Hello there,i was wondering wether anyone had noticed that the usingthe LNAV mod, the Auto Pilot doesnt follow correcty the pathif waypoints are more than 100nm distanced.On short flights, like 1.30 hour, it's ok since waypoints onrnav routes arent usually less than 40nm away from each other.But, if you try some long distance flight over ocean for instance,if you want to perform a transatlantic flight, then you go from10 degrees to 10 degrees, which is 600nm...It will then get away from the route.I was thinking about this because even with such long distances,the FMS should follow the patch better than it does.When the 747-400 will come out, this question will have to be solved,because the 747-400 often flies oceanic routes with 600nm long waypoints legs.Maybe it should be correct for the 737NG before?Regards.Arnaud Solvay, LFPG

Please sign your name, it is a forum requirement. Thank you.As to you concern, this is a known issue with the 737NG LNAV logic. PMDG has stated that the 800/900 will have improved logic in this regard.To mitigate this issue, make sure your waypoints are no more than 40-50nm apart, and you will be just fine.

...And to elaborate a little on Fred's post.The A/P is not inaccurate in following the LNAV path. As I have understood it it is the ND that is inaccurate drawing the right path.But now when I'm writing this I got a bit unsure about this... What happens if you make a long flight path leg and compare that to raw data? Ah, will have to try..TGIF,

Thank you very much for your answers, gentlemen.I first apologize for posting on a topic that had already been discussed, I havent found that with the search mode.I'm somehow satisfied being not the only one having this problem.I thought maybe the drawing on the ND was false becuase of a toopoor refresh rate, but I let is at default settings, so.I would be worthy indeed to try a 600nm long leg flight, andcheck if at the end you get to the correct coordinates.It would prove that it is the ND, and not the path that is incorrect.Will try to do that and report back later.Arnaud SolvayP4 2.4b ATI 9800XT 2561go DDR 333Asus P4P8X i865p

Arnaud,You will get to the right point in the end of a long leg. THe problem here is that the earth is round. And thus you are flying along a great circle as opposed to flying along a straight line which they did when the earth was flat. But in the middle of the leg you have the biggest discrepancy between the path flown and the path drawn. This is where it would be interesting to see if raw VOR/DME data is consistent with my position on the ND map. Cheers,

Dear Mats,I understand perfectly how round is the earth, as actually my jobin real life consist in making flight plans for some huge carrier.But the draw on the ND is kind of 'drawnfrom the top' or form above,so it shouldnt display apart from the path.I had the great opportunity to fly once into the cockpit of a real tranatlantic flight onboard a 747-400 from Europe to Miami,and the drawing was straight all the long of the trip, from 10 to 10 degrees.On another note, I was wondering this since on another payware add-onsfor fs2002, the ND displayed the path correctly even on longer legs than 600nm.Maybe I didnt understand what you're pointing at.regards,Arnaud

Well,I was probably a tad unclear in my post. ;-)What I was trying to get through is that if either the path or the aircraft position on the ND is drawn with regards to the old earth, the flat one. And the rest is drawn taking into acount the new round earth, there must surely be a discrepancy even when displayed on a flat display unit? I'm sorry if you felt offended by my great circle explanation but actually I was just trying to lighten it up a bit... :-)Cheers,

Dear Matts,Dont worry, hehe, I havent been offended at all, of course!We're just trying to make this point clear.Up to now, I dont really know what's wrong with the LNAV path display.But so far, I dont think it has anything to do with the flat or the real roundness of our poor Planet.I rather think it is a small issue in the programming of the simulatedEFIS display of the 737NG.As Fred stated in the first answer, I guess the brilliant team out thereat PMDG had already solved this for the nearing release of the 800/900 series.All the best,Arnaud S

What has been explained previously is the the A/P is flying a great circle route, while the ND is simply displaying a straight line between the two waypoints. There will obviously be a discrepancy. I have always understood that the autopilot is flying correctly, and the 800/900 update should correct the ND to display the flight path correctly.Aaron

Aaron,You sir are correct. I was remiss in not mentioning that in my post.