOX  (UPPER STOP 
NOT SHOWN BUT 
EQUIVALENT) 
UMBRELLA 
ASSEMBLY 

PLATE COVER 
STA CABLE STB CABLE  GIMBAL LOWER 
FWD  CABLE DRUM GEARBOX 
B STABILIZER ACTUATOR MECHANISM 

Horizontal Stabilizer Trim Control System - Component Location
Figure 1 (Sheet 3)

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