VNAV not functioning

[sddjd 117]

Can anyone provide some tips on why my VNAV button will not operate? I've entered the route/legs including spd/alt settings per the instructions. LNAV operates properly; I'm sure I've missed something... I've also received "crz alt unable" (or close to that), though in the last case it was only set for 10500. ??

[Mats_J 2]

Hi nameless, ;-)Please sign your posts with your real name as per the forum rules pinned at the top.With this little information it's hard to say what's going wrong. I would suggest you to download and go through Tim Metzinger's excellent tutorial/check-out course. It's a pretty big download but well worth the effort. You can find it here:http://library.avsim.net/sendfile.php?DLID=43841After going through the five stage course I'm sure you'll be able to tell what was wrong with your VNAV! :-)Hope it helps,

[Noel 4,494]

I have the same issue. VNAV doesn't want to operate. I'll bet someone here knows what is probably amiss. I load up gross wt, route, clmb and cruz, and all works fine, except VNAV doens't activate.Cheers,Noel

[AirbusA340300 0]

I had that happen today on a flight i did. VNAV said alt ubable, it was the first time it happened, but after i climbed past 10000 or so i was able to engage VNAV, i dont know, maybe it was a SID restriction or something.Adam Kaleniak

[Guest Olivier]

I thinkg VNAV will not work for two reasons--> The MCP is not set at the right altitude. VNAV will never make the plane go higher than what is set on the MCP. If your cruise level is at 33000 feet, then set your MCP to 33000 feet before clicking VNAV (or set it to the next restriction). It does not seem to be the case though.--> It seems that, in your case, it can be unable to reach its altitude. There can be many reasons. Perhaps, the cruise level is too high considering the length of the flight. The speed can be too high and the plane will simply not be able to climb at that speed (not enought power).I would suggest you to check your climb speed (usually 293 knots). Also, make sure your flight is long enough for your cruise altitude (if you put FL330 for a 100 NNM flight, it will not work).Also, sometimes, there is a altitude restriction that is impossible to match. For instance, if you put a 20000 foot restriction to your first waypoint and this waypoint is 10NM from the runway, then VNAV will tell you that it is unable to do this.Good luckOlivier

[Noel 4,494]

Ahh, very nice Olivier. I will follow up with these considerations.Cheers,Noel