FLAPS NOT UPS245
Horizontal Stabilizer Trim Control System Circuit
Figure 2

A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
 B.  Normal electrical trim control system consists of control switches, cutout switches, a column-actuated cutout switch, trim control relays, flap switch and relay, limit switches and an electric actuator. The PSTA contains an electric motor, two electromagnetic clutches, a speed change relay, torque limiting clutch and output shaft to drive the jackscrew gearbox. The motor is operated by 3 phase AC power and the relay and clutches by DC power. The PSTA is controlled by the trim control switches, control cutout switch, limit switches and cutout switches. The control switches, located on the outboard horn of each control wheel, have two momentary positions, nose up and nose down, and are spring-returned to the center off position. The column-cutout switch located beneath the cockpit floor is actuated by forward and aft movement of either control column. Stabilizer limit switches are located on the bulkhead aft of the jackscrew gearbox and actuated by a striker to limit stabilizer leading edge up and down travel. The cutout switches on the control stand are used to remove power from the PSTA.
 C.  Normal electric trimming of the stabilizer is done at one of two rates as controlled by flap position. Trim rate with flaps retracted is 1/3 the trim rate with flaps extended. High speed autopilot rate is equal to the normal electric low speed rate. The low speed autopilot rate is 1/2 the rate of the high speed autopilot rate.
 2.  Horizontal Stabilizer (Fig. 3)_____________________
 A.  The horizontal stabilizer assembly consists of a left and a right section attached to a center section. The center section jackscrew support, hinge supports, front spar, and rear spar members are all truss-type forgings. Good accessibility for maintenance is provided by the use of a star-shaped pattern of beams for the center section.
 3.  Stabilizer Forward Control Mechanism (Fig. 1)____________________________________
 A.  The stabilizer forward control mechanism provides the means of manual control for the stabilizer in the event of electrical malfunction. The forward control mechanism extends from a stabilizer trim wheel on each side of the control stand to a forward trim mechanism in the lower nose compartment. The stabilizer trim wheels and a sprocket are splined to a control wheel shaft that extends through the control stand. Rotation of the stabilizer trim wheels transmits motion to a forward trim mechanism spocket by a chain assembly. The forward trim mechanism spocket drives the forward cable drum which is cable-connected to the stabilizer jackscrew gearbox cable drum. The stabilizer jackscrew gearbox cable drum drives the jackscrew to position the stabilizer. The forward trim mechanism is attached to the lower nose compartment by support linkages which can be adjusted to align the trim mechanism and obtain proper chain and cable tension.
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737-300/400/500
MAINTENANCE MANUAL

STABILIZER (LH SHOWN) 
JACKSCREW SUPPORT 
FWD  SUPPORT HINGE  STABILIZER ATTACH FITTINGS 

Horizontal Stabilizer
Figure 3

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 4.  Stabilizer Position Indicator (Fig. 1)
_____________________________
 A.  A stabilizer position indicator provides continuous indication of stabilizer trim. The stabilizer position indicator operates through a flexible shaft connected to the forward trim mechanism, a jackshaft, and a linkage to the indicator. The flexible shaft transmits motion to the indicator linkage through the jackshaft and positions the indicator. During normal electrical operation, the stabilizer trim cables drive the forward trim mechanism which operates the trim position indicator and rotates the trim wheels. A scale on the control stand is calibrated in units of trim to indicate the proper takeoff stabilizer range and is referred to as the GREEN BAND.
　
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