Jump to content

777 fmc fix page


Vic1

Recommended Posts

I am having issues getting the pred eta-alt function to work correctly. specifically the option to enter an altitude or flight level. this sppears to malfunction .sometimes no green circle appears at all on the majenta line other times it does but in a position that bears no accuracy or is even close to the point it should be and additionally this point keeps jumping forward and remains a fixed distance from the currant ac position.in other words the aircraft never reaches it. this is also confirmed on the fix page under pred where this distance remains static . Im not sure if im doing something wrong or if its a bug?

 

tks

 

 

kav

Link to comment
Share on other sites

  • Commercial Member

 

 


Im not sure if im doing something wrong or if its a bug?

 

My bet is that you're just not doing something correctly, since I've never had an issue with it.  Hopefully someone else can confirm.  I might be able to tonight.

 

Have a look at FCOMv2 11.42.35 and .36 for a little explanation.

Kyle Rodgers

Link to comment
Share on other sites

i have looked at fcom  already yes. and following those instructions i cannot achive what is described.logically in my mind..surely the green arc should align precicely over the top of the predicted altitue green circle if that altitude is identical to what is in the mcp altidude window during either climb or decent am i right?..............this is not however what i am seeing in my sim.not even close even when the climb or decent is stabilized and the green arc is stable on the majenta line

 

during climb as described in my first post above and during decent the green circle apears along the track in no logical locations.and the ac passes that point way before it reaches that altitude.yet despite this the entry disappears as the ac passes that point even though the actual altitde may be many thousands of feet above it.

 

 

tks

 

kav

Link to comment
Share on other sites

 

 

surely the green arc should align precicely over the top of the predicted altitue green circle if that altitude is identical to what is in the mcp altidude window during either climb or decent am i right?

 

Hi,

 

If I'm right and according to what I see, The green arc and the prediction circle don't constantly match over time. Sorry for my english, I mean the green circle is reflecting the point your are going to reach the desired altitude/flight level with your current vertical speed and is constantly updated. The green circle prediction is the point your are going to reach the desired altitude/flight level taking into account speed and altitude restrictions that are entered in the MC (either in leg page or the 250kt/10000ft) 

It becomes clear in the exemple below:

 

You are at FL200 descending to 5000ft. You set the prediction fix box to 5000ft and the altitude on the MCP to 5000ft also. Normally, your green arc should be displayed closer to your position than the prediction circle because you are currently descending at let say 2000fpm but the green arc doesn't take into account the reduction to 250kt at 10000ft while the pred circle does.

So you will have a discrepancy most probably until you have passed 10000ft. Then both should match again.

 

At least it is what I see on my sim. Maybe I'm wrong?

Romain Roux

204800.pngACH1179.jpg

 

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

Link to comment
Share on other sites

i hear what you are saying yes .taking in to acount constraints etc etc but this still isnt explaining what i am seeing unfortunately.

 

Give it a try at an altitude where there are no fmc constraints or waypoints etc .just doesnt work for me

 

 

kav

Link to comment
Share on other sites

Hi, 

 

Ok sorry, in my case it works fine. I just did a flight yesterday from KAUS to SANT yesterday and I observed also discrepancies during the climb from 12000 (after acceleration) to FL330 but it is because the green arc doesn't reflect the expected loss of vertical speed while you are gaining altitude and these discrepancies appear when your set altitude on MCP and fix pred is far from your actual altitude.

If I set the MCP and fix pred only a few thousand above in stable climb, they match as you won't have a lot of change in your vertical speed.

I also always enter the wind datas as accurate as I can. Maybe is is also a factor, specially for the descent?

 

A question, if you set your fix pred altitude to the cruise altitude during a climb, does it match with your T/C?

Romain Roux

204800.pngACH1179.jpg

 

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

Link to comment
Share on other sites

Hi,

 

I have just taken two screenshots to illustrate what I have and I explain above.

 

I have removed the fix pred because it is superposed to the T/C.

Is it what you have?

 

Discrep%20prd%20fix.JPG

Discrep%20prd%20fix2.JPG

Romain Roux

204800.pngACH1179.jpg

 

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

Link to comment
Share on other sites

thanks for the shots.ive not tried that scenario myself but anyway thats not the situation that im talking about really.   i am finding the aircraft is reaching the pred alt circle in a stable climb yet no where near the actual pred  altitude.....and in a climb the pred alt circle keeps moving along the majenta line and the aircraft never reaches it despite reaching that altitude in reality. .stuff like that.

 

 

 

so this makes me ask the question to anyone that knows out there....what is this feature actually meant to be used for in a real 777? and can you use it accurately when climbing or decending in v/s or does it only work when in vnav? and if so whats the real point of it?

 

 

tks

 

 

kav

Link to comment
Share on other sites

Ok, sorry to misread your issue, just trying to help: )

 

Regarding the function, I'm not a pilot but the use I see in sim is to know where your vertical profile will cross an altitude between two waypoints.

Let's take an exemple:

My FMC leg page tells me I'm gonna cross Osuka at fl260 and the next waypoint Limla is already at fl300... where am I gonna cross fl280? Of course, it is in the case the pred is accurate.

