VNAV, unstable descent

Hey y’all,

Today as I was descending into KSFO I can’t say I was staring at my altitude, but generally have my device by my side with the volume up and always in my sight so I can keep watch on my descent.

Today as I turned my VNAV on 3nm from my descent at 40,000ft, I notice halfway through that I’m ascending around 65nm out from my destination at 27,000ft, than descending at a much lower altitude. My flight plan didn’t overlay itself, and I only had two altitudes set, both on my final approach. Here is my flight plan:

I can’t seem to understand why the VNAV was going up and down. Did it have a set glideslope that it was off track of? Did the VNAV get confused with the flight plan going around the bay, when I was already close to the airport? No waypoints were sending it off track, although I believe a waypoint turn caused the VNAV to change.


I will generally record all controlling sessions, and any bugs I encounter. Here I started recording whilst the AP was on and as soon as I realised. Here is the initial recording:


I also have attached the replay below. I did not tamper with any of the arrival procedure, neither did I touch the autopilot until that part of the recording on descent. You will notice the Autopilot climbing and descending at thousands of feet at a time way past the 3000vs limit the VNAV has set in place.

Sharemyinfiniteflight.com could not handle the replay size.

If anyone can reproduce this issue then that would be great, it’s my first time experiencing any problem with VNAV. PYE3 and I28L arrival and approach at KSFO, experienced on BA A380.

Details

Device: Samsung Galaxy A70

Android Version: 9 (these 2 aren’t important I think)

Infinite Flight version: 20.01.02

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I was on the same route in the same aircraft and the same thing happened. I just ignored it and manually entered the vs

I see it flashing because it is over -3000. You could have been going too fast for the descent to make a valid descent. Was this video right after vnav was enabled? Mach 0.80 is pretty quick for a descent when you are going to gain speed which may be more depending on speed.

My speed was Mach 0.86 at the time (meant to be 0.85, but this wouldn’t affect the VNAV at this speed anyway).


It may be an issue with how the arrival procedure is structured around the airport, maybe the altitude VNAV got confused with how my fpl was long, but direct path to the altitude was short. That’s a far out random thought.

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This was after the VNAV started a ‘bobbing’ action after what seemed a normal descent. The replay should prove this.

Generally I travel a cruise speed from descent to about 14,000ft and below. Never have I encountered this issue when flying the A380 easily over 10-15+ times after 20.2’s release at this speed.

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Oh ok I see now. I’ll take a deeper look.

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Thank you @Ecoops123

@Chris_S any response? Apologies to bump, just to keep the topic open from system 😅.

I’ll try and recreate this when I’m next free.

Hey ecoops well you can do one thing is to maintain a speed around 300-280kts
Well sometimes when you don’t have many altitudes set I have seen vnav going crazy…
Some stars dont have restrictions at all…
So in case it happens again maybe u can slow down to get ROD back uo

Thanks for the suggestion Hari. This has never been a problem to me and believe it is something to do with how the the procedure ‘wraps’ around the airport. At the speed I was at there shouldn’t have been any issue with the descent, and has not been since 20.1’s release, where I fly 3-7 times per day using VNAV. 🙃

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Yep ☝️…

I just tried to reproduce this but unfortunately (or, fortunately perhaps) I was unable to, following all the details you provided. Maybe you can try to reproduce this again youself? If not I think it’s safe to say it was a ‘one-off’.

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