captain420

Why won't my 777 Descend?

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I am flying with Pro-ATCX and its requesting me to descend down to 26000 feet... My plane currently in LNAN/VNAV mode... I dial the requested altitude 26000 in the MCP and push the button in, but the plane doesn't descend at all. Am I suppose to hit FLCH instead? I'm confused. I remember doing it how I normally do and thats to dial my desired altitude and push the button and the plane will start descending, but this time it didn't at all and I'm not sure why.

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4 minutes ago, captain420 said:

 My plane currently in LNAN/VNAV mode... I dial the requested altitude 26000 in the MCP and push the button in, but the plane doesn't descend at all. Am I suppose to hit FLCH instead?

If you were in both LNAV and VNAV, what button did you push in if both of those buttons were already on? 

You could go down using either FLCH or use VS. Worst case, take manual control and fly the plane down to 26000.

You might want to read the manuals that come with the aircraft as they will give complete descriptions of what the different autopilot functions do and how they operate, that way you don't get caught off guard next time you need to make a course correction or altitude adjustment that was not previously programmed into the FMC.

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The same knob where you dial in the altitude, thats the knob that I push in to initiate the descent. I have been doing it this way for ages, but this time it didn't work for some reason.

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Pete, I don't recall... but wouldn't setting the altitude in the MCP and pushing in the button to initiate a descent override what's programmed in the FMC?

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Two options.

1  Take your plane out of VNAV and descend using FLCH or VS

2  go to FMC descent page and select des now  This is presuming that ATC is descending you prior to your fmc calculated TOD

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Shawn, yes ATC is telling me to descend about 80 miles before my actual fmc calculated T/D.

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"the button"  just is an alt intervention that you would use to void your next alt constraint in the fmc (while climbing through a plan for example, or to keep you from leveling off at a constraint if you need to reach an assigned altitude.)  I don't believe it does a thing if you are in level flight when you hit it.  all this is to the best of my knowledge, which is not complete.  

Generally I ditch VNAV and use the FLCH and vs modes to stay ahead of the descent path if possible, but using des now (and then execute) will start your descent on a v/s to intercept the vertical profile.

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So if I'm currently at FL350 and ATC tells me to descend to FL260, whats the best way to do this? Go to the VNAV page and hit DES NOW or dial the ALT to 26000 and then hit FLCH?

So you're saying if I do the former method, it will stay within the altitude limits programmed in my FMC for each of the waypoints?

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I don't know about the 777 as I don't own it in P3D, but in the 737 if you override the FMC and try to descend the Off Path Descent light will come on, you must always use the DES NOW button if given a lower ALT by ATC.

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4 hours ago, captain420 said:

So if I'm currently at FL350 and ATC tells me to descend to FL260, whats the best way to do this? Go to the VNAV page and hit DES NOW or dial the ALT to 26000 and then hit FLCH?

So you're saying if I do the former method, it will stay within the altitude limits programmed in my FMC for each of the waypoints?

You dont need to hit FLCH when you select DES NOW. However what I generally experienced in this case is that descend rate will be so small and shortly after your FMC calculated T/D point, you would be following the original FMC descent path.  

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16 minutes ago, costamesa said:

what I generally experienced in this case is that descend rate will be so small and shortly after your FMC calculated T/D point, you would be following the original FMC descent path.

That's the idea...

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It is well known and documented the ProATC-X is not that accurate on when your aircraft descends.  It could tell you to descend from FL240 to 160 and that would be wrong as the AIRAC and FMC's will tell you something completely different.  But, if ProATC-X tells you to descend, you need to initialize the descent.  Normally, if the FMC is properly programmed, it will begin the descent based on the info it received in the latest AIRAC from Navigraph or Aerosoft as in real life.  If you see the aircraft is trying to descend and ProATC-X hasn't told you to descend yet, you need to go into the ProATC menu and select Other Requests and then Request to Initialize the Descent.  ATC will approve it and you can change your altitude to what ever ProATC controllers tell you.  Just don't exceed what is shown on the FMC as they had already stated the flight levels are not accurate.  You could slam into a mountain or tall building if you followed the instructions of ProATC!

