Flying the 772 Freighter from PANC to KSEA, I received a descend clearance a bit early (some 45nm from TD), so I applied a DES NOW after switching MCP ALT from FL410 to (suggested by ATC) FL320. As nothing happened, which I frowned, I waited sometime till a point where she really must be descending. Then I switch to FLCH and she started down. After sometime, i tried to re-engage VNAV, as I did many times before after DES NOW procedures. The VNAV button lit for 2 seconds and went off. Just to make things short, until I landed at KSEA, I couldn't use VNAV again.

 

The option for override A/T is put to NEVER. I use FS2Crew 777 button, but I can't remember the FO messing descendings. I also have SPAD Next with a default PMDG 777 profile.

 

What could be preventing the use of VNAV?

Heron,

 

Typically if VNAV is not engaging you should be seeing a message in the FMC Scratchpad telling you why.  Any messages in the Scratchpad?

First thing that comes to mind as alluded to by Paul is that you do not have a descent path profile.

 

Also find interesting that you're in a freighter, usually heavy, and you are at FL410. In years of flying this model I've never been light enough to get that high.

Paul and Dan,

 

No messages on FMC or EICAS. The strange detail was the LNAV/VNAV indication on ND. I had a descent profile at that point because I had prepared an ILS34R approach via MARNR5 and there were no glitches or holes along the built path.

 

The freighter was really light due to the company's shortage of 757s at that moment in Anchorage.

 

I will be back to Alaska and will report here how things went.

 

Thanks folks,