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amalishkin

Question About VNAV Behaviour

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Hello,

 

first off hats off to PMDG on this awesome bird... 

 

my question might be noob user but anyways, what i am experiencing is not what tutorial shows, i setup my bird for any flight i do use acceleration feature and Pause at TOD, however i get paused at TOD at which point i set my MCP next altitude which is below my cruising altitude, however as soon as I unpause the flight, and bird passes the TOD it does not begin descent even though its flying in VNAN/LNAV mode, is there additional step i am missing or something?  in tutorial didn't see anything stating to hit any buttons to let the bird begin descent.

 

 

Anyways it might be me missing something but wanted to ask experts first.

 

 

Thanks for help

 

 

Andrei Malishkin.

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You have to set a lower alt before you get to TOD. Otherwise the FMA will show VNAV ALT, in which case you'll have to push the alt dial after selecting a lower alt.

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In my experience with both NGX and T7, you need to set the MCP altitude before you reach T/D. If you're late, you need to give it a nudge. In T7, I do that by pressing level change button, see it start to descend and then re-engaging VNAV. I do it this way because pressing VNAV to turn it off and back on, which worked for me in NGX does not seem to work in T7 (pressing VNAV does not turn it off).


Jordan - keep in mind that pressing the altitude knob is altitude intervention and may delete the restriction (note: not sure if it happens in *this* particular case). If you delete all the restrictions, VNAV can no longer calculate the descent, unless you've entered the destination runway, as it works "upwards" in T7.

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is there additional step i am missing or something?

 

Unpause - then wind the altitude down.

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No expert.... But I would suggest ALL descent/approach planning be done at least 100 miles prior to TOD so VNAV can do its thing.

 

In your case, unattended flight, press ALT INTV after winding down the knob.... Preferably after you get winds and enter the confirmed arrival and approach (which could be different than your planned- now that several hours have passed).

 

This is not RW behavior and VNAV will need a hand.....as Alt would normally be rolled down BEFORE TOD.

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First place to look when you ask "why is she doing this?" is the FMA annunciator. Check your pitch and roll status there, it will tell you why it is holding altitude instead of descending on vnv pth.

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I think most of you missed what i typed, so to refresh my question:

 

777 in cruise flight...gets to PAUSE on TOD >>> enabled via option its about 30 or so miles out from TOD point

 

I change altitude to my descent path lets say 4500,  unpause sim

 

SIM still shows VNAV/LNAV, the mcp shows new altitude, however 777 overflies the TOD and keeps going via original altitude i have to hit FLGCH to get decent going, after which i can hit VNAV again and it will decent correctly etc.

 

just curious if anyone seen this with PAUSE option enabled etc.

 

 

Thanks.

 

Andrei.

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I think Kyle may have addressed that. Unpause and *THEN* change the altitude. I'm not sure why that would matter, but if he said it, there must be a valid reason for it.

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I think Kyle may have addressed that. Unpause and *THEN* change the altitude. I'm not sure why that would matter, but if he said it, there must be a valid reason for it.

Yes that seems to be what is important.

 

But.....the reason I am reacting.....can you change your Avatar please?

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Ill test this tonight and let you know, on NGX this definitely behaves differently.  I can set new altitude while in pause mode and it will pick it up and on TOD will begin decent.

 

 

Thanks for info.

 

 

 

Andrei.

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But.....the reason I am reacting.....can you change your Avatar please?

Why would I do that? :-)

 

 

 

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I think most of you missed what i typed, so to refresh my question:

 

777 in cruise flight...gets to PAUSE on TOD >>> enabled via option its about 30 or so miles out from TOD point

 

I change altitude to my descent path lets say 4500,  unpause sim

 

SIM still shows VNAV/LNAV, the mcp shows new altitude, however 777 overflies the TOD and keeps going via original altitude i have to hit FLGCH to get decent going, after which i can hit VNAV again and it will decent correctly etc.

 

just curious if anyone seen this with PAUSE option enabled etc.

 

 

Thanks.

 

Andrei.

 

Andrei,

 

Just tried several times where I let the sim pause before T/D, rolled the MCP down while paused, then unpaused and I cannot reproduce what you're seeing here. It descended normally each time.

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I think Kyle may have addressed that. Unpause and *THEN* change the altitude.

 

It was just a hunch.  I forget which functions (even with the default stuff) get messy with trying to adjust them while on pause, but there are a few things that won't adjust, or will show adjusted, but the sim won't pick up on it until you adjust it again after pausing.

 

e.g.:

Sim pauses

I roll the ALT down to 10,000.

Unpause

Sim doesn't react.

I flip the ALT to 9,900 and back to 10,000 and then it starts to react.

 

Again, not sure if this has ever affected any of my PMDG fleet, but it's happened before in something I've flown.

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Ryan thanks for checking,

 

more info on this, so here is whats going that i can replicate with no issues, the flight was PANC to KLAX planned flight level 350 and step to 390,  during the flight I stepped to 390, approached TOD, sim paused, i changed my MCP to new descent altitude and let it go, plane reached TOD, continued like nothing was programmed however, 15 miles out from TOD it started descent however the descent profile was off by 5000 feet, i continued the flight until LAX approach plate for 24L however i was 3000 feet over the approach level, so had to hold to lower to right altitude.  so not sure whats going on, but will try more tests.  i am pouring the over the tutorial right now to make sure i didnt miss but basics are same as NGX and had never had this with NGX, almost to me felt like TOD initiation phase begins at wrong point, like it shows right TOD for stepped up altitute, however it still calculated distance based on initial flight level or something.

 

ill keep you guys posted, but overall awesome bird.

 

 

thanks

 

 

 

Andrei. 

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I've changed MCP ALT during pause numerous times, then un-pause and descent starts at T/D normally. You step climbed from 350 to 390 on the relatively short PANC-KLAX trip? These are FMC computed steps I assume.

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