I think it may have been discussed before but I can not find any recent posts regarding this matter.When flying in LNAV mode, the aircraft tends drift slightly off the intended track and parallel the route. The longer the leg, the more error is present. Not only is this visible on the ND but on the PROG PAGE 2 of the FMS as well. I have seen X/TRK errors in excess of 4nm. I have attempted upgrades of FSUIPC as well as a complete reinstallation of FS2004 and the 737NG while trying to troubleshoot with no success.Any ideas on how to correct this problem.Thanks in Advance.PS: Aside from this issue, this airplane rocks!

Please sign your name to your post, forum rules require it. Thanks :)As to your question, the issue is more of 'what you see is not what you get.' Let me explain. The LNAV autopilot function is flying the correct course, factoring in the great circle route. However, the ND does not factor in great circle corrections; thus you have an autopilot flying the wrong thing, but your ND is telling you something different. This is why on longer legs, the error is more prevelant :)This display issue is slated to be fixed in the 800/900.

Thanks for the reply.I sincerely apolgize for not following your forum guidelines. I am happy to hear that the problem is known, and slated to be corrected.Thanks Again.Alecs Bains

>thus you have an autopilot flying the wrong>thing, but your ND is telling you something different.Should read: "thus you have an autopilot flying the right thing, but your ND is telling you something different."BTW, they aren't 'my forum guidelines' sir, they are PMDG's.