A320 difference in predictions on FMGS/ND

[IFLY_INDIGO]

hi guys,

I wonder why there is always a difference of 1 minute in the predictions on the FMGS and on ND for the upcoming waypoint...

thanks in advance

[SuperRanger]

at least for the B777, one uses current spot wind, while the other uses forecast winds entered in the RTE DATA (at least beyond 200nm but that's another story!)

SR

[Ndicho Moja]

In a nut shell......FMGS is the dream and the NAV display is the reality.

[Reimers]

The ND is a rather simple prediction. It uses current wind and GS, whereas the FMGC will use the wind entered into it for calculation beyond 130 NM. It should be more accurate, but it can only be as accurate as the data entered into it.

[kijangnim]

Greetings and happy new year

NONE OF THE ABOVE :ouch:
The ND To waypoint prediction is based on the (real-time ) bearing between the aircraft and the To waypoint, whereas the MCDU predictions are based on the FPLN trajectory, i.e active leg :ok:

[bobrun]

Could it be that the FM rounds up the minutes to the closest minute, up or down, while the ND simply display the minute regardless of the seconds?

For instance, if the eta is at 0612 and 40 secs, the FM will display 0613 and the ND will display 0612? Once the waypoint has sequenced, the actual minute is kept, so always rounded down (in this case, the FM would show 0612 for the last waypoint sequenced at 0612 and 40 sec).

:confused:

[kijangnim]

Greetings
For sure it is rounding up to the nearest minute, however the computation occurs as described above.

[Hot Rod]

Reimers description is correct. The ND is using current GS and the FMGC uses the entered winds for predictions. Sometimes the difference can be several minutes.

This also makes it good to use "abeam points" when going direct, otherwise you take away the wind predictions.

[kijangnim]

Greetings
I remind everyone that the RUMOR part of PPRUNE is NOT in the technical, furthermore ,I recommand every one then to read the FCOM, it is Black and white :ouch:

[waren9]

Without looking, I agree with Reimers and Hot Rod.

kijangnim- how about posting a fcom reference then? make yourself look very cool.

[kijangnim]

Greetings
Since without looking you agree with others, why would you need references, spoon feeding habit? :bored:
I gave info, do your homework :ugh:

[SFCC]

And learn to spell Rumour whilst you're at it.:}

[Rod Eddington]

NONE OF THE ABOVE
The ND To waypoint prediction is based on the (real-time ) bearing between the aircraft and the To waypoint, whereas the MCDU predictions are based on the FPLN trajectory, i.e active leg

That doesn't explain why theres a difference when u are direct to a waypoint (which the almost always is) which is what was originally asked. Pretty sure it's because ND assumes the current GS remains the same until the waypoint is reached and MCDU uses predicted figures based on input winds & planned changes in speed. Which is what has been said above.

[kijangnim]

Greetings,
there are no two sets of prediction computation in the FMS.
Predictions are computed using blended wind.
the wind used for that purpose is a weighted average for the next 100 nm between the actual wind and the predicted wind entered by the crew.
the FMS applies a percentage of the result as per the distance between the aircraft and the waypoint, i.e, at 100 nm the blended wind will be composed by 100% actual at 90 NM, 90% of actual wind and 10% of the weighted average as defined above, at 80....
and That wind is used to compute the waypoint displayed on the ND, but will use the distance based on the bearing between the aircraft position and the waypoint, whereas on the MCDU it is based on the active leg (which could be great circle, rhumb line, Arc ....)
This why selecting the ABEAM Function on the DIR TO is good because it will transfer the prediction wind to the abeam waypoints.

I leave the spelling to SFCC :}

[waren9]

Couldnt find word for word what kijangnim has said in FCOM, however, since he's from France he may very well be right. I did however find the following which may also help the original poster

FCOM 1.31.45 indicates that current G/S only is used to give the ETA for the next waypoint shown on the ND.

This from FCOM 4.02.20 for FMS time/fuel predictions
"IN FLIGHT : The system updates the predictions and the current ECON speed, using the measured wind at the present position. It mixes actual wind and forecast winds to compute the wind ahead of the aircraft but this is totally transparent to the crew."

