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 3.  Alternate Electric Trim Switches
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 A.  The alternate electric trim switches function is similar to the manual trim switches and are provided as an emergency override in case the manual trim system is malfunctioning. The alternate electric trim switches are powered by the 28 volts dc battery bus and the signals are sent directly to the STCMs to a separate set of coils on the trim control solenoids. Alternate electric trim will function even if both SRMs are disabled or removed.
 4.  Stab Trim Cutout Switches
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 A.  The two stab trim cutout switches on the pilots' control stand are three-position guarded switches; switch positions are ON, AUTO, and CUTOUT. One switch is for the No. 2 hydraulic system (right channel) and the other is for the No. 3 hydraulic system (left channel). When the switch is in the ON position, the motor operated value (MOV) on the STCM for the specific channel is open and allows hydraulic fluid to flow into the STCM. In the CUTOUT position, the MOV is closed, the STCM is inoperable, and the specific channel is disabled. In the AUTO position, the SRM will automatically open the MOV and then close the MOV automatically when unscheduled trim is sensed.
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MAINTENANCE MANUAL

Automatic/Manual Stabilizer Trim Control
Figure 4

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MAINTENANCE MANUAL

 5.  Flight Control Computer (FCC)
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 A.  The three FCCs provide stabilizer trim commands to decrease the load on the elevator. Each FCC sends both digital and analog data to the SRM and receives analog data from the SRM and FCU. The left and right FCCs provide dedicated outputs to the L and R SRMs; the center FCC provides control to either the L or R SRM. The C FCC controls the L SRM unless the L SRM fails, then the C FCC will switch to control the R SRM. The FCC provides the following output digital discretes on an ARINC 429 bus: FULL-RATE AUTOTRIM (FRAT), TRIM UP ARM (TUA), TRIM UP CONTROL (TUC), TRIM DOWN ARM (TDA), TRIM DOWN CONTROL (TDC), ENGAGE SRM L, and ENGAGE SRM R. FRAT is a mode of autotrim that utilizes both trim channels during a dual channel autoland approach. TUA, TUC, TDA, and TDC are trim commands from the FCCs to the ARM and CONTROL microprocessors in the SRM. ENGAGE SRM L and ENGAGE SRM R are used by the C FCC to change SRM control. The FCC will output the analog discrete AUTOTRIM ARM when the FCC is engaged. The FCC receives the analog discrete AUTOTRIM VALID from the SRM and the analog input STAB POSITION from the FCU.
 6.  Stabilizer Trim and Rudder Ratio Module (SRM)
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 A.  The two SRMs provide the following functions pertinent to AST: stab trim mode selection, stab trim limit change logic, stab trim rate control, motor operated valve (MOV) shutdown logic, and failure monitoring. The SRM also provides rudder ratio changer computations (Ref 27-21-00/001) and outboard aileron lockout logic (Ref 27-11-00/001).
 B.  Stab Trim Mode Selection
 (1)  When the left, center or right autopilot is engaged, the appropriate SRM enters autotrim mode and accepts trim commands from the FCC. When all autopilots are disengaged and the airspeed is below 220 knots, the SRM enters the Speed Trim mode and the Mach Trim mode is entered above 0.86. The SRM uses Computed Airspeed from the DADC and stabilizer position from the FCU to adjust the stabilizer to increase static speed stability for speed trim and cruise longit udinal static stability for Mach trim . If the manual trim switches are used, the SRM will enter manual trim mode; manual trim has priority over Mach/Speed Trim and single channel autotrim. With multi-channel engaged in autoland, autotrim mode has priority over all modes except alternate electric trim.
 C.  Stab Trim Limit Change Logic
 (1)  ELA ELA, ELB; the TUA and TDA signals are sent through the stab trim limit switches to limit stabilizer travel authority; under certain conditions, changes to these limits are desirable. The SRM will change the trim command path to a different limit switch in two cases, one for the nose up limit and one for the nose down limit. FRAT mode will change the nose up limit and landing gear position, Flap position or Speed Trim mode will change the nose down limit.
　
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