Unexpected descent with VNAV off

Hello community!

As I ‘was’ flying the Delta A359 from Shanghai to Detroit, I woke up with 3 speed violations as the aircraft decided to descend on its own. VNAV is always off to prevent this kind of issue, yet it happened

I have never experienced this issue in over 2000 flights nor did the aircraft perform such action in any flight.

All systems worked fine, flight went smooth until I woke up being violated.

Did anyone else experience this? If yes, what happened and what triggered the issue as I’m very curious but also angry/sad?

If anyone is interested to see the replay, click on the link below, I also added 2 pictures showing the systems operations


NOTE: keep it nice, any hostile comments will be reported!
Also note: I’m not too heavy if anyone tries to blame it to the aircraft weight.

Happy flyings and a nice weekend!

Kevin/KJKLM281

Interesting and rather ominous. AP was set to 4,000 feet with a decent rate that VNAV would get, but vnav was off

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Are you using any 3rd-party apps? These kinds of descent rates are only possible with either VNAV, or a 3rd-party app using the Connect API - the end user cannot set a vertical speed of 1460ft/min, or any of the other descent rates that were seen in the replay. The user may set 1500ft/min, or 1400ft/min - they only have control of the hundreds and thousands in IF, and would have to turn to VNAV or a 3rd-party app for the more precise descent rates recorded.

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Agreed with the above assessment, coupled with the fact that the ALT Target change to 4000ft coincided with the turn to the next waypoint. Seems like the workings of a third party VNAV functionality where you may have inadvertently entered the wrong altitude for that particular waypoint within that app.

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I tend to use IFA a lot, yet it shuts down everytime halfway trough the flight so I need to restart it which I havent done. I usually open the app again 15 minutes before TOD but I wasn’t even close to TOD at that time.

And as far as I know, IFA can only be used to step climb. To descent, VNAV needs to be armed (even when using IFA & custom altitudes) which was turned off.

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That shouldn’t be happening.
Which device are you using?

That’s not the case.
IF-A’s Alerts/NAV feature can be used for both climbing and descending.

That’s not the case either.
IF-A’s Alerts/NAV feature is activated when LNAV is engaged.

What @AviatorAlex states is true.

Taking all of this into account, it’s more than likely that IF-A’s Alerts/NAV feature was still controlling the aicraft’s altitude.

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I’m using the Iphone 15 pro max.
The app shuts down quite often and shuts down everytime when my alarm goes off or someone called me on the same device.

Also when the app is working, the climbs do not always tend work. It doesn’t climb at all or skips a step. From FL320 to FL360, skipping FL340 for example.

Outside all of this, I noted that the V speeds for the Embraers do not work as well.

That explains it.
IF-A gets killed when that happens. :)

Haven’t noticed that myself.
Will try to do some tests soon.

That’s a known issue as well.
It’s on the list of things to add/fix.
Hopefully it can be done soon.

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