(b)
Software monitors test hardware functioning, interfaces, and data reasonableness. The various types of software monitors are: 1) Activity monitors. 2) Wrap-around test monitors. 3) Arinc 429 sign status matrix monitors (SSM). 4) Command/response monitors. 5) Discrete I/O monitors. 6) Detent comparators. 7) Operational monitors. 8) Signal selection fault detection voting circuits (SSFD).
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(2)
Interface Messages
(a)
Interfaces are tested at power up and during ground tests. Messages "NO INFC LRU/LRU" will not be reported as flight faults. Flight faults will report group faults which can be related to a ground test NO INFC message.
(b)
If the MCDP generates a NO INFC message for FCC, TMC, or FMC on power up, the MCDP will not interrogate that LRU for flight faults and will not exercise ground tests for that LRU.
(c)
The FCC and TMC will report NO INFC messages if an activity monitor fails to detect any change in data received over the interface for specified period of time.
(3)
Mode Errors
(a)
Mode errors are reported for flight deck effects caused by crew procedures. For most mode errors there is no fault and no need to replace hardware. 1) Typical crew caused mode errors include:
a) Failure to select TRUE heading when flying near the north pole. b) Attempting to change ILS frequency after capturing glideslope. c) Failure to disconnect a single channel engaged autopilot prior to flight and touchdown. d) Pilot input via wheel/column that overrides a single channel engaged autopilot.
(b)
Mode errors with diagnostic codes 247 or 143 may be hardware problems and require corrective action.
(4)
Sign Status Matrix (SSM) Faults
(a)
LRUs transmitting ARINC 429 data provide SSM information in data. The SSM provides status of normal operation, functional test, no computed data, or failure warning.
(b)
FCCs, FMCs, and TMC will normally store any SSM other than normal operation in non-volatile memory as fault data. This data can be read in the shop.
(c)
SSM data forwarded to the MCDP will normally consist of only failure warn. Three situations can occur that will result in SSM of no computed data to be forwarded to the MCDP. 1) Loss of data from IRU can result in diagnostic codes 57,
62, 75, 76, and 247 associated with NCD from IRU.
2) Loss of ILS data at or after capture for glide slope or localize can result in diagnostic codes 199, 200, 202 and 203 associated with NCD from ILS.
3) Loss of FMC data caused by NCD from IRU can result in diagnostic codes 120 and 121 associated with NCD from FMC.
(5) Signal Selection Fault Detection (SSFD) Faults
(a) Signal selection fault detection is a process used to identify and eliminate a suspected source of faulty data. SSFD is used when dual or triple data sources are available. Any one data source can be voted out as faulty or if that sources data compares unfavorably with a computed median data value.
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(b)
SSFC monitors have a process they go through to not let the autopilot and flight director use the bad data from a defective sensor to calculate the data they will use. SSFD monitors come on when the airspeed is more than 60 knots and an autopilot is in use, or if one or two of the flight directors are on. The monitors continue to compare data values independently of how many autopilot channels are in use. The diagnostic code intermittent bit indicates which of the three FCCs has bad data. This FCC, because of the bit, will not be permitted to have a say as to what data is good or bad. Without this bit, this FCC will possibly have that say and permit a data output from an FCC that has bad data.
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