[VNAV] Disconnect during descent

[mfarmercytz]

Just had an unusual issue with the IXEG 737 on cruise from KSFO to KPDX. During descent the VNAV turned off and produced a message in the FMC saying "VNAV DISCONNECTED". I tried to reconnect but it would light up for a second and disconnect with the same message. Did a V/S descent instead.

Anybody else seen this or know what could cause it?

 

Log attached.

IXEG_FMS_debug.txt

Log.txt

GizmoLog.txt

[Litjan]

I would think that you were too fast - VNAV will only allow your speed to go 10kts faster than planned - if you (for any reason) are faster than that, it will disconnect (saying: "hey, pilot, I am in over my head. Do something for your salary and get me back on profile, then we can try again").

Once you are back at the (or close to) the planned speed, you can re-enable VNAV.

Jan

 

[mfarmercytz]

Thanks Jan! I couldn't find an explanation in the documentation. I thought I was well under speed but at some point I just reverted to a V/S descent. Will keep a closer eye out next time!