Why won't my 777 Descend?

[captain420 3,099]

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.

[cmpbellsjc 2,960]

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.

[captain420 3,099]

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.

[pete_auau 1,158]

what  was  the fmc  alt  set  at

[captain420 3,099]

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?

[ShawnG 1,823]

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

[captain420 3,099]

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

[ShawnG 1,823]

"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.

[captain420 3,099]

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?

[SierraHotel 976]

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.

[costamesa 88]

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.  

[skelsey 765]

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...

[Jim Young 2,356]

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.

[captain420 3,099]

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.

 

[ShawnG 1,823]

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.