Why no VNAV ALT?

[Benjamin J 751]

Recently, the PMDG 747-400 has started behaving very weirdly regarding VNAV. When reaching the altitude that's set on the MCP and VNAV is enabled, the 747 goes to ALT mode, not VNAV ALT. I then have tea difficult getting VNAV engaged again, sometimes I can't get it engaged at all after it has leveled off in ALT mode and I have to climb using flight level change. For as far as I know, everything entered correctly into the FMC also. What's wrong? It used to work very well. Must say this really bumms me out... Thought I had everything working in FSX, but now stuff anyway seems to fail...

[sheepfu 3]

Does your MCP altitude match the altitude shown on the CRZ page?

[Benjamin J 751]

Well no, but that shouldn't be a problem. I follow ATC's instruction to climb to FL so-and-so, and as such I set the MCP altitude to whatever they clear me to. The plane then levels off at that altitude as per ATC's instructions. At that point, it should go into VNAV ALT mode instead of VNAV SPD, but instead it goes to ALT mode. So, VNAV is completely disengaged! Then when I want to climb again, I can't reengage VNAV, but I have no idea why it disengages in the first place when leveling off. Also when at cruise altitude I can't re-engage it (at this point it should be in VNAV PTH).

[vc10man 732]

Benjamin, as per the previous answer given to you, if the altitude is not set correctly in the FMC, you will experience problems. I'm aware that you are a fan of the default FSX ATC, and even if as you say you are following the ATC instruction to be at a certain FL, why don't you just put that FL in the FMC. That way you will eliminate any VNAV(which takes its info from the FMC) ALT issues. I've just completed 100 hours(more than that in total) with the both versions of the 747-400/F as I cycle through my hangar trying to build up logbook hours, and I've not come across this problem, but then I do not use the default ATC. Instead I rely on FSC+EFB combination.

[Benjamin J 751]

No, I'm sorry, but that is not true. I have been flying like this for ages, without setting every constraint ATC gives me in the FMC, and it has never, ever been a problem. The A/P should go into VNAV ALT, until I reset the altitude on the MCP to the next flight level. It then used to automatically go into VNAV SPD, until it reached the next ATC given constraint. then finally, at cruise altitude, it would go to VNAV PTH. If you wish, check the tutorials (specifically type rating course 4) by RSR for the 747, and you'll see that he does exactly this, except that it works for him beautifully. Heck, he has an entire chapter devoted to VNAV ALT, saying exactly when it should activate: when the crew intervenes in a climb or descent path by setting an altitude in the MCP which is different than the altitude VNAV planned to climb or descent too, which is what I'm doing. Again, what the plane is doing is that at level-off, it disengages VNAV completely and engages the normal ALT HOLD mode, which is not what should happen. it should activate VNAV ALT mode. This is not a problem of default ATC or whatever, it is a problem in the plane or the way I set up the FMC, but it should work fine. EDIT: I might have found something in previous threads that could explain this strange behavior. I follow up on that and post here what happens. But, if anybody has a solution, you are of course more than welcome to post!

[Benjamin J 751]

Well that's strange :S The FMC reminded me to reset the MCP ALT, and when I did, VNAV PTH engaged and started descending the aircraft, as if it was on all along, even though I was in ALT HOLD mode all the time... Gonna test this a bit more over the coming weeks. So, I'm watching the plane reach FL230, where there is an MCP set alt restriction. It switches back and forth between VNAV PTH and ALT, going completely crazy. VNAV PTH sort of wins, and continues the descend passed the alt restriction, but ALT mode is suddenly engaged! I then lower the MCP altitude, and see VNAV PTH engage again. I have absolutely no idea what is happening, but a reinstall might be in order... Also, the altitude warning horn is going off like a madmen. EDIT: Opened a support ticket. This is so weird that I'm quite sure now that there is a problem with the software. Hopefully support can tell me what to do...

[vc10man 732]

Thanks for those updates. Look forward to your further report into this as this is certainly an odd behaviour that I myself have not particularly seen with my 744, but is worth following for the future.

[thebigmanwiththeplan 0]

Hi,I had this very same problem and I fixed it by doing the following. Firstly load a default cessna at your desired departure airport. Next, turn off everything in the cessna; fuel, magneto, battery etc. Then open the aircraft menu and load the PMDG 747 and you should be set.Hope it works.

[Benjamin J 751]

So I had contacted PMDG support via a support ticket, and in the end a simple reinmstall cured the issue. I also reinatlled FSDT KLAX, As I was getting some strange couatl error messages, whiuch disappeared after reinstalling FSDT KLAX. So, all set to fly again.