If the aircraft does not begin to descend and the FMC and ProATC states you need to be at a lower altitude, then you simply click on the VS key and lower the rate of descent to whatever you feel is appropriate.  When you get to the required altitude as shown in the FMC, just hit and enable VNAV again.

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Jim, thanks for that info. I wasn't aware of that at all. I hope they will address this issue as it seems like an important one. 

I am still on AIRAC 1706... Too lazy to update all my apps. So with your method, is this realistic at all? I am trying to make my flying experience a bit more real, which is why I decided to use PATCX. But now that you mentioned that it's descent is flawed, I'm not sure if I should stop using it until they fix it or just simply use your method.

 

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Every possible option has problems.  I use pro atc x Since it is reasonably accurate and gives an element of realism in that it can change your star on the fly as weather changes, and give you vectors when needed. You are the pilot though, and if the descent is incorrect, you can request differently.  I haven't had too many issues on my flights.

 

 

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I also find that often times during approach, PATC will tell me to descend, but I'll end up not capturing the GS because the descent that it gives me is not low enough.

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22 minutes ago, captain420 said:

I also find that often times during approach, PATC will tell me to descend, but I'll end up not capturing the GS because the descent that it gives me is not low enough.

Please post the issue on the ProATC support forum.

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1 hour ago, captain420 said:

Jim, thanks for that info. I wasn't aware of that at all. I hope they will address this issue as it seems like an important one. 

ProATC-X is updated once a year and believe the next update is March 2018.  If they "fixed" the issue for one flight plan, it would not be fixed for another member using another flight plan so they just provide descent levels and hope you just follow the FMC data (or your charts).  They are pretty close to the top of descent shown in the FMC but cannot see how they can perfect that.  In the real world, you or your co-pilot would be monitoring the charts and navdata and, if ATC is not telling them to descend, they will contact them for instructions and ProATC allows you to do this by requesting that descent be initialized.  Fun stuff!

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in my experience as long as you are flying a STAR pro-atc clearances should generally match the altitude restrictions in there. in fact they match a little too closely as some arrivals it will give you clearances every few nm to match each restriction which leads to the vnav wanting to level out at each restriction even if you intend a more constant descent. it is safe to assume that you will get those clearances though so generally you can just fly it like normal.

need to be careful if it is vectoring you however, as it will gladly put you into a mountain in that case :biggrin:

cheers,-andy crosby

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16 hours ago, captain420 said:

I am flying with Pro-ATCX and its requesting me to descend down to 26000 feet... My plane currently in LNAN/VNAV mode... I dial the requested altitude 26000 in the MCP and push the button in, but the plane doesn't descend at all. Am I suppose to hit FLCH instead? I'm confused. I remember doing it how I normally do and thats to dial my desired altitude and push the button and the plane will start descending, but this time it didn't at all and I'm not sure why.

The method you are doing to try and descend is correct and I am not sure why it didn't work for you. Did it only do it once or can you duplicate it? Selecting a lower altitude in the MCP altitude window and pressing the selector should initiate a descent in the same manner as selecting DES NOW on the FMC VNAV DES page. It will go into a cruise descent until intercepting the normal descent path or the selected MCP altiude (whichever occurs first). Is this the Capt Sim 777 or PMDG one?

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Steve, I'm using the PMDG 777.. it's how I've always initiated a descent. So I had to descend the plane by hitting FLCH in order to get it to go down. Cause the way I've been accustomed to doing it doesn't seem to work last night on my flight. 

Do you guys usually start descending when you see T/D shown on your MFD? Or do you descend a bit earlier? For those flying without ATC that is. Curious to know what others do in this scenario. When flying offline and without ATC, I usually initate my descent perhaps 10-20nm before the green T/D circle.

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Was doing some reading on some older posts, and I guess your issue might have been that the alt intv button isn't active before 50nm from tod?  I just read that on another post and have been trying to verify in the docs, but an fcom and fctm isn't an easy place to find a quick answer, lol

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If that's the case then that would explain why it didn't work. I believe PATC had me initiate a descent about 80+ nm out from the actual T/D that shows up on my MFD.

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If more than 50 nm from TOD VNAV will begin a cruise descent when pushing MCP alt selector (at ECON cruise speed not descent speed) .  A new T/D will be generated if the altitude is captured well before the descent path.

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