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2) An unscheduled stabilizer trim is detected by the SRM when a position change in excess of 1.0 degree is detected without a corresponding valid trim command from either of the two SRMs. The faulty STCM is determined from the hydraulic brake release signal; the motor with the brake released is shutdown. Both SRMs are in control of the automatic shutdown simultaneously. The shutdown command is a pulse of 10-second duration. The shutdown results in the display of the amber "STAB TRIM UNSCHD" EICAS message until the MOV has closed. When the MOV has closed and the affected system is shutdown, the "UNSCHEDULED STAB TRIM" message is inhibited, and the the STAB TRIM 2 or STAB TRIM 3 message appears on the EICAS display. If the MOV failed to close, the "STAB TRIM UNSCHD" message would remain for 30 seconds. In each case an appropriate fault message is stored in CMC memory.
(b) FCC Input Monitoring
1) Continuous monitoring of FCC bus information is provided by the SRMs. The left SRM monitors L FCC and C FCC inputs and the right SRM monitors R FCC and C FCC inputs. The ARINC 429 bus inputs are monitored for parity and failure status: four incorrect or failure words out of eight successive samples causes a fault to be set for the corresponding FCC. The monitor also checks the TUA, TDA, TUC, and TDC command signals for proper coincidence, i.e., (TUAíNOT(TUC)) + (NOT(TUA)íTUC) + (TDAíNOT(TDC)) + (NOT(TDA)íTDC) + (TUAíTDAíTUCíTDC) = 0, if this equals logic 1 for
1.5 seconds, the FCC fault is set.
2) If there is a trim command without the proper ENGAGE SRM L or ENGAGE SRM R command or AUTOTRIM ARM signal, the FCC fault will be set. If the ENGAGE SRM L and ENGAGE SRM R bits are both true in the same FCC word for two seconds, the corresponding FCC fault will be set. All faults will result in disabling autotrim mode by setting ENABLE low and generating a CMC fault message.
(c) Stab Trim Fault Monitors
1) These monitors detect errors in processing, mode coincidence, output coincidence, power and the like, that may seriously affect proper trim command outputs to the STCM. The monitors detect: differences in modes between the ARM and CONTROL microprocessors; differences between hardware wraparound output commands and software commands, and differences in trim command between the ARM and CONTROL microprocessors; any of these differences detected will result in the message "Onside Stab Trim/Rudder Ratio Module Fault" sent to the EIUs. Also, software subroutine errors, other CPU monitoring, and power up test failures will result in the message "Offside Stab Trim/Rudder Ratio Module Fault" sent to the EIUs.
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(3) Monitors in the FCC
(a) FCC Trim Fault Monitoring
1) The FCC detects three types of trim failures. Dead trim is defined as a trim command without stabilizer motion, runaway trim is defined as stabilizer motion without a trim command, and out-of-trim runaway is defined as stabilizer motion in the opposite direction as commanded. These trim failures will result in the following actions and EICAS annunciations. For L or R FCC engaged in single channel operation, dead trim causes PITCH MODE FAIL and autopilot CAUTION light annunciations. Runaway trim causes UNSCHED TRIM annunciation. Out-of-trim runaway causes PITCH MODE FAIL and autopilot CAUTION light annunciations. For C FCC engaged in single channel operation, dead trim results in internal autosequencing if C FCC is driving R SRM: the annunciation is identical to L or R FCC single channel operation. In multi-channel engaged operation, dead trim will result in internal autosequences and NO LAND 3 is annunciated after touchdown. The annunciation is the same as for single channel operation for runaway trim and out-of-trim runaway except no annunciation will occur below alert height and NO AUTOLAND is displayed after touchdown.
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