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757
MAINTENANCE MANUAL
Selects the active trim source; either the Flight Control Computers (FCCs), Mach/Speed function of the SAM, or the column-mounted trim switches.Outputs both ARM and CONTROL electrical commands to the Stabilizer Trim Control Modules (STCMs).Provides redundancy management, failure monitoring, and annunciation.Each module includes two parallel, non-identical microprocessors (ARM and CONTROL) that develop simultaneous trim commands. These commands are based on input data from the three trim sources.The SAM provides logic circuits for system engage/disengage, stabilizer trim mode, and trim-source command selection. Other internal circuits are digital data ARINC 429 receivers, analog discrete input and output buffers, and cross-channel and self monitoring. Built-In-Test (BIT) fault indications are output to the Engine indicator and Crew Alerting System (EICAS) display unit and annunciator panel.The front panel of the SAM contains 12 fault balls that indicate specific failures within the stabilizer trim system. All except the SAM fault ball are set by faults identified within the CONTROL microprocessor. The SAM fault ball is set by either ARM or CONTROL microprocessor. Fault balls cannot be set if the SAM fault ball is set. The fault balls are black when not set, and yellow when set. Fault balls do not latch for input faults when airplane is on ground. The following fault ball indications are provided: a) FCC - set by a left, center, or right flight control
computer input fault. b) ADC - set by a left or right Air Data Computer (ADC) fault discrete, or ADC input comparator fault.
c) STCM - set by a fault detected by hydraulic pressure switch monitor, loop function monitor or unscheduled trim monitor. (Fault ball can only be set if either of the hydraulic pressure discrete from the associated hydraulic system pressure switch indicates high hydraulic pressure.)
d) COL trim SW - set by a manual-trim coincidence monitor detected failure (manual electric trim switches fault).
e) MAN LEVER SW - set by a standby manual-trim lever switch fault during power-up test (the trim lever operated at power-up or the switch is closed).
f) FSPM/SPM - set by a failed output from the stabilizer position monitor for more than 2 seconds. g) AIR-GND RELAY - set by first or second air/ground fault.
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757 MAINTENANCE MANUAL
h) L. AIL LOCK ACT/ELEV ASY L. ACT - set by an elevator assymmetry limit actuator fault. i) R. AIL LOCK ACT/FSEU - set by a first or second flaps retracted fault. j) HYD PRESS SW - set by a system hydraulic pressure switch fault.
8) The SAM monitor sets the SAM faultball when the following faults are detected: a) Channel code discrete and airplane type code discrete
faults (If discrepancy persists for more than 2 seconds). b) Mode coincidence, background, and ARM/CONTROL watchdog faults c) Elevator asymmetry limit actuator and power-up test faults
9) The SAM is able to reset itself if it is not in control and a SAM fault occurs. The SAM can also reset itself if a SAM fault has occured and the other SAM picks up control. If a SAM fault occurs again within one minute of an autoreset, the SAM will not be able to perform another autoreset. The SAM fault ball can reset itself if an autoreset is successful and no other SAM faults occur within one minute.
(b) When pressed, the RESET switch on the front panel of the SAM clears (resets) all fault balls, unless the fault is still present.
G. Stabilizer Trim System Block Diagram (Fig. 5)
(1) Stab Trim Manual Control
(a)
The stabilizer ball-screw actuator consists of two independent hydraulic motors and brakes coupled to a mechanical drive device. The assembly moves the stabilizer at half-rate trim when powered by either the left or right Stabilizer Trim Control Module (STCM). Pressure supplied simultaneously from both STCMs produces full-rate trim. The brakes prevent air loads on the stabilizer from either back driving or overruning the actuator.
(b)
Each STCM has an ARM and a CONTROL hydraulic valve. The manual stabilizer trim controls (P10) are connected to the STCMs. One control activates the ARM valve in both STCMs and the other control activates the CONTROL valves. Actuation of the manual trim controls trim the stabilizer at full rate and overrides all electric trim commands.
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