(b)
The FCCs generate trim commands in response to an elevator deflection exceeding a preset authority threshold. Trim commands applied to the SAM cause the stabilizer to assume the elevator trim load. Stabilizer position signals are fed back to the FCCs from the associated Stabilizer Position Modules (SPM). Trim commands stop when the elevator is within the elevator authority threshold limits.
(c)
The FCC generates a digital trim engage discrete and a separate analog autotrim discrete. These cause the SAM to select the FCC as the active trim source. With any one FCC controlling, a single STCM hydraulic output drives the stabilizer motor at half-rate speed (0.1 degrees/sec to 0.25 degrees/sec). The elevator feel computer determines the pressure applied at the STCM, which determines stabilizer rate.
(2)
Automatic Stabilizer Trim FCC Logic (Fig. 6)
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MAINTENANCE MANUAL
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STABILIZERPOSITIONFROM SPM
FCC CROSS
CHANNEL
BUSSES
ELEVATOR FEEL POSITIONFROM ELEVATOR NEUTRALSHIFT TRANSDUCER
LABEL 273TO SAM(S)
AUTOTRIM
VALID
FROM SAM(S)
AUTOTRIM
ARM
TO SAM(S)
Automatic Stabilizer Trim FCC Logic
Figure 6
C99429
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757
MAINTENANCE MANUAL
(a) Autotrim FCC Operation
1) The FCC provides trim commands when the autopilot is engaged and manual trim mode is not selected. The engaged FCC generates a digital trim-engage discrete and a separate analog autotrim arm discrete. The ENG L SAM or ENG R SAM discretes are separate bits in ARINC 429 word 273. The AUTOTRIM ARM signal is the analog discrete. Both signals are provided to each channel of the SAM. When the SAM accepts the autotrim engage request, an AUTOTRIM VALID analog discrete signal is transmitted to the FCC.
2) The three FCCS communicate with the SAM via ARINC 429 buses. Protocol logic within each FCC determines which computer trims the stabilizer. The FCCS monitor each other. In the autotrim mode, the command response monitor in FCC performs loop monitoring to detect autotrim failures. Autotrim failure occurs when stabilizer does not respond to computed commands, stabilizer movement is inconsistent with computed commands or when analog AUTOTRIM VALID from SAM becomes invalid. When a single autopilot is engaged, a detected autotrim failure which cannot be cleared by internal autosequencing (center FCC only) disables the active trim control path by disengaging the appropriate SAM. Since the center FCC interfaces with both SAMs, sequencing to another trim channel following a trim failure can occur only if the center FCC is engaged. When the center autopilot is engaged, the left SAM is always selected. A subsequent trim failure requires the FCC to inititate sequencing to the right SAM. When more than one autopilot is engageed, sequencing to another trim channel is attempted when the current SAM-driving autopilot disconnects, or the active trim control path fails as detected by the local command-response monitor. The FCC autosequencing cycle is left - Right - Center - Left.
3) Elevator feel position and neutral shift compensation for ground effects determine trim commands. Trim up or trim down ARM and CONTROL signals are digitally outputed to the SAM. Trim motion is then monitored by command - response monitor in each FCC. If the TRIM VALID signal is not valid, AUTOTRIM ARM, and ENG R SAM or ENG L SAM signals are not transmitted.
(3) Automatic Stabilizer Trim Schematic (Fig. 7)
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757 中國航空網 www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：757 AMM 飛機維護手冊 自動飛行 AUTOFLIGHT 2(82)
