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Error between FMC heading/course and Nav display

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    Error between FMC heading/course and Nav display

    I have an issue with the AP tracking a given course/heading. FIRST, my weather is totally FSX. No xwinds, no winds, no turbulence nothing except cloud info and draw distance set in custom draw for gfx. There is a problem with the 737NGX drifting back and forth along a heading. I have yet to see a solution but have a few observations.

    When setting up a flight plan in the FMC, the projected headings, altitudes etc. enter just fine and all data is correct. However, once in the air the FMC will show a course/heading that is different from the Navigation display on the front panel. This usually shows up with long distances between waypoints and the difference seems to be around 25 to 30 degrees. It appears the FD then tries to make the difference between the FMC and the Navigation display. This discrepancy results in the airplane constantly drifting back and forth trying either follow the FMC heading or the Nav display heading. So, is there a solution here? Am I missing something.

    I never had this problem on my old windows 7 machine. Now using a brand new windows 10, great processor, gpu, ram etc. No problems with my machine. ANYBODY, ANYTHING?

    frank cox
    Last edited by frankc99; 11Feb2021, 06:09.

    This is to sign my original post


      Could you show some screenshots of the problem? Would be very useful to see what you see.


        OK, I think I may see what's going on. Most flights I notice this on originate at KAMA, Amarillo, Tx. Amarillo has no SIDS or STARS. All flights are handled by approach and departure. When programming the FMC, I usually go direct to the transition point for the STAR for wherever I'm going. Usually that initial leg is a long one. I just finished a flight to KLAX and ran a flight planner and manually entered all the waypoints. The first leg was about 100 miles. The FMC showed a heading of 274, but the ND showed a track of 260. But once I crossed that first waypoint all the numbers lined up throughout the flight. Do I need to initialized my IRS at the beginning?

        Frank Cox


          Could you explain your question a little bit more? Do you not align the IRS before flight?

          Jim Erwin
          Jim Erwin


            OK all, I figured this out a few days ago and wanted to test it well before posting another message. First in response to aligning the IRS. That has nothing to do with this. Forget it. Now I have to admit I've been entering the FMC navigation wrong, for years. Did not notice any problems until Windows 10.

            Not blaming Windows 10 but perhaps there was enough coding of some different math between 7 and 10 for this problem to crop up. 10 has been around some time now and when searching for a solution I read a lot of posts from years ago, like 2012, 2013 etc. Never did find a solution but, at least in my case I found it.

            The basic problem, or complaint was "My airplane ( note a PMDG airplane) will not follow a path from the FMC. It drifts back and forth, sometimes radically and headings between the FMC and the ND are sometimes quite different. Here's the cause and solution, at least in my case.

            My departure airport does not have either SIDS or STARS procedures. So, when I would program the FMC I would go from picking a departure runway straight to the arrival STARS, pick a transition point and fly the route and when I upgraded to Windows 10 the problems started.

            What was happening, mathematically anyway, was the first leg of the route, point A to point B had no fixed point A. Point A was the airplane itself. Not a fixed point but a moving point that in effect was constantly changing the line. I guess that would be illogical because the airplane is now defining the line as it moves instead of simply following the line as intended.

            So when entering routing info, go to page 2 of the RTE, enter a fixed point A. I use the departure airport. Then program the enroute and STARS as normal or run your line from your airport, point A to the transition, point B, then finish out as you normally would. The basic fix to this problem is, SET A FIXED POINT AT THE START!


              Sorry, forgot to sign last post.

              Frank Cox