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 C.  The rate control valve solenoid produces high flow rate when energized and low flow rate when deenergized; therefore, each STCM provides both directional and rate control to its hydraulic motor. The gearbox in the STDM is a differential type that sums the inputs from the L and R hydraulic motors. If both hydraulic motors are operating, full rate trim is obtained; if only one motor is operating, half rate trim is the result. Figure 6 shows the different trim rates possible using one or two motors, combined with the different trim rates (rate control solenoid valve).
 8.  Power (Figure 8)
_____
 A.  The left SRM receives +5 volts dc, +15 volts dc, and -15 volts dc from dual FCE power supply modules (PSM 1L and PSM 2L). The right SRM receives +5 volts dc, +15 volts dc, and -15 volts dc from a single power supply module (PSM 2R). The PSMs provide the stable DC supply voltages (rail voltages) to the arm and control microprocessors in the SRMs. Both left PSMs receive 115 volts ac, 400 Hz from the ac standby bus (sub-bus of ac bus No. 3) and 28 volts dc from battery bus No. 1 . The right PSM No. 2 receives 115 volts ac from ac bus No. 2 and 28 volts dc from dc bus No. 2.
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SRM Power
Figure 8

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 B.  By pulling the appropriate circuit breakers, various trim functions provided by one SRM can be disabled, while allowing the other SRM functions to continue normal operation. Opening the STAB TRIM L RATE or STAB TRIM R RATE circuit breaker disables the rate control function (defaults to LSTR). Opening the STAB TRIM L S/O VALVE or STAB TRIM R S/O VALVE circuit breaker disables the MOV. Opening the STAB TRIM R CONT circuit breaker disables right channel autotrim and Mach/Speed Trim. Opening the STAB TRIM L CONT circuit breaker disables left channel autotrim, Mach/Speed Trim, and manual electric trim.
 9.  Operation (Figure 9)
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 A.  Single Channel Engaged
 (1)  
During single channel engaged operation, the engaged FCC will generate stabilizer trim commands to reduce the elevator load. The ELEV SERVO CMD is the signal that commands the elevator to move. This signal is sent through the nose up and nose down threshold detectors. If the ELEV SERVO CMD exceeds, for example, the nose down threshold (THS), the nose down autotrim commands (TDA, TDC) are set high. These autotrim commands are sent to the SRM, which then delays the response for 3.5 seconds and sends the trim command to the STCM. As the stabilizer moves in the commanded direction, the elevator load is reduced. The ELEV SERVO CMD then drops below the threshold (THR) and the discretes (TDA, TDC) are reset low again. The SRM then immediately stops the stabilizer time command to the STCM. The sequence of events are identical for nose up trim except that the nose up threshold detector limits are negative.

 (2)  
The AUTOTRIM ARM analog discrete and ENGAGE SRM L, R digital descretes will remain low until the FCC is engaged This forces AUTOTRIM MODE Logic low in the SRM. After a FCC is engaged, the SRM will enter AUTOTRIM MODE if the FCC input monitor sets ENABLE high. Also, after AUTOTRIM ARM is received by the SRM, the AUTOTRIM VALID output becomes sensitive to manual trim and FRAT command requests. In other words manual trim input or FRAT input will cause the AUTOTRIM VALID to go low. Stabilizer trim faults will also cause AUTOTRIM VALID to go low and with single channel engaged, loss of AUTOTRIM VALID will cause the autopilot to disengage.

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