Hi, I just Copied and paste from AIRBUS document (.pdf) named WISE
The original doc is very clear to your answer, but here is looking a little bit difficult to read. If there is a way in PPRUNe to send original .pdf, I can
What can explain an Estimated Time of Arrival (ETA) discrepancy between NDs and MCDUs?
SolutionActually, the FMS has two different channels to compute the ETA. One is used for display on MCDU, andthe other on ND. That explains why a difference may exist between ND and MCDU:- Time displayed on ND is: CFDIU time + FMS 'Time to Go',- Time displayed on MCDU is: FMS time + FMS 'Time to Go'.In the case both MCDUs show different ETA compared to the ones on both NDs, it may be explained bythe two below causes:- First, the CFDIU time is based on the clock time or the internal CFDIU time (depending on the validity ofthe clock time).Whereas the FM time is based on the clock time and/or GPS time (depending on the validity of the clocktime and the GPS time).- Moreover, ND display is based on 'Time to Go' computed by the FMS assuming a direct distance fromthe aircraft position to the next waypoint, whereas the distance displayed on the MCDU assumed adistance along path which can be very different in some specific cases.In the case both MCDUs show themselves different ETA (between MCDU#1 and MCDU#2), it may point toa prediction computation discrepancy between FM1 and FM2. Such a difference could have been inducedby:- A distance difference induced by 2 different paths (different transitions for example),- A reference time difference (clock or GPS time),- Different cruise winds between FM1 and FM2,- A frozen time reference on one side,- A refresh issue on MCDU flight plan page...An In-flight Print can be very helpful to further analyse this kind of event.Additional NoteSee also:The ETA Airshow was about 20 minutes different from the ETA on the MCDU.FMS issue GPS Predictive - Anomalies on ETAA380 ETA parameters not sent from FMSEstimated Time of Arrival (ETA) for the TO and-or the NEXT set to zero in ADS report