A weird VNAV bug

Hi all,
This morning, whilst descending towards Nantes Atlantique (LFRS), I noticed that the VS had increased strangely in the last minutes or so to over 1400ft/min which forced me to deploy airbrakes before reaching 10.000ft.
I then checked the map and noticed that the VNAV banana for my 3000ft aim was about where I drew it in blue in the following pic :

Whereas it should have been further given that there was two sharp turns before the aimed 3000ft waypoint.

Strange thing is that I flew the very same approach two days ago and the problem did not happen.

Any idea what the reason could be?

Sorry for my ignorance on this, but I don’t know specifically what VNAV banana means. So my first guess was the arc that shows your VNAV altitude intercept point. But that can’t be where you drew the blue(?), because the alt intercept point seems to be there beyond the base of your flight path triangle? And that seems to be roughly right for the 3000ft setting?

Or am I misunderstanding?

Different winds could make a difference in descent rate due to causing different ground speed relative to IAS.

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You guessed right, the arc is the banana, and if I recall correctly, the word was used somehow when the feature was introduced…
And yes, the arc was indeed where I drew the blue arc… (I made the picture after stopping VNAVand getting the VS right). So it is weird.

And winds are not the issue here. Definitely!
And to explain maybe better, it was as though VNAV got the route wrong : left side rather than right side! Which is of course impossible. Or is it nit?

I say it’s because of the sharp turns on approach 🤔

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Thanks for clarifying. That makes sense. I was just oblivious to this (which is not surprising).

Although as you say VS increased, 1400ft/min does not sound abnormal, even though it required airbrakes (differences in weight and density altitude are two other possible factors in the horizontal speed achieved relative to vertical speed).

I think I see what you’re saying, but that doesn’t seem likely from my memory. But I need to test it to make sure my memory is in keeping with my experience!..

Yeah, that’s where my assumptions might be fuzzy…

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The approach should be circular kinda like this

I usually take the VNAV off and HDG and turn the aircraft manual on approach like this. Like GCRR Lanzarote sharp turns

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Yes this is the route, but 2 days before, and in other similar cases, the VNAV should follow the NAV and not take a shortcut!

I also like this in Macau :

But I shall now remember VNAV can do funny things!

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