Flight Controls -System Description
Pitch Control Schematic

SPEED TRIM
FAIL

A
B
POSITION

Flight Controls -System Description 737 Operations Manual

Stabilizer
The horizontal stabilizer is positioned by the main electric trim motor controlled through either the stabilizer trim switches on the control wheel or by the autopilot trim servo motor. The stabilizer may also be positioned by manually rotating the stabilizer trim wheel.
Stabilizer Trim
Stabilizer trim switches on each control wheel actuate the electric trim motor through the main electric stabilizer trim circuit when the airplane is flown manually. With the autopilot engaged, stabilizer trim is accomplished through the autopilot stabilizer trim circuit. The main electric and autopilot stabilizer trim have two speed modes: high speed with flaps extended, and low speed with flaps retracted. If the autopilot is engaged, actuating either pair of stabilizer trim switches automatically disengages the autopilot. The stabilizer trim wheels rotate whenever electric stabilizer trim is actuated.
The STAB TRIM MAIN ELEC cutout switch and the STAB TRIM AUTOPILOT cutout switch, located on the control stand, are provided to allow the autopilot or main electric trim inputs to be disconnected from the stabilizer trim motor.
Control column actuated stabilizer trim cutout switches stop operation of the main electric and autopilot trim when the control column movement opposes trim direction. When the STAB TRIM override switch is positioned to OVERRIDE, electric trim can be used regardless of control column position.
Manual stabilizer control is accomplished through cables which allow the pilot to position the stabilizer by rotating the stabilizer trim wheels. The stabilizer is held in position by two independent brake systems. Manual rotation of the trim wheels can be used to override autopilot or main electric trim. The effort required to manually rotate the stabilizer trim wheels may be higher under certain flight conditions. Grasping the stabilizer trim wheel will stop stabilizer motion.
Stabilizer Trim Operation with forward or AFT CG
In the event the stabilizer is trimmed to the end of the electrical trim limits, additional trim is available through the use of the manual trim wheels. If manual trim is used to position the stabilizer beyond the electrical trim limits, the stabilizer trim switches may be used to return the stabilizer to electrical trim limits.
Stabilizer Position Indication and Green Band
Stabilizer position is displayed in units on two STAB TRIM indicators located inboard of each stabilizer trim wheel. The STAB TRIM indicators also display the TAKEOFF green band indication.
Copyright . The Boeing Company. See title page for details.
9.20.8  December 1, 2000

Flight Controls -System Description 737 Operations Manual
The trim authority for each mode of trim is limited to:
.  
Main Electric Trim
Flaps retracted 2.5 to 12.5 units
Flaps extended 0.25 to 12.5 units

.  
Autopilot Trim 0.25 to 14.0 units

.  
Manual Trim 0 to 17.0 units

The green band range of the STAB TRIM indicator shows the takeoff trim range. An intermittent horn sounds if takeoff is attempted with the stabilizer trim outside the takeoff trim range.
Speed Trim System
The speed trim system is designed to improve flight characteristics during operations with a low gross weight, aft center of gravity, high thrust. It monitors inputs of stabilizer position, thrust lever position, airspeed, and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. It operates most frequently during takeoffs and go–arounds. Conditions for speed trim operation are listed below:
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Flaps not up . N1 above 60%

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Airspeed 100 – 300 KIAS . Autopilot not engaged

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10 seconds after takeoff . Sensing of trim requirement

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5 seconds following release of
trim switches

Yaw Control (before Rudder System Enhancement Program (RSEP) modification)
Yaw control is accomplished by a hydraulically powered rudder and a yaw damper system. The rudder is controlled by displacing the rudder pedals. The yaw damping functions are controlled by the yaw damper rate gyro.
Rudder
The rudders provide yaw control around the airplane’s vertical axis. The A and B FLT CONTROL switches control hydraulic shutoff valves for the rudder and the standby rudder.
Each set of rudder pedals is connected by cables to the main and standby rudder PCUs through the rudder feel and centering unit. The main rudder PCU is powered by hydraulic system A and B while the standby rudder PCU is powered by the standby hydraulic system. The standby hydraulic system is provided as a backup if system A and/or B pressure is lost. It can be activated manually through the FLT CONTROL switches or automatically. (Refer to Chapter 13, Hydraulics, Standby Hydraulic System)
　
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