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737 MAX Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management Computer
11.32.8 MN-FLT-OH-201
Dual FMC Failure
If both FMCs fail, the FMC alert light will illuminate. The MENU page will
appear on both CDUs. VTK will appear on both navigation displays. LNAV and
VNAV will disengage. After 25 to 30 seconds, both navigation displays will
display failure information.
Software Exception Handling Logic
Note: A Software Exception is the disruption in the normal flow execution of the
software code; i.e. an attempt to solve an unsolvable map calculation.
The software exceptions handling logic will prevent a complete FMC restart in the
event of most software exceptions. If performance data is entered, an exception in
most processing will result in a downgrade of FMC operation, not a complete loss
of function.
In the downgrade mode:
• LNAV and Map are retained
• FMC Predictions are halted
• VNAV is disconnected
If the software exception occurs in processing of the active flight plan, an alerting
level message, VNAV INVALID - PERF, will be displayed for the pilot indicating
re-entry of Cost Index is required in order to restart the internal software
predictions process, which is required before reengagement of VNAV. Predictions
can be restarted allowing VNAV to be reengaged after entry and EXEC of Cost
Index on the PERF INIT page.
When a software reset occurs in the MOD PLAN and not the ACT PLAN, the
FMC will delete the MOD PLAN. An advisory message, INVALD MOD PLAN,
will be displayed for the pilot indicating that this has occurred. LNAV and VNAV
will remain engaged for this case.
When a software reset occurs in an INACTIVE PLAN, the FMC will delete the
INACTIVE PLAN. An advisory message, INVALD INACTIVE PLAN, will be
displayed for the pilot indicating that this has occurred.
This improvement will not prevent all restarts; it does not cover those that may
leave the FMC’s memory in an inconsistent state.
March 1, 2021