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TRIM COMMANDSTO SRM(S)ì TUA, TDA, TUC, TDCì ENGAGE SRM L, Rì FRAT
STABILIZER
POSITION
FROM FCU(S)
THR = 0.25`
THS = 0.4` OR 0.6`
AFTER G/S CAPTURE.
AUTOTRIM ARM
TO SRM(S)
AUTOTRIM VALID
FROM SRM(S)
FLIGHT CONTROL COMPUTER SOFTWARE HARDWARE
Automatic Stabilizer Trim Operation
Figure 9 (Sheet 1)
A
747-400
MAINTENANCE MANUAL
TRIM COMMANDS FROM FCC(S)
AUTOTRIM ARM FROM FCC(S)
AUTOTRIM VALID TOFCC(S)
MANUAL ELECTRIC TRIM COMMANDS TRIM COMMANDS TO STCM
STABILIZER TRIM/RUDDER RATIO MODULE
Automatic Stabilizer Trim Operation
Figure 9 (Sheet 2)
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
22-22-00
ALL ú
ú 04 Page 15
ú Oct 10/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
B. Multi-Channel Engaged
(1)
The major differences with multi-channel engaged are the stabilizer trim bias, the FRAT capability, the failure annunciations, and the AUTOTRIM VALID logic function.
(2)
The purpose of the stabilizer trim bias is to intentionally introduce a nose up trim command on a dual autoland approach; this causes the FCCs to command elevator in the opposite direction to compensate for the stabilizer trim. If the elevator fails to compensate, the FCCs will command the elevator neutral and the stabilizer flares the airplane to land.
(3)
With the multi-channel and radio altitude less than 500 feet, the engaged FCCs interrogate the SRMs for FRAT capability; this causes the AUTOTRIM VALID to go low, which signals the FCC that there is FRAT capability. During multi-channel engaged, AUTOTRIM VALID transition to low will not cause the autopilot to disengage. The self-test, which lasts 4 seconds is conducted and trim commands are inhibited during the test.
(4)
The stabilizer trim bias and FRAT will only be used during dual-channel engaged autoland - LAND 2 annunciated. The trim bias will occur at 120 feet. During FRAT, both channels are driven separately by the two engaged FCCs and the SRMs do not delay the command to the STCM. However, if only one of the trim channels are found to be available during testing for FRAT capability such as when AUTOTRIM VALID goes low prior to initation of test or when no AUTOTRIM VALID transition to low 4 seconds after FRAT test is initiated occurs, then the stabilizer trim bias will be initiated at 190 feet.
(5)
Stabilizer trim commands are inhibited under the following conditions: Rollout or G/A is engaged. Multi-channel autotrim commands have priority over all other trim commands except alternate electric trim.
C. Faults and Monitoring
(1)
The AST system has many monitors and tests that detect failures and abnormal operation of the components. A failure detected by the monitors can result in internal interface switching, CMC fault storage, EICAS message display, and/or autopilot disengagement, depending on the significance of the failure.
(2)
Monitors in SRM
(a) Automatic Shutdown of the Stabilizer Trim MOV
1) The Stabilizer Trim MOV can be shutdown (closed) by positioning the appropriate stab trim cutout switch in the CUTOUT position. Also, if the switch is in AUTO position, the SRM will automatically close the MOV when an unscheduled trim is detected. The automatic shutdown system consists of the two SRMs, the L and R STCM MOVs, the L and R Stabilizer Position Transmitter Modules (SPTMs), and the L and R FCUs. Stabilizer position is sensed by an RVDT in the SPTM and is converted to a dc signal in the FCU before being transmitted to the SRM.
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