A320 mcdu predictions
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Joined: Jan 2013
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From: usa
A320 mcdu predictions
The mcdu is used for long term predictions when we fly in full managed. The question is, it uses the entered wind for predictions. Supposing the winds are different than entered by how much will the predictions differ. The optimum altitude is catered for by current conditions, what about fuel and constraints?
Next. Does the airbus assume anti ice ON descend and calculates TOD? Will there be a marked difference in profile with eng anti ice on/off...any relation to idle factor+delta?
Next. Alternate predictions in engine out, we must revise the alternate fuel right? Any way to change the default alternate crz altitude from 310 and 220 in wind page?
Next. Does the airbus assume anti ice ON descend and calculates TOD? Will there be a marked difference in profile with eng anti ice on/off...any relation to idle factor+delta?
Next. Alternate predictions in engine out, we must revise the alternate fuel right? Any way to change the default alternate crz altitude from 310 and 220 in wind page?
Joined: Feb 2000
Posts: 4,705
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From: 500 miles from Chaikhosi, Yogistan
Can't give you an answer as to how much they are different as every case is unique, but have seen it around 4 minutes difference in ETA from the ND to the MCDU on a reasonable sector. The wind uplink function erroneously loaded ~100kt winds as quartering tailwinds instead of actual (predicted) 75 kt headwinds.
Joined: Aug 2008
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From: Far from a continent.
Mcdu does not take anti ice into consideration for descent, but a fully managed descent gives room for airspeed increases to cater for the anti ice. This is usually enough.
As for the winds verses actual. Whatever you load into the Mcdu will be taken as gospel by the computer. Rubbish in = rubbish out! Correct the loaded winds if a marked difference is apparent. Update the isa difference to actual for a more accurate opt/max prediction.
ND uses actual wind for prediction. Always reliable.
Hope this helps.
ps. no way to change the alt crz lvl. Play it safe, add extra couple 100 kilos for diversion gas!
As for the winds verses actual. Whatever you load into the Mcdu will be taken as gospel by the computer. Rubbish in = rubbish out! Correct the loaded winds if a marked difference is apparent. Update the isa difference to actual for a more accurate opt/max prediction.
ND uses actual wind for prediction. Always reliable.
Hope this helps.
ps. no way to change the alt crz lvl. Play it safe, add extra couple 100 kilos for diversion gas!
Last edited by cameljockey; 24th January 2013 at 04:42.
Joined: Dec 2000
Posts: 141
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From: Canada
FCOM volume 4 has all the information on how the FMGC works with winds. There is an integration formula that applies a weighting factor of current winds as measured to entered winds based on distance. This is considered when looking at OPT altitudes, but remember the FMGC is a very, very primitive tool for OPT altitudes due to very limited data and simple algorithms when compared to a flight planning system. Sorry I don't have the exact reference at hand at present. There is also a good explanation of MCDU time predictions and their relationship to what you see on the PFD in the FCOM too. The simple answer is PFD uses current ground speed and the MCDU uses entered data, with corrected wind adjustments.
