AP got lost in LNAV mode

[jixeg737]

Hello together,

yesterday I did this flight:

EDDS TEDG2H TEDGO UL607 UTABA UM738 NATAG DCT BORMI DCT LUMAV UP131 LIKNO UQ95 UTENO DCT KASTU DCT TEA TEA1A LIRN

During approach and before the LOC capture, the AP (LNAV mode) suddenly got lost (laterally) and did not follow the magenta line on the ND anymore (the AP flew some curves).

So I had to revert to the HDG mode.

What could have been to cause of that?

There were no discontinuities on the LEGs page.

Thanks a lot for your help.

Kind regards

Jens

[Litjan]

Sometimes the LNAV code still faults when there are very complex routings - with big track changes and closely spaced waypoints. I see that you flew to Italy - their SIDs and STARs are notorious for not playing well with modern airliners´ FMS.

The real FMS has a limitation called "triple bypass bug" - when the plane can´t make three consecutive waypoints (because it can´t make tight enough turns), the code folds up. We didn´t model this, but I just mention this to show you that the real LNAV code isn´t foolproof, either.

You did the right thing - when the plane doesn´t fly the way you want it to, take action. Thats why there are still pilots in the cockpit, these days.

Cheers, Jan

 

[jixeg737]

Hello Jan,

yeah, I had a bypass notification on the CDU. So, that's it.

During the flight, the notification was new to me. I didn't pay attention to the notification.

Thanks for your answer.

Kind regards

Jens