757 MAINTENANCE MANUAL
 AUTOMATIC STABILIZER TRIM SYSTEM - DESCRIPTION AND OPERATION
____________________________________________________________
 1.  General (Fig. 1)_______
 A.  The automatic stabilizer trim system provides automatic control of horizontal stabilizer position. When the autopilot is engaged, the flight control computer outputs stabilizer position commands. There are no line replaceable units, only automatic stabilizer trim components. All components are part of the Horizontal Stabilizer Trim Control System (Ref Chapter 27).
 B.  This section covers autotrim operation. A general description of horizontal stabilizer trim system components is added to help understand operation. For component details and complete system description, refer to 27-41-00/001, Horizontal Stabilizer Trim Control System.
 C.  The stabilizer trim system maintains the airplane in a longitudinally trimmed condition by varying the incidence of the horizontal stabilizer and provides automatic Mach and speed stability. The optimum pitch attitude is maintained during various flight conditions. The stabilizer may be trimmed manually or automatically. The stabilizer trim system has the following operational modes.
 (1)  
Manual and manual electric modes - the stabilizer is trimmed by pilot commands.

 (2)  
Autotrim mode - The autopilot Flight Control Computers (FCCs) trim the stabilizer to relieve sustained elevator loads.

 (3)  
Mach trim mode - The stabilizer is trimmed as Mach increases.

 (4)  
Speed trim mode - The stabilizer is trimmed at lower airspeeds when the flaps are down.

 D.  Stabilizer Trim Mach Speed Stability Component Locations (Fig. 2)
 (1)  Flight Deck Components
 (a)  The following stabilizer trim system components are in the control cabin. Stabilizer trim (STAB TRIM) manual trim levers, position indicators, and hydraulic cutout switches are adjacent to the throttle quadrant on the control stand (P10). The annunciator panel on overhead panel P5 contains the amber STAB TRIM, UNSCHED STAB TRIM and MACH/SPD TRIM lights. The stabilizer trim control switches are on the outboard horn of the captain's and first officer's control wheels. The control column stabilizer trim cutout switches are under the cabin floor.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

SEE

A

LEFT AND RIGHT SAM

STABILIZER TRIM SYSTEM

AUTOMATIC STABILIZER TRIM SYSTEM ONLY MACH SPEED STABILITY SYSTEM ONLY
Automatic Stabilizer Trim System
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL

OVERHEAD PANEL (P5)
. ANNUNCIATOR PANEL

MAIN EQUIPMENT CENTER. LEFT STABILIZER TRIM AND ELEVATOR ASYMMETRY LIMITMODULE (SAM) (E3-1). RIGHT STABILIZER TRIM AND ELEVATOR ASYMMETRY LIMIT
MODULE (E4-1). LEFT TRIM LIMIT SELECT RELAY (REAR OF E3-1). RIGHT TRIM LIMIT SELECT RELAY (REAR OF E4-1)
STABILIZER TRIM SEE A TRIM CONTROLSTABILIZER
CONTROL SWITCHESSWITCHES
CONTROL COLUMNCUTOUT SWITCHES(UNDER FLOOR)
QUADRANT STAND (P10)
. STAB TRIM MANUAL TRIM LEVERS

STABILIZER
(OR ALTERNATE STAB TRIM SWITCHES)
. STAB TRIM POSITION INDICATORS
. STAB TRIM C AND R HYDRAULIC CUTOUT SWITCHES

STABILIZER TRIM ACTUATORSTABILIZER TRIM CONTROL. HYDRAULIC BRAKESMODULES (STCMs) (LEFT AND RIGHT)(LEFT AND RIGHT)
STABILIZER TRIM ACTUATOR
. HYDRAULIC MOTORS
(LEFT AND RIGHT)

STABILIZER TRIM LIMIT SWITCH
AND POSITION TRANSMITTER MODULES (STLMs)
(LEFT, CENTER, AND RIGHT)

A
Stabilizer Trim Mach Speed Stability Component Locations
Figure 2

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