737-800 VNAV disconnect

[biggles72]

What are the criteria for a VNAV disconnect in the NG? Searched FCOM Volume 2 and did not find any answers.:ugh:

What causes VNAV to disconnect, and what are the criteria?

Thanks :)

[Shiny side down]

Out on a limb... but I understand the following-

VNAV will cause a disconnect whenever it is unable to comply with either
the VNAV profile and/or any constraints set.

for instance.
You descend at M0.79/280kts, then decelerate to 250kts at FL100. Ahead there is a level (FL70 at waypoint) or further speed restriction. During the descent, it (VNAV) has increased speed to achieve the VNAV-path because the profile was wrong.
Now it needs to decelerate to 250 kts, but cannot also make the FL70 restriction at waypoint. VNAV disconnect.

If speed is the issue in descent, this will usually be preceded by 'Drag required' messages or similar.

Likely causes-
A significantly adverse wind not entered in the forecast page.
Being held at a level beyond the descent point.
An intermediate level off, and not being able to regain the VNAV-Path during the further descent.

Not sure if this is what you are after?

[A and C]

Without looking at the books I would say that Shiny as just about said it all!

[_FL600_]

One cause of a VNAV disconnect is that when your airplane flies at 15 kts faster than scheduled speed.

To explain it better, FMC calculates a descent path with your choice of speed to meet your desired altitudes over your programmed routing. To aid the FMC, you need to enter some additional info like tail winds and anti ice altitudes in the FORECAST PAGE.

If you have a sudden strong tail wind or possibly you didn't program the FMC with forecast tail winds, FMC will ofcourse tend to catch up with the VNAV path with no matter to the speed. Now, if you exceed the scheduled speed by 10 kts, you'll get DRAG REQUIRED. If you exceed by 15 kts, you'll get VNAV DISCONNECT.

Hope that clears it,

Happy flying,,

[biggles72]

Thanks FL600. That was a nice simple answer.

For some reason I thought there were more reasons to get a disconnect. I am sure I have been on a RNAV app for example, and had a disconnect when I was on speed, but maybe it was a glitch with 10.6 software?
Anyway cheers. :ok:

[VH-ABC]

FCOM Vol 2 page 4.10.6 is the reference.

Vnav mode is terminated by any one of the following...

selecting another pitch mode.
glideslope capture.
reaching end of Lnav route.
transition of glideslope intercept waypoint if G/S is armed.
crosstrack deviation exceeds twice the rnp value during PTH descent for an active leg with a database vertical angle and Lnav not engaged.

[TolTol]

Doesn't VNav also disconnect after reaching the runway waypoint in the legs page (ie RW05) during a VNav approach.

[BongleBear]

yeah i had a vna disconnect late on in a vnav approach, the speed was racing away so assumed it was that. didnt think it was as much as 15kts, maybe 10kts above and it cant slow down without a human input (ie flaps or gear)