A320 MCDU Difference in Distance to Go

[Eurotraveller]

Hi,

A few weeks ago during descent after a 4 hour sector on the A320, we noticed that the 'Distance' (ie distance to go, track miles) at the bottom of MCDU 1 was about 5nm more than that indicated on MCDU 2.

In addition, we were slightly high on profile - PROG page on MCDU 1 (and donut on Captain's PFD) indicated about 450ft high on profile, whereas FO's PROG page and donut indicated about 1800ft high on profile. AP 1 engaged, and managed descent seemed to be taking its lead from the Captains side as you'd expect.

The Position Monitor page indicated that the IRS's hadn't drifted any more than normal (less than 2nm each), aircraft remained in GPS Primary, NAV Accuracy High at all times in the descent, and a manual check of nav accuracy on both sides showed we were where the map thought we were. We put a nearby VOR in the PROG page and both MCDUs indicated the same distance to the VOR, just the flight plan track miles were different.

FMS 2 A321, getting on a bit.

Any ideas what could cause this? We were stumped, but it was 5 in the morning....

Cheers,
ET

[capt. solipsist]

happens in one of our (old) aircraft fairly regularly as well, though dtg difference is never more than 2miles, but the donut thing is similar to yours. corrects itself around TMA (though not really sure abt this).

we just go to open descent for a bit, and once we go back to DES (if we remember :}) it's all sorted out.

[Last Ditch]

Yep, I have seen this too.
Even the donut is out in both PFDs at times.
Make the system recompute and everything reverts to normal.
Few methods-
1. cycle through open descend and descend, like mentioned earlier.
2. change CI.
3. change selected speed on FMGS.
I am sure I can think of a few more..

[Eurotraveller]

Thanks guys. If I see it again I'll try switching to OPEN DES then back into managed to see if that makes a difference.

ET

[Jonty]

You need to make it recompute, so give it a "direct to". Even doing this to the point your already navigating towards will force the FMGCs to think again.

[Microburst2002]

When that works, it means some erroneous computation was going on.

If it doesn't work, then it means that FMGC 1 and 2 positions are significantly different, which can happen in long flights in non GPS fitted models. You can compare the miles to destination in the bottom F-PLN page to check the extent of the difference. Even for a slight difference the non AP coupled FD bar will appear too displaced in DES mode.

[rcoward]

Another way is to update the wind (or temperature as well, I think) in the Approach Performance page. This then recalculates the profile and the residual differences 'tend' to disappear. That's not to say it resets any position errors, as Mircoburst2002 suggested you may be experiencing, but if your aircraft are GPS equipped and are linked into the FMGS (and not just the EGPWS, as some of the older aircraft are), then there are a few other things and techniques you can do.

1. FD's off, AP off. Give it 5-10 seconds. FD's on, AP on, speed/hdg/alt knobs all push! (back to managed) = essentially, recalculates for the FD's coming back on.

2. Swap AP's. So, AP off then engage the other AP.

3. Check positions of the 3 IRS's and GPS (if fitted). If one is significantly out then flag up on aircraft tech log on ground and try not to use that IRS. Beware if it is IRS-1 that you suspect, that engaging APPR mode on the FCU and both AP's (CAT 3 DUAL), you 'could' get a map-shift effect. Either leave it with just AP-2 and APPR mode or fly a selected heading to intercept localiser/Non-Prec inbound track.

4. Don't do any FMGC resets in the air unless absolutely necessary. See FCOM 4.06 for details.

5. Some people will be tempted to do a position update over a beacon. Try to resist this as much as possible as this is NOT a very accurate way of doing it! The Cone of Confusion overhead will extend and remain in the cockpit for quite a while afterwards, especially if you've used the wrong beacon!!! :ugh:

6. What Jonty and Last Ditch were saying is also good as it forces the FMGC to recalculate the descent profile. If you have any doubt over which FMGC is out, try going to TRK/FPA (rather than HDG/VS, but keep the AP mode still in fully managed) and seeing which side is showing a 2.5 degree descent. That is the one which is correct.

I agree with capt solipsist, that once in close proximity to an airfield, the differences 'should' disappear. Should, being the operative word. A quick recalc as described or fly selected heading to the localiser will keep you safe. Don't do anything drastic. It's not THAT bad...

Hope that helps.