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747-400
MAINTENANCE MANUAL

 4)  Once ALT HOLD mode is engaged, it can be disabled if CMD is disabled or the F/D is turned off. ALT HOLD will also be disabled by selection of the following modes: V/S, FLCH, VNAV, APP, or GA.
 B.  System Monitoring and Fault Detection
 (1)  
There are 21 different types of monitors, with many different monitors within each type, and each different monitor may check a number of airplane sensors and interfacing components. In order to limit the scope of this coverage, it will focus on the monitors that check systems external to the FCCs. A fault detected by one of these monitors will result in a CMC In Flight Fault.

 (2)  
FCC/Airplane Sensor Monitoring

 (a)  
LRU Activity Monitors: The ADC, FMC, IRU, ILS, and RA are monitored for activity. These detect if data is not being updated to the FCC.

 (b)  
MCP Path Invalid Monitor: The FCC outputs two test word patterns to the MCP, if the MCP fails to complement the words and return it to the FCC, a fault is recorded to the CMC.

 (c)  
Sensor Sign Status Matrix (SSM) Monitors: The ADC, FMC,IRU, ILS, and RA SSMs are checked for acceptable codes. The SSM is three bits of the data word that is sent from each sensor. Coded into the three bits is the status of the sensor or the data in the word for example the SSM can indicate that Non Computed Data (NCD) is being sent or that the sensor is in the test mode. Only fail/warn SSMs are reported to the CMC.

 (3)  
System Monitoring

 (a)  Detent comparator: Servo position is compared to the surface position to detect detent piston failures. Monitor only active when the servo is engaged.

 (4)  
Servo Monitoring

 (a)  
Aileron Exceedance Monitor: Determines if aileron servo authority limit is exceeded

 (b)  
Servo Synchronization Monitor: Compares aileron and elevator servo position to surface position to determine if the servos are synchronizing during the engage process.

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747-400 MAINTENANCE MANUAL
 (c)  
Command Response Monitor: Servo operations are monitored by comparing the servo command from the FCC to the servo position of the respective servo. Monitor only active when the servo is engaged.

 (d)  
Servo Current Drive Monitor: Performs a check to verify EHSV interface. Excessive current sets fault.

 (5)  Interface Monitoring
 (a)  
Null Fail Monitor: Detects if the Stabilizer, Flap or Speed Brake Handle position sensors voltage level falls below 1 volt dc.

 (b)  
Hot Short Monitor: Detects if the ARM or ENGAGE solenoid valves are drawing current without the appropriate ARM or ENGAGE commands.

 (c)  
Servo Power Loss Monitor: Detects if the FCC is generating ARM or ENGAGE servo commands but no current is being drawn by the solenoid.

 (d)  
FCC Power Loss Monitor: Detects if 28vdc that supplies servo engage power is lost.

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747-400 FAULT ISOLATION/MAINT MANUAL
AUTOPILOT/FLIGHT DIRECTOR SYSTEM
COMPONENT  102 FIG.  QTY  ACCESS/AREA  REFERENCE AMM 
SHT 
ALTIMETER - (FIM 34-33-00/101)
 C LOW RANGE RADIO, B507
 L LOW RANGE RADIO, B73
 R LOW RANGE RADIO, B74
ASSEMBLY - (FIM 31-51-00/101) 
MODULARIZED AVIONIC AND WARNING ELECTRONIC, 
　
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