VNAV fault

Hello guys! Hope you are doing well during the Covid-19. I had experienced this issue for 3 times on the Boeing 737NG. When I turn on the VNAV and the plane is starting to descend with the calculated descending rate, the plane goes normal in the first several minutes maybe 10 mins but then the plane starts climbing itself with the positive rates(eg. I am descending at -1600 and the plane suddenly start climbing with +1600)
My cruising ALT is FL355 at 0.77 mach, sensitive setting is 35% of roll, 23% of pitch, 50% of yaw, trim is at 5
Do you guys have the same problems?

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That should not happen
Maybe it is because you are going to fast or to slow

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I am flying in an 737-800, will see about this


And this belongs on #support

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ok no problems, thank you very much!

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no prob I am changing it now

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Getting VNAV ready now…

not really, I was following the descend charts information

try looking here

Can you give more details of your parameters ?

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so basically, I had set all the ALT on the navigation point with the numbers showed on the approach chart and had double checked it before takeoff and in the mid-flight. As I am about to descend, I turned on the VNAV and the plane start to descend with the assigned altitude. But about 10 min later(or maybe around FL250) the plane suddenly stop descending, instead it start climbing with the positive rate(eg the computer calculated the plane should be at -1600 to reach the next ALT and my plane suddenly start climbing with +1600)
This problem had occurred three times, all on the Boeing 737NG. But the VNAV worked just fine on the B772

Maybe because it was turning? So the plane did not finish descending into a way point but then another way point with another altitude came in and the plane responded by climbing?

I am not sure though

Are you sure that you aren’t missing something that can confuse VNAV ?

But according to the update VNAV isn’t supposed to climb it’s only supposed to help with descent

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VNAV is used for descent phase.It may act like this to correct something.

Do you have a list of waypoints and alts so your flight can be replicated?

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yes I know that, the AP shows the V/S is on the negative rate both on the glare shaded panel and the AP panel but plane is responding as climbing with no reasons.

I am pretty sure because I had done the exactly same route with the same data on the B772 and the VNAV worked normally

you mean… the waypoints written as letters?

just like how you copied from the