I have a test bed in which the FMC and MCDU are hooked up to one another.

FMC menu was populated on MCDU and was able to be navigated.

After making some changes to the FMC GPS #2 lines, the FMC started acting up and would then fail.

Since failure, the FMC menu would no longer display on the MCDU even after the fail was cleared.

Just wondering what is it that I’m missing.

No wiring from FMC to MCDU was touched and only GPS & IRS busses 1&2 were ever replaced.

We can tap into the FMC and see that it is sending some data to the MCDU but the FMC menu still fails to populate.

  • 2
    $\begingroup$ What PN are you talking about? Are you using OEM hardware? So many questions.... $\endgroup$
    – Afe
    Mar 3, 2023 at 11:32
  • 1
    $\begingroup$ Welcome to Aviation Since not everyone here is an expert, can you add more details to your question? $\endgroup$ Mar 3, 2023 at 13:12

1 Answer 1


Assuming the MCDU is ARINC 739A compliant (almost all are) the logic behind the menus is entirely within the FMS. The MCDU is just a dumb terminal.

When you press a button on the MCDU it just sends a message to the FMS that that specific button was pressed. The data displayed on the MCDU is just ASCII characters sent from the FMS within a command to write the text on the specific display location (line/column).

Your best option is to swap in a known good MCDU and see if anything changes. I expect the failure is most likely within the FMS.


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