Just flying from EFHK to EGLL, the flightplan has been created with FSBuild 2.1.During CRZ, I refiled my FP for FL340 - in order to do that, I disengaged VNAV, set FL340 in FMC, set FL340 in MCP and tried to renable VNAV, but got the error msg "check MCP alt" - ok, this is the known problem I thought, let's continue with LNAV...Looking at the map (see attachment), I find myself drifting away from the plotted course (about 10 NM), although LNAV is enabled - ATC even asked me to correct it. I disabled LNAV, got back on the plotted course and enabled it again: LNAV directed a turn off the course.Looks like either the export frm FSBuild didn't work correctly or LNAV has a problem as I changed CRZ alt.I've been following the discussion about having another heading as the one set in MCP when using HDG SEL, but this sould not be the case when using LNAV?ADDENDUM: I just passed DANDI - FMC initiated a turn, followed the plotted course, then started to drift again...http://forums.avsim.net/user_files/42003.jpg

I've noticed some drifting from the course with LNAV, but not as much as 10 miles.It has seemed like connected to crosswind situations.. perhaps the crosswind calculation for the course isn't working properly?some far fetched speculation, but maybe it tries to keep flying at the certain direction, but doesnt check for the waypoint location and slowly drifts away from the centerline between two waypoints.

Well... I'll try another flight and program the FMC myself to exclude FSBuild from the issue. Hopefully I get a 40kts crosswind again (or set it, if not) - interesting is, that the waypoits were met, just during the (long, around 300 nm) legs it drifted of the course. Could as well be a problem of going direct instead of using the airways?Nik Zanolari

Nik,You just said the magic words! ;-)>interesting is, that the waypoits were met, just during the (long, >around 300 nm) legs it drifted of the courseThis is due to a graphics bug not drawing the path lines on the ND according to great circle courses. You are travelling along a great circle, but the ND draws a straight line. Hope it clear things up,

Thanks for the clarifications, Mats!As I am rather new to airline sims, handling a such sophisticated product as the 737NG is a real challange, thus hard to know if I did a mistake, I don't understand something or if there is a bug involved.Funny though, VATSIM ATC complained about a course difference - but this was at the stage when I switched VNAV off to do (or try) the CRZ alt-change, probably I got off the plotted course there due something else.Nik

Nik,It is a complicated piece of equipment, this lady. But IMO that's half (if not more) the fun of it. Discussing these kinds of issues on the forum. Regarding your VATSIM flight I would believe that the ATC was watching you drift when you tried to get to the magenta path again. And as stated earlier the magenta path is not on course. You were probably heading spot on all along. ;-)Regards,