And this from 4.04.25
"Once in flight, the FMGS considers the actual measured wind up to 200 NM ahead of the aircraft to permanently update the wind profile. This updated wind profile is used to compute the predictions and the performance data, but is not displayed to the crew."

As other posters before me have already said, what I have cut and pasted here may be some of the reason for differences shown between ND and FMS predictions.

Edited to add some FCOM references

[kijangnim]

Greetings

Indeed the computed blended wind is not displayed to the crew, but the ground speed for ND display and MCDU predictions is the same one since the NEXT Waypoint is common.
Please note that 200 nm is for the pegasus/legacy HWL FMS/FMS2 whereas it is 100 nm on Thales/Smiths FMS2 despite what Airbus says.
because the FMP was developped by Smiths (B737)
:ok:

[Aikon]

kijangnim,

If it uses the same wind, how can you explain the time difference between the ND and the MCDU?

You were saying the ND prediction uses the distance based on the bearing between the aircraft position and the waypoint, whereas on the MCDU is based on the active leg (which could be great circle, rhumb line, Arc ....). I have noticed the time difference even the waypoint is just 10/20NM away. How can the actual distance or great circle distance be so much different when the distance is so near?

[kijangnim]

Greetings,
I happen to know Engineers whom have worked on the FMS2, the info I have released are genuine, and some are not in the FCOM, however 1 minutes time difference at 20 nm is too much (it will mean that the distance has at least an extra 8 nm :eek: so the ground speed difference would be huge, considering the proximity of the point)
as far as I am concerned I have never witnessed such difference (Which doesnot mean that it doesnot happen) and if I had, I would have logged it and send it to Airbus.
:ok:

Now for the record the FMP Flight Management Processor performes computation (Smiths side of the FMS) and the IDP Interface and Display Processor (THALES side) displays the results on the ND and the MCDU.

[TheGorrilla]

First:

I recommand every one then to read the FCOM, it is Black and white http://static.pprune.org/images/smilies/shiner.gif

Then:

I happen to know Engineers whom have worked on the FMS2, the info I have released are genuine, and some are not in the FCOM

Make your mind up and stop spouting whatever nonsense first enters your mind! Grief!! :rolleyes:

[waren9]

as far as I am concerned I have never witnessed such difference


Well, I have and so have many others. In my last job where some of the legs were well over 200nm it was quite common to see a 2 or 3 min split between times on the ND and in the FMS. Particularly where you might be entering/leaving a jetstream between those 2 waypoints.

Do you think we should all write to Airbus?

[kijangnim]

Greetings
The point I am taking about is the NEXT WAYPOINT, displayed on the ND versus the active leg.
For the JET case, as long as you are not yet within the jet stream it makes sense, the FMS is blending the wind using an actual wind much weaker therefore the wind average is far less that what it should be, and it affects your predictions, until you are within the jetstream then ...

[CONF iture]

FCOM 1.31.45 indicates that current G/S only is used to give the ETA for the next waypoint shown on the ND
Thats it !

Best exemple is to take a first waypoint something like 200 NM after takeoff and look at the difference of ETA between ND and MCDU.
Take that reading early after takeoff when GS is still pretty low and the difference will not be counted in minutes ... but in minutes X 10
As soon as GS increases, difference shrinks.

[Aikon]

kijangnim,

The point I am taking about is the NEXT WAYPOINT, displayed on the ND versus the active leg.
For the JET case, as long as you are not yet within the jet stream it makes sense, the FMS is blending the wind using an actual wind much weaker therefore the wind average is far less that what it should be, and it affects your predictions, until you are within the jetstream then ...

I thought you said the difference is because of
and That wind is used to compute the waypoint displayed on the ND, but will use the distance based on the bearing between the aircraft position and the waypoint, whereas on the MCDU it is based on the active leg (which could be great circle, rhumb line, Arc ....)

So which is which now?!

[kijangnim]

Greetings, everyone

The least I can say is that I made you open the books, and kept you very busy :E

[scorpilot]

i still havent opened the fcoms, reading the thread has drained me!

[TheGorrilla]

i agree.... ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ :zzz::zzz::zzz::zzz::zzz:

Time to get a life.