It can be useful to check if your are going to meet a restriction when it is not at a waypoint specified in your leg page, like when sometimes given according to a DME.

 

I let real pilot or more informed simmers confirmed that point.

 

To conclude, I'm sorry not to be able to help you more to solve your issue.

Romain Roux

204800.pngACH1179.jpg

 

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

Link to comment
Share on other sites

no probs and thanks for your reply.

 

so as far as i can see this function is mainly used in order to see weather you will make a crossing restriction from atc/ now thats fine when you are desending in vnav  but if it shows you wont make it at the currant desent rate you will need to leave vnav and adjust your rate manually in V/S. therefore the pred -alt function should work flawlessly in what ever mode your are currantly in and constantly adjust to that particular mode.in other words the green circle should move back or forward along the track to account for your adjusted decent rate.and smoothly. and also the calculated distance to that point should change too in the fmc so you can adjust your decent to reach that particular alt at a particular distance away as is often the atc instruction .

 

but this is unfortunately not what i am seeing in the sim at all..........so this is what makes me ask the question.what is the purpose of this feature if it doesnt work this way.or is it broken in the pmdg 77 and should work this way.or am i using it wrong?

 

 

any realworld pilots of the 777 out there i would really appreciate your advice and thoughts on this

 

tks

 

kav

Link to comment
Share on other sites

I guess that if you leave the vnav mode to another pitch mode, the system "thinks" you don't want to follow the vertical profile anymore, so you don't need prediction related to your vertical profile. In that case you would rely only on the green arc.

I'm curious of what real pilots would tell though.

Romain Roux

204800.pngACH1179.jpg

 

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

Link to comment
Share on other sites

T/C and T/D circles are computed based on performance, weight etc etc. With everything filled in correctly, including winds, you will be very close to reaching cruise at T/C. Not only that but you can see in the LEGS page at about what altitude you cross every point along the route. This is for top of climb and tops of descent calculations only. 

The green arc gives you distance to go to reach any MCP entered altitude. It comes in handy in ATC requests and to visualize/plan you visual approach.

 

The green arc is most used. The T/C only gives me where I will eventually level off if all goes well. The green arc is a more ad hoc short term planning tool.

 

 

Xander

Xander Koote

All round aviation geek

1st Officer Boeing 777

Link to comment
Share on other sites

  • Commercial Member

I guess that if you leave the vnav mode to another pitch mode, the system "thinks" you don't want to follow the vertical profile anymore, so you don't need prediction related to your vertical profile. In that case you would rely only on the green arc.

I'm curious of what real pilots would tell though.

 

Bingo.

 

 

 

so this is what makes me ask the question.what is the purpose of this feature if it doesnt work this way.or is it broken in the pmdg 77 and should work this way.or am i using it wrong?

 

I'm getting the impression it's just that you don't understand the purpose of this tool.  Its purpose is to show you where you'd hit a particular altitude along your track, given your LNAV and VNAV Path assumptions.  That's it.  Nothing more, nothing less.  If you were using V/S mode, you could clearly see the predicted altitude by using V/S or FLCH with the green banana and selecting the appropriate altitude.  What you were giving an example of is a function already covered by the green banana.

 

As far as going off an LNAV path, how is the system supposed to know how you plan on getting from Point A to Point B (and where it should draw the green circle)?  How it is to know that you're not going to make S-Turns all the way down?  It doesn't, so it can't give you any info there, either.

 

It's only there to depict where/when VNAV is planning to put you at a particular altitude.  Otherwise, if you're in a different mode, that's all under your control, and is visible with the banana.

Kyle Rodgers

Link to comment
Share on other sites

the only problem with the green banana/ arc is that it can be hard to accurately judge the track distance when its between distance/radius arcs on the nd.

 

hence why i wondered if you could use the pred alt feature to do this more accurately . clearly not though. yes it will give you the distance but there is no way of using it to accurately adjust your decent or climb rate in vnav so that distance is also not ajustable in vnav.

 

"As far as going off an LNAV path, how is the system supposed to know how you plan on getting from Point A to Point B (and where it should draw the green circle)?  How it is to know that you're not going to make S-Turns all the way down?  It doesn't, so it can't give you any info there, either."

 

in exactly the same way it knows where to place the green banana/arc.......and it could calculate where that arc crosses the majenta line  in that present moment and state the distance away from currant pos and that would be very useful and kyle i never mentioned going off the Lnav path i said vnav path in order to ajust desent rate in V/s

 

 

so with that said.......what is the best method to use when you are given an atc crossing restriction.eg desend to fl210 by 30 nm. ideally it would be great to be able to place a marker on the majenta line thats 30 nm ahead of currant pos.......and then adjust your decent rate using the green banana/arc so it falls prior to that mark otherwise i find i tend to lose sight of that imaginary point and nearly always miss the mark and bust my altitude or end up diving as fast as i can because i lose track of that point and it becomes a roiugh guess. either way its not ideal.

 

 

so how to do this?

 

kav

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...