I am having issues with the LNAV function on the PMDG 747, it seems like it's having a hard time maintaining a straight line following the planned route on the FMC. It starts to swerve to one side, and then overcompensates to the other, until my roll is almost 90 degrees. It seems to only happen when I fly in or out of Malpensa (Milan) which is a shame because that's one of my favourite airports.Any ideas folks?

Probably would be better in the 747 Forum. Questions:Does the FD guidance call for the extreme bank?Have you tried reinstalling your navdata (Since it's only at one airport)?What weather were you using? (Active sky wake turbulence has been known to show up in weird spots)

I have not really paid much attention to FD. I will let you know.Actually, it happened for the first time on a flight that has nothing to do with Milan. I just flew Vienna to Amsterdam and it started to do the same thing. Where can I download "navdata"?Thanks

Do you use any kind of add-on weather programs, and even if not are you using FSUIPC with real weather to perform wind smoothing? If you are not using FSUIPC to smooth winds with real weather the wind shifts erradically and will cause the aircraft to fly as described. I have had issues over time with the J-41, MD-11, Concorde, and other aircraft. I finally broke down and got FSUIPC and with the wind smoothing that fixed the S-turning, and veering off course issues when the wind would suddenly shift, or gust up... I suggest whenever it starts acting erratic to go in the weather settings and cancel out all your wind and see if it straightens right up. If it does then that is your problem. This is just my experience though. There are also sometimes issues with the AP systems, but it varies from system to system. Hopefully this helps.

Hi Hirgab, Please sign your posts with full name, per forum rules, thanks.I agree with all of the above, particularly with Steve's about winds. Another thing that comes to mind from Joseph's comment is my experience with the MD-11 (not the 744, but probably the same) is to not set the Autopilot if you're not tracking the course. So to put it in example, when I take off I manually fly the route for a short time following the Flight Director. If you are way off from what it says and press CMD the aircraft may not settle on the route normally and you can recreate what you're seeing here with the wild S-turns. I try to get it as perfect as possible before engaging the AP. IF, I get the turn and burn issue, I disengage the AP and take manual control again until I'm satisfied with my alignment with the flight director and try the AP again. If it still eventually starts the turning I do exactly what Steve said and use my preset keyboard command from FSUIPC to clear the winds.I have seen both instances many times and these steps fixed the problem.