(b)
The PSMs supply power to the following components:
1) Stabilizer Trim/Elevator Asymmetry Limit Module
2) Spoiler Control Module
3) Yaw Damper Module
4) Rudder Ratio Changer Module
5) Excitation for the above LRU LVDTs and RVDTs
(c)
Each PSM provides the following outputs: 1) |15v dc power distribution buses from six separate dc power regulators. 2) |5v dc power distribution bus from a single +5v dc power
regulator.
3) 26v ac power distribution.
(d)
DC power output lines from the PSM on the same shelf are connected inside each CSEU so that a loss of either PSM will not cause the loss of any CSEU.
(e)
26v ac output lines are connected to the change-over circuitry inside the two PSMs on the same shelf. If one PSM fails, the change over circuitry will switch over to the other PSM and continue to supply 26v ac to the CSEU.
(f)
Four ac power output lines from the PSM supply 26v AC to the demodulator circuits in the SCMs, the YDMs, and the RRCs. These lines also provide 26v AC excitation to the control wheel transducers (3), the speedbrake lever transducers (3), the spoiler power control actuators (6), the yaw damper servo (1), and the rudder ratio changer (1).
(g)
Front panel mounted circuit breakers in each of the two sides of the 26v ac line protect the PSM from shorted components and help to troubleshoot these faults. But an open circuit breaker does not set the PSM MODULE faultball to yellow.
(h)
Under normal conditions, the "A" subchannel of the SCM controls the spoiler panels. The 26v ac excitation voltage for the spoiler PCA RVDTs comes from the PSM circuit which supplies the SCMs "A" subchannel. If the SCM makes a "switch" decision, control of the spoiler panels and PCA excitation are switched to the "B" subchannel.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(4)
Stabilizer Trim/Elevator Asymmetry Limit Module (SAM)
(a)
The stabilizer trim system controls the airplane pitch axis during flight by up or down movement of the horizontal stabilizer leading edge. Movement up of the horizontal stabilizer will move the airplane nose down. Movement down of the horizontal stabilizer will move the airplane nose up.
(b)
The stabilizer trim system has these trim modes: 1) Manual Electric Control Trim Mode - pilot command's to the control wheel trim the stabilizer manually. 2) Alternate Manual Trim Mode - pilot command's to the center control stand trim the stabilizer manually. 3) Auto Trim Mode - the autopilot flight control computers (FCC) trim the stabilizer automatically.
4) Mach/Speed Trim Mode - the stabilizer is trimmed as the mach increases or at lower airspeeds when the flaps are down. The Mach/Speed Trim System is active when manual, electric, and automatic trim commands are not present.
(c)
Refer to 22-22-00 for more information on autotrim mode.
(d)
Refer to 22-24-00 for more information on mach/speed trim mode.
(e)
Manual electric control trim mode:
1) In manual electric control trim mode, the stabilizer control is from the dual electric trim switches on the control wheels. When you push these switches up or down, you send commands to the two SAMs which then transmit the signals to the control and arm solenoid valves in the left and right STCM. When these valves operate, the hydraulic pressure is applied to release the secondary brake to the applicable hydraulic trim motor and drive the stabilizer jackscrew.
2) The electric trim switches will send the necessary signals to the two SAMs to drive the stabilizer at the full rate. Stabilizer movement will continue until you release the switches, or until the electrical limit switches operate.
3) Manual electric trim has priority over the other electric trim modes, except for the multichannel autoland mode (when the autoland mode is selected for two or three FCCs).
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