Aircraft changes course on LNAV when flight plan a few thousand miles out is altered

Hi everyone,

I noticed that when I alter my flight plan for waypoints that are at least hundreds if not thousands of miles out, the aircraft which is on LNAV will start turning left or right.

Would appreciate if someone can shed light on this phenomenon!

If it’s your next waypoint - the aircraft needs to adjust its heading to account for any current winds or if it is now “off-track” it needs to get back “on-track”.

It’s not my next waypoint though. There are at least 20 waypoints between the one I’m currently flying towards, and the one I am amending.

Make sure that the pink bar “the active leg” doesn’t change at all while you are modifying your FPL otherwise your aircraft will change heading.

Yes, I did made sure of that.

In the NAV menu (above the A/P menu) is the NAV Source GPS?

Yes, it is.

I’m not sure then whats causing this…hopefully someone else can figure this out

If you could upload a photo, it would be the quickest way to help you.

Thank you for helping me nonetheless.

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Here’s an example. It’s not the flight I performed just now because I didn’t take a screenshot :(

As you can see, I am heading towards waypoint LEDOX. What I did was to delete a waypoint further down, say IGOGU. When that happens, the aircraft will start turning left or right. The same happens when I add a waypoint in place of IGOGU.

I was flying an Airbus A350, A/P on and LNAV engaged.

Hope this helps.

When you delete your flight plan or add something to your flight plan which is the next waypoint, this happens, not sure why but it happens.

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Thanks for uploading. Unfortunately, I can’t tell from your image because, as far as I can understand, it doesn’t show a re-creation of your described error.

My entire experience with what you show here, that the path to your next waypoint LEDOX remains the active leg, is that nothing will alter the aircraft’s course until a different leg is activated in its place. As @BennyBoy_Alpha stated

There are triggers to LNAV changing the active leg which might take some getting used to, but unless the active leg actually changes, I have personally never experienced any course change (from altering downstream waypoints).

The only other thing I can think of is that some aircraft track the active leg more smoothly than others. For example, I was in the TBM yesterday tracking on an active leg, and it was continuing to over turn, back and forth, a bit to keep on the active leg.

If such assertive tracking turns occurred while you were adjusting waypoints further ahead, it’s possible you could suspect the adjustment caused the turn, even though the two events were unrelated.

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