A320 FMGS Question

[High Flyers]

I have a tech question on the A320 FMGS. The question concerns the EXTRA/TIME section of the FUEL PRED page. I know from the manuals how the EXTRA/TIME is calculated before the engines are started, but how is it calculated during flight? The manuals I have talk about using BLOCK-(TAXI+TRIP+RSV+ALTN+FINAL), but in flight surely it must be based somehow on FOB vs fuel to destination? Any help would be appreciated.

[OLBA18]

Extra time I beleive is FOB minus fuel to reach dest as per fmgs i.e incl approach that's inserted and taking note of any step climbs etc + the fuel needed for the inserted altn flight plan using the inserted altn fpl winds, weight and flt level. No 5 percents or extra fuel is incl , hope this helps

[High Flyers]

Thanks OLBA18. This is what I thought would be the case. Are you sure the RSV is not included?

[AffirmBrest]

Just to clarify,

Any amount that is in the RTE RSV box is taken account of when calculating the EXTRA fuel displayed, at least it is in our aeroplanes.

This comes in handy when (as is often the case) the FMGS ALTN fuel prediction is somewhat less than our flight planned ALTN fuel (different allowances/procedures etc.) - you can put the difference into the RTE RSV to give an ACCURATE prediction for extra fuel.

And it definitely does use current FOB minus fuel to dest+altn+rsv to work out 'extra' fuel - on long flights you can clearly see the prediction change as you change level/take short cuts etc.

[High Flyers]

Thanks AffirmBrest. That's what I thought. Now a happy chappy with FMGS fuel predictions, my line trainers will be happy . Happy landings with plenty of reserve fuel in hand!