Vnav issues

[Adolfin]

I was flying this thing today and ran into some issues. After the aircraft reached top of decent it would start decending (as normal) but won't go the hole way as in the flight plan. It would drop to 8000ft rather than 4000ft and than max throttle even tho it's meant to slow down. FMC gives message Invalid crz altitude 

[poodster]

Did you change your altitude bug to 4000?

[Adolfin]

2 minutes ago, poodster said:

Did you change your altitude bug to 4000?

Yeah, I did that 

[Slowdive]

I had this exact same issue tonight with version 1.0.7. It started when I dialed in 13000 on the MCP in anticipation of the descent on the CLARR TWO arrival into KLAS. It initiated the descent fine, but stopped at FL240 after Misen and never descended after that until I manually switched to LVL CHG. It did some of the other odd things you mentioned further down the arrival.

*edit* I just read on another thread that the VNAV is a little flakey. No problem, I'll fly the descents manually from now on. However the strange "Invalid Cruise Altitude" warning is still something to look at perhaps.

Edited November 28, 2016 by Slowdive

[Litjan]

Yeah, we need to run a solid quality pass on the VNAV - it works ok for standard situations, but once the descents get more intricate (with lots of restrictions, etc.) it doesn´t do so well.

The "UNABLE CRZ ALT" warning is usually a sign that you are asking too high a crz alt for the route distance, i.e. the plane can´t make it up and down in time. This may be compounded by restrictions on SID and STAR that force a flatter profile...

The "CHECK ALT TGT" message says that your MCP alt is set in the opposite direction of what your FMS expects. So if you are at 5000, your cruise alt is set to 10.000 but you set the MCP alt to 4000, you would get the message ("huh? don´t we need to climb to cruise alt, first? Why do you want to descend already??")

Jan

 

[Rodeo]

7 hours ago, Litjan said:

The "CHECK ALT TGT" message says that your MCP alt is set in the opposite direction of what your FMS expects. So if you are at 5000, your cruise alt is set to 10.000 but you set the MCP alt to 4000, you would get the message ("huh? don´t we need to climb to cruise alt, first? Why do you want to descend already??")

You do have a knack for explaining this kind of stuff, IMO