J.
Stabilizer Trim Shutoff Switches
(1)
Two guarded stabilizer trim shutoff switches allow system hydraulic pressure to be removed from the STCM's. The switches control motor-operated shutoff valves in the STCM's. The right system shutoff switch controls the right STCM. The center system shutoff switch controls the left STCM. The switches are mounted on the control stand.
K.
Stabilizer/Elevator Asymmetry Limiter Module (SAM)
(1)
The SAM is part of the integrated package of flight control electronics, the control system electronics unit (CSEU). Each of the two (left and right) CSEU's contains a SAM. The SAM's provide electric stabilizer trim control and associated failure monitoring. Each of the two SAM's provides:
(a)
Stabilizer trim mode priority logic for manual electric trim, autopilot trim, and mach/speed trim
(b)
Stabilizer trim engage/disengage and auto transfer logic
(c)
Unscheduled trim fault detection
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A
757
MAINTENANCE MANUAL
FIRST OFFICER'S
CONTROL COLUMN
CAPTAIN'S
CONTROL
COLUMN SEE A
TORQUE
TUBE
SEE A
COLUMN PIVOT STOP BOLT (TYP)
SWITCH ACTUATING
STABILIZER TRIM
BOLT (TYP)CUTOFF SWITCH (TYP)
FWD A
SPRING
CAM ROLLER
CAM
SWITCH (TYP)
STOP (TYP)
STOP BOLT (TYP) SWITCH ACTUATING
FWD BOLT (TYP)
OPERATIONAL SCHEMATIC
Control Column Stabilizer Trim Cutoff Switches Figure 5
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A
757 MAINTENANCE MANUAL
(d)
Manual-electric trim monitoring
(e)
Fault messages for the caution and warning system as well as for maintenance
(f)
Mach/speed trim function
(g)
Rudder ratio changer command computation
(h)
Elevator asymmetry limiter control
(2)
Fault balls on the SAM's display line replaceable unit (LRU) status. A manual RESET button on the SAM clears intermittent failures.
3. Operation_________
A. Functional Description (Fig. 3, 6 and 7)
(1)
AIRPLANES WITH STAB TRIM LEVERS ON THE CONTROL STAND; stabilizer operation can be commanded in any of three electrical modes or in the manual-mechanical mode. The electrical modes are manual-electric, automatic stabilizer trim and mach/speed. The automatic stabilizer trim mode is described in MM 22-22-00. The mach/speed mode is described in MM 22-24-00. The stabilizer is hydraulically driven in all modes. Right and center hydraulic systems supply power to the right and left STCMs respectively.
(2)
AIRPLANES WITH ALTERNATE STAB TRIM SWITCHES ON THE CONTROL STAND; there are four modes that can be used to operate the stabilizer. They are 1) manual-electric, 2) automatic stabilizer trim, 3) mach/speed, and 4) alternate-electric operation. The automatic stabilizer trim mode is described in MM 22-22-00. The mach/speed mode is described in MM 22-24-00. The stabilizer is hydraulically driven in all modes. The right and center hydraulic systems supply power to the right and left STCMs respectively.
(3)
The stabilizer trim system requires application of hydraulic power from the right and center systems to achieve normal trim rate. Normal trim rate varies from 0.5 degree/second to 0.2 degree/second. Trim rate with only one system operating is from 0.25 degree/second to 0.1 degree/second. A rate control valve in the STCM controls the flow rate to and from the arming and control valves as a function of airspeed and horizontal stabilizer positon. Between high and low trim rates, the rate varies linearly as a function of elevator feel pressure.
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