A320 difference in predictions on FMGS/ND
Beau_Peep
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A320 difference in predictions on FMGS/ND
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
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
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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.
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Greetings and happy new year
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
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
Last edited by kijangnim; 9th Jan 2009 at 14:33.
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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).
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).
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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.
This also makes it good to use "abeam points" when going direct, otherwise you take away the wind predictions.
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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
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
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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
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
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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
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
Last edited by waren9; 13th Jan 2009 at 21:05.
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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)
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)
Last edited by kijangnim; 14th Jan 2009 at 04:17.
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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?
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?
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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
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.
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.
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
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.
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.
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First:
Then:
Make your mind up and stop spouting whatever nonsense first enters your mind! Grief!! 
I recommand every one then to read the FCOM, it is Black and white 
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
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as far as I am concerned I have never witnessed such difference
Do you think we should all write to Airbus?