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Unpredictable FMC behavior, is it RL, PMDG or me ?

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    Unpredictable FMC behavior, is it RL, PMDG or me ?

    I programming following route: ESSA/08 BAPAP2L BABAB T316 GOTAL GOTAL1S ESSV/03 and start the flight.
    Before GOTAL I entered direct VSB at the top of LAGS-page and then entered hold 020/R at VSB at the second line. There is now a route discontinuity after VSB hold, but that's looks fine to me.

    Some minutes later before GOTAL (while still going direct VSB, 40 nm remaining) following unexpected happen, The direct VSB at the top disappear and the AC does at right turn approx 30 deg heading nowhere. The hold become on the top of LAGS-page and still appear the map but there is an empty gap to the hold. I repeated exactly the same behavior twice and assume I can repeat how many times I like to.

    My pilot work around reaction was to reenter the direct VSB and that works, but I'm unable to reenter the hold and had to fly the hold manually as it still appear om the map and in HOLD-page, but not in the LEGS-page. Trying entering a new hold in the LEGS-page failed to me.

    Does any body know where the fault is and how to be able to fly the hold via AP ?

    Best regards Lennart Vedin
    Last edited by lvedin; 09Jul2019, 23:07.

    Holding logic (and to be honest, LNAV in general) has never worked correctly for me in the NGX. I was never sure if it was a matter of limited resources available in my computer, or what... but hold entries are typically random and not what the HOLD page indicates for an entry, and complex LNAV paths are only kind of followed approximately. The real plane never ever deviates from the magenta line, ever.
    Andrew Crowley


      The LNAV capability of the PMDG addons is based on a very old code that is actually more or less obsolete today. PMDG are currently working on an overhold logic to update their fleet with latest LNAV capabilities but this will still take some time.

      I can‘t say if Lennart‘s issue has something to do with it, screenshots would be great. But the autopilot will generally be updated by PMDG in the „near“ future.
      i7-6700k, GTX 1080TI, 32GB DDR4 RAM @1600MHz, 4k


        Just a note that maybe is pedantic, but I think is kind of a meaningful distinction: it's not so much the autopilot that needs to be fixed, it's the flight guidance. The autopilot just follows the flight guidance, and the PMDG autopilot does that just fine. But these problems with tracking an LNAV course exist while handflying and following the flight director as well. The LNAV flight guidance in the NGX sim is just sloppy, and occasionally unpredictable.
        Andrew Crowley


          Correct me if I'm wrong that I read somewhere that PMDG for NG3 will have a while rewritten LNAV code
          Alex Kulak
          PMDG Studier and flyer
          Ramp Agent


            It is not the AP, the issue is the magenta line and the fix VSB that was on the top of LNAV, disappeared 40 nm before I reached it, on top become now VSB-hold, but how should the FMC fly a hold 40 nm before the hold.

            Repeated twice, but now I cannot repeat on third and fourth try, maybe I do some action different.

            / Lennart Vedin


              Yes, that‘s what I meant.. Talking about the AP was pure laziness to dig deeper
              i7-6700k, GTX 1080TI, 32GB DDR4 RAM @1600MHz, 4k


                Well, the real b737 has really bad VNAV but good LNAV and PMDG has good VNAV and bad LNAV.
                Omar Josef, from Spain
                Started with FS5.0 back in the day
                737/757/767 Type Rated
                I make a living flying 737s across Europe
                Still use the damn simulator


                • DDowns
                  DDowns commented
                  Editing a comment
                  I'm not sure I'd give VNAV a good rating for PMDG NGX. The 777 is better but they are reworking it to bring up LNAV/VNAV to at least good and make the big transition to ARINC 424 navdata.