ALT HOLD instead of VNAV ALT

[mccoy007 0]

After install the server pack for PMDG744X, I think I find out a little mistake in A/P model. When use VNAV climbing to an assigned CRZ altitude, if the MCP ALT setting is lower than the CRZ ALT which be assigned in FMC CRZ page. The PITCH Command mode in PFD will annunciate ALT instead of VNAV ALT, also on the MCP, ALT HOLD light will be enlightened. VNAV light will extinguish. But the FMC still have control to the speed. I can still either use CI value to change speed or by input a speed manually. The SPD light is not enlightened on the MCP.This seemed abnormal. If engage the ALT HOLD mode, the SPD should be controlled via MCP not FMC.This behavior I haven't meat before, and especially during the online flight, I always have to maintain a constrained altitude before I am able to climb to my CRZ ALT, and after I have clearances to a higher level, I have to use VS to control the vertical speed, the VNAV mode can not be engaged.I check the Big Boeing FMC Guide, and I find out there is a detail explain on this point, to cut the things short it could be sum up in this. "While VNAV profile guidance is constrained by the MCP altitude, VNAV ALT is annunciated."I also double check this by viewing my DVD flight video which recorded by ITVV, Virgin 747-400 Cockpit flight video from EGLL-KSFO. They had a similar situation after take-off, the MCP ALT was first assigned to FL290, not FL330 as their planned CRZ ALT, I see the VNAV ALT on the PFD, not ALT.Also in the previous PMDG744X, there isn't such a kind of behaviors, the VNAV ALT will annunciated instead of changing to ALT, if there is a constrained altitude.However this can be resolved by selecting a new CRZ ALT on the MCP.Hope you can check on this.Thanks and RegardsYufeng QI

[rsrandazzo 6,353]

Yufeng-I ran a couple of tests on this end and I am not seeing the behavior that you described.VNAV works as it is expected to.The only thing that would take the airplane into ALT hold as you describe would be pilot action, intentional (hitting the button) or non intentional (a set of key commands transmitted by a hardware device, some type of scripting device, or an inadvertent keyboard combination).Fortunately, the airplane cannot be controlled by $20 computer hardware in the real world. ;-)

[Guest Junius]

I am having the exact same problem after installed the service pack. The VNAV light on the MCP and VNAV ALT annunciation in PFD will turn off after passing the transition altitude defined in FMC, then the MCP will automatically switch to V/S mode instead. Oddly the SPD light is not illuminated.This never happen to me before.Regards,Junius.

[rsrandazzo 6,353]

Junius-Can you give me some parameters I might use to try and reproduce what you are seeing?I've just tried and had no luck.Also- it would help me if you could be more specific in describing your modes, too. Three is no way for the airplane to cross the transition altitude while in VNAV ALT mode, because VNAV ALT mode is a level flight process. If the airplane were climbing or descending at a very high rate, it might switch to VNAV ALT during the level off- but i don't think that is what you are describing...Do you have a specific airport/departure/flight plan you are following where this happens consistently?I've made a half dozen attempts to duplicate what you are seeing, and I'm just not seeing it. I'm also not seeing how it is possible based upon what we know of how it works.

[ccaughie 0]

I saw exactly the same thing last night when going through type rating course #4, part 5. I think I followed the instructions to the letter (so if you do the same you should be able to replicate the problem!), and was very confused when after reaching 6000' (which was the MCP ALT setting) VNAV disengaged entirely, the ALT HOLD button illuminated and the PFD said ALT instead of VNAV ALT.Not only that but nothing I did allowed me to re-engage VNAV, even after raising the MCP ALT. I had to use FL CH to get up to cruising altitude and only then could I get VNAV back on.I've installed SP1, but I never tried this lesson before installing it so I don't know if it would have done this before.Hope this helps,Colin

[mccoy007 0]

Hi.Take a look at thisAs you can see from here.I will give you more images later, because I just got up...Yufeng

[mccoy007 0]

Here's some moreAs you can see from the picture above, my MCP constrain the ALT at FL290, not FL350 as my planed, it used to have VNAV ALT instead of ALT. This should be a bug, because if the ALT HOLD engage, I can not control the SPD in FMCThen in the following picture I continuous my climb. You can see I can control the plane's speed in FMC, or speed window. I am not able to reengage the VNAV now.Then if I redefine a CRZ ALT in FMC, the VNAV will automatic come back...So... you can see there should be some logical problems on this.Hope you can take a look.Regards,Yufeng QI

[Guest Junius]

Look through the pictures it seems like I am having the same problem as you, my MCP has the same behavior during climb and decent. I have tried few differents flight plans (build by FS Build 2.3) and this happened everytime.I remember everytime the VNAV mode would stay normal until approximately 15000-20000ft then automatically disengaged and turn to the V/S mode, so I suspect the the source of problem is the FMC's TRANS ALT number (default set to 18000ft). I'll intall the 2.10.0040 update and try again to see if problem still occur.Regards,Junius

[ccaughie 0]

I just tried running through the TRC 4 lesson again (part 5: "Let

[mccoy007 0]

This shouldn't have anything to do with Flight plan but the constrain of the altitude in certain waypoint and the flight PATH.You can see in my example the VNAV disappear because I set a constrained FL instead of the CRZFL which being per input into the FMC. It shouldn't disengage the VNAV mode.And what is strange here is VNAV mode seemed to be partial disengage, the speed still being control by FMC, but the vertical part is override and to be controlled manually. If VNAV completed disengaged I should be able to control the speed on MCP and no need to override manually by click the speed knob.Of course if everything in tutorial way there will be nothing wrong, but during the online/real flight we have to follow the ATC...

[Guest jumbojock]

Strange seeing V/S with the speed window closed.David "jumbojock" Robertson

[Guest kiwiwanabe]

For what it's worth...I've seen the same behavior on 2 out of 2 flights since installing the update....Kind regards,-Russ

[mccoy007 0]

The latest SP, which is released on MAY2708,747-400X V2.10.0040 still have this issue, seemed only fix the issue which is listed on the website, not including VNAV ALT, so are you working on this now, PMDG group?Yufeng

[Guest mukesh]

I have a same exact problem after i update in PMDG FS2004 747-400.do you think i uninstall and reinstall most current version of this product and does not require any update to current can solve the problem.Thanks,Mukesh

[mccoy007 0]

I don't think so.I think we'd better wait for PMDG developing team to answer this right now, since this may be caused by the code update.Hoping they can solve this problem as soon as possible.RegardsYufeng