Different ETA ND/MCDU on Bus

[Dutchman]

Why is there a difference of 2 minutes sometimes in the ETA displayed on the MCDU for TO waypoint and the ND TO waypoint (top right)? and which one is correct as causes quite some confusion giving estimates

[safelife]

The correct one is the one on the ND as the one in the MCDU is rounded up to the next minute.
Prove: when you sequence a waypoint the time in the MCDU will drop by a minute in most cases.

[CONF iture]

The correct one is the one on the NDNot really (http://www.pprune.org/4650259-post22.html)

[JABBARA]

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

[jtr]

Really dragging the brain cells out here but I thought it was because the ND ETA shows estimate based on current groundspeed whereas MCDU has wind factored in based on entered info.

As an e.g., going oceanic with 460+ nm between waypoints, when crossing each waypoint you needed to give the estimate to ATC based on the MCDU info as the ND would be out by a good amount if there was a wind change during the next leg.

[Airmann]

^^^yup this is what I was taught.

MCDU: uses winds entered or uploaded into the box (CLIMB/CRUISE/DESCENT) to compute GS and ETAs. I.e. previously measured or forecasted winds.

ND: uses actual/current winds (as shown on ND, furnished from ADIRS) to compute GS and ETA.

Hence ND is more accurate, uses actual conditions, MCDU uses planned/forecasted conditions

[nitpicker330]

Nope. ND simply works out current distance to waypoint at current G/S and gives you an ETA. Simple slide rule stuff.

FMGEC Is a bit smarter than that, it calculates the ETA using your Selected/managed profile ( CLB CRZ DES ) and uses the forecast winds you've entered PLUS it blends in actual winds to arrive at accurate ETA predictions for all down route waypoints including Dest.

Hence over long sectors ( 450 miles ) the ND and FMGEC Flt Pln ETA's for your active waypoint may be a lot different due to the current wind being different to the forecast up ahead of you. :ok:

Same on the Boeing's I've flown.

[nitpicker330]

I just found this in FCOM 4 regarding the ETA on the top right of the ND and I quote from the FCOM:---


Predicted time of arrival at the TO WPT (upper right hand corner of the ND), assumes that the aircraft flies direct from present position to the TO WPT at present ground speed.

[nitpicker330]

Also found this in the FCOM 4 regarding down route ETA predictions displayed on the MCDU:--

WIND PROFILE

To obtain the best predictions, the pilot must enter the wind for various flight phases and specifically for waypoints in cruise. ON GROUND : During flight planning initialization, enter the winds for climb and cruise phases using HISTORY WIND and WIND pages. You will enter, manually or with ACARS, different wind values in climb and cruise phases. The system will compute a wind for all waypoints of the F-PLN using linear interpolation between manual/ACARS entries. This wind profile will be displayed on F-PLN B page and is called forecast wind profile. Pilot or ACARS entries are displayed in large font, system computed winds in small font. The forecast wind profile will be used to compute fuel and time predictions as well as econ speed/Mach targets.


IN FLIGHT : the system updates the predictions and the current econ speed, using the measured wind at 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.

[nitpicker330]

Also found this in the FCOM 4:--

OPERATION RULES CONCERNING PREDICTIONS

— The pilot must keep the flight plan data properly updated during the flight in order to get accurate and meaningful predictions. — The pilot should rely on the navigation display for short-term predictions. It indicates what the aircraft will do under the currently engaged modes (selected or managed). — The pilot should rely on the MCDU for long-term predictions when managed guidance is active or about to be re-engaged.

ie if your active waypoint is close by and your groundspeed won't change much then you can use the ND for guidance. If you are climbing or descending into a changing G/S then use the MCDU. :ok:

[misd-agin]

Two minutes? Try up to 7-10 minutes. :sad:

Current GS vs. FMC knows future winds and mach profile.

You can see this if you're given a fix a long way away while you're still at slow airspeed. "Say XXXXX estimate". ND will lie, FMC will be more accurate.

[FLEXPWR]

Even on a waypoint relatively close, you can get 1 minute, sometimes 2, of difference between ND and MCDU.

ND indeed looks at the ground speed, while MCDU make predictions based on last overhead waypoint wind (stored), blending with actual for the current waypoint estimate.

Even when the clock time from GPS and MCDU are identical, the MCDU will round up to the next minute past 30 seconds. So 10:39:32 will show as 10:40 in the FMS predictions.

The ND will use time without rounding up, so 10:39:59 will show as 10:39 on ND predictions. It is the same on the lower ECAM, the time displayed will wait past 59 seconds to switch to the next minute, you can easily compare with GPS clock on the right.

For the MCDU predictions, it can be seen when selecting vertical revision, then RTA. The time predicted to the waypoint will be displayed with the seconds. If it shows 10:39:29, then the FPLN page will display 10:39. If it shows 10:39:30, the FPLN will display 10:40.

Since the ND and FMS do not use the same references for their predictions, if the ND prediction is 10:38:59 and FMS 10:39:31, then the difference should be 2 minutes.

I guess this is the last proof to date that pilots have really nothing to do but sit on their hands, while the bean counters make the company work! :rolleyes:

Flex