LNAV veers off course

[Guest haveblue777]

I'm having problems with LNAV veering off course for no apparent reason:I've been running the 4-4 type rating lesson flying from Phoenix to LA. Everything goes fine through takeoff to establishing cruise. Along the way I am using LNAV to track the horizontal course. During the cruise, LNAV is tracking the magenta course line perfectly, and then during a segment without any upcoming turns or banks, LNAV suddenly banks the aircraft and veers way off course! I disconnect the A/P, fly it back in line with the magenta line, then re-engage LNAV and A/P. It tracks for a little while, then veers way off course again. At these moments I am touching absolutely nothing, and up until then there were no unusual issues or warnings. Has anyone else experienced this? I can't seem to find any threads regarding this type of LNAV problem...Sincerely,Pierre E. Yap

[Tabs 2,790]

Pierre,Do you by chance have FS minimized (or alt-tabbed out) or have it out of windows's focus as the primary window in some way? There's a sporadic issue we're aware of where FSUPIC stops communicating with the panel and causes things like this to happen - we can't replicate it with any consistency though so we're not really sure how to fix it. If you're actually in FS with the sim as the active window when this happens though, it's something else going on and I'll probably ask you to send me a save state to try myself to see if i can reproduce it...

[Guest haveblue777]

FS is not minimized or is it out of window's focus. It is definitely the primary window.It's strange, and only seems to happen on flights with longer straight line course segments. LNAV just breaks away with no warning and refuses to track the course even if I get the plane perfectly lined back up with it. I then need to hand fly the rest of the way down.I can try to reproduce the deviation (since it happens pretty consistently to me) and upload a snapshot. Stand by.Pierre E. Yap

[Guest haveblue777]

Ryan,Once I reproduce the problem, how do I send you a save state? Is that a function within FS or PMDG? Or are you just referring to sending a screenshot?Pierre E. Yap

[Chaos81 110]

http://ops.precisionmanuals.com/wiki/PMDG_...to_someone_else

[mcbellette 145]

Could this be just the FMC making minor corrections to the course to compensate for crosswinds? I mean, of you have crosswind moving from the left to the right across the flight path, then the plane would veer slightly left to stop it from being pushed too far to the right.Just wondering if this is what is happening in this instance.Regards,Matt

[michal 44]

Judging from his description the 747 totally looses the track and falls off course - this would never happen during adjustments for wind corrections. Michael J.

[Guest planeman223]

Unfortunetly guys this is just going to happen. In the 737 this is also a problem. I am guessing you are talking about waypoints around 700 or 800 miles apart? The airplane will meet the next point. For some reason the ND displays the curvature of he Earth so it looks curved. Also you can just put in the heading bug what you need to be crabbed. Come back and check on your plane once in a while!! haha Nothing to worry about just another annoying fs bug.Alex

[Guest bthinn]

I have been having a similar problem. On all my recent long-distance flights (eg London-Bangkok; Auckland-LA), the plane completely veers off-course (usually when I'm having my sleep-break) and goes into a series of spirals before finally runing out of fuel. Up to that point (say six or seven hours into the flight) everything has gone perfectly. I also run Active Sky weather and FS Passengers. It's restricting me from making any long-distance flights for the meantime as it has happened on several occasions now.Baz

[cowpatz 243]

I have noticed on the later stages of the initial climb out on the odd missed approach that LNAV would remain engaged but the autopilot would send the aircraft into an orbit. Reselecting the active waypoint made no difference as did trying a different roll mode like hdg select or hdg hold. I did notice a slightly out of trim condition at the time even though the trims were centered and thrust assymetric. The only way to resolve the issue was to disconnect the AP and re-engage it. This cured the out of trim condition as well as the random orbit.Maybe if this happens to you again try disconnecting the AP and then reselecting it again. I know that it is not satisfactory but it may help with fault diagnosis.Cheers Steve