757 MAINTENANCE MANUAL
(e) 11J21 or 11J22, ELEV POS RIGHT
(2) S 864-031Put the L, R, and C FLT CONTROL SHUTOFF switches on the right side panel, P61, to the ON position.
(3) S 444-032Do the activation procedure for the spoiler/speedbrake, if it is necessary (Ref 27-61-00).
(4) S 864-033Remove electrical power if it is not necessary (Ref 24-22-00).
(5) S 414-034Close the access door for the forward stabilizer compartment, 311AL (Ref 06-42-00).
(6) S 414-035Install these access panels for the position transmitter and PCA bellcranks (Ref 06-42-00): (a) 335BB and 335DB for the left elevator (b) 345BB and 345DB for the right elevator.
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757 MAINTENANCE MANUAL
HORIZONTAL STABILIZER TRIM CONTROL SYSTEM - DESCRIPTION AND OPERATION
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1. General (Fig. 1)_______
A. The overall function of the stabilizer trim system is to maintain the airplane in a trimmed condition and to provide automatic mach and speed stability. The system consists of: (1) several alternative sources of trim command signals; (2) two independent actuation devices which position the stabilizer in response to source commands; and (3) an electronic module which selects a particular trim signal source and also energizes one or the other actuation device.
B. AIRPLANES WITH STAB TRIM LEVERS ON THE CONTROL STAND; the three sources of electric trim command signals are: (1) the flight control computer (FCC); (2) the mach/speed trim function within the stabilizer/elevator asymmetry module (SAM); and (3) the manual-electric trim switches on the control wheels.
C. AIRPLANES WITH ALTERNATE STAB TRIM SWITCHES ON THE CONTROL STAND; there are four sources of electric trim command signals. They are: 1) the flight control computer (FCC), 2) the mach/speed trim function in the stabilizer asymmetry module (SAM), 3) the manual-electric trim switches on the control wheels, and 4) the alternate-electric trim switches on the control stand. There is no mechanical control mode for the STCMs.
D. The description and operation for the stabilizer asymmetry module (SAM) is contained in this section. The removal and installation procedure for the module is contained in MM 27-09-00/201. The selection of the active trim source is done by logic circuits in the SAMs.
E. The SAMs accept input signals from the FCC, the mach/speed trim function in the SAM, and the control wheel switches. The output signals from the SAM are sent to two STCMs.
F. There are two independent channels of trim command and actuation. Each channel consists of a SAM, an STCM, and a hydraulic motor and brake on the stabilizer ballscrew actuator. The two channels are known as trim channel left and trim channel right. When an FCC or mach/speed function is the active trim source, only one trim channel is activated, providing half-rate trim. When the electric trim switches are energized, both trim channels are activated simultaneously, and the stabilizer is driven at full-rate trim. The trim rate varies with airplane speed and horizontal stabilizer position as determined by hydraulic pressure output from the elevator feel computer (Fig. 7). The variable hydraulic pressure output from the elevator feel computer is fed to each STCM. In each STCM a rate control valve regulates actual hydraulic pressure available to drive the horizontal stabilizer.
G. AIRPLANES WITH STAB TRIM LEVERS ON THE CONTROL STAND; in addition to the three electric trim signal sources, stabilizer position can be controlled by manual-mechanical trim levers. The levers operate hydraulic valves inside each STCM by mechanical cables. They can override all three electric trim signal sources.
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757
MAINTENANCE MANUAL
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