Althought there have been a number of posts reporting problems
with VNAV, allow me to add myself to the list of complainers.
Since I got the software, I have done 6 flights, with the VNAV
function performing once correctly. The first time I used
it, it was with the FMS tutorial. The plane refused to climb
above 3400ft (alt at MCP was 6000). In most of the cases,
weird things happen. I engage VNAV above 400AGL as suggested,
and most of the times above 1500-2000 AGL. The plane usually
does unbelievable things. First, even without the engagment of VNAV, TO/GA power seems to all the sudden die at 1500AGL (and
I don't mean because of the CLMB mode). Manual throttle is
applied, but ussually the plane is in stall territory.
Let's forget that and let's say I engage VNAV at 500AGL. Then,
I click AP on and guess what, HDG is engaged, Level change is engaged and TO/GA mode disengages with the engines throttling back to idle. It has happened a number of times.
Since I believe that most people are going to write this off to
my inexperience, let me tell you that I get a purple track in the map mode, everything is actived in the fltplan, I have an active flight plan. It is 2AM EST in US and I am still trying to figure it out.
I would appreciate any input on the matter. For the development
team, is this a known issue or not. If it is not, could you please provide us with a flightplan checklist, that would ensure that VNAV works as advertised? What is it that we have
to make sure it happens so that VNAV does not give us grief?
If this is related with an erroneous flight plan, why then the
engagement of the autopilot triggers the engagement of FLT change and HDG given that VNAL and LNAV are not engaged.
Again, please provide some input.
P.S. VNAV works fine after you level off at the first altitude