Not sure if this belongs in the PFPX forum but anyway. I planned a flight to LSZH to Rwy 14 but mid flight the runway changed to 28 so i selected in the ARR page rwy 28 and the approach TRANS GIPOL28 my RNAV STAR NORTH was BLM2G.

 

My fuel prediction on arrival went from 2.4t to 2.1t a penalty of -300kg. I found out during the TRANSITION approach that the database coded in used FMC SPD a much shallower descent path rather than idle thrust. This what caused me to loose 300kg of fuel. But i changed PFPX to Rwy28 after the flight to see if it compensated for a non idle path descent but only got an extra 10kg for the release fuel.

 

So it seems PFPX doesn't take into account extra thrust required during approach. Is this true?

PFPX's predictions are only as good as the profile used for a specific aircraft, and even with a very detailed profile, I don't know if its calculations would be fine-grained enough to differentiate between a VNAV/SPD vs. VNAV/PATH descent for a specific approach. Even the aircraft's own FMS may handle VNAV differently from day to day for a given approach depending on factors like wind speed and direction, temperature, aircraft weight etc.

I saw in a payware video of a real flight (cannot remember the editor) of Air France from LFPG to KJFK, that during the briefing before the flight when the pilots confirmed the fuel request, they added 2000kg of contingency fuel over the flight plan handed over by the dispatcher.

 

I guess this is where the pilots' proficiency and experience come to place to cover situations that may not be taken into account in the flight plan.

 

Even though I'm not a real pilot, I don't fully rely on pfpx for the fuel calculation and sometimes add some extra fuel when I expect some heavy traffic at arrival with possible extra hold or when I'm not really sure of what will be the arrival in use at the destination airport.

Sometimes, the arrival used for the calculation in pfpx may be the shortest one and counting only on the fuel calculated may lead to being short of fuel if another procedure is used or if the ATC extends the approach in case of traffic.

 

There may also be unexpected delays like yesterday afternoon at EHAM on IVAO where a traffic declared an emergency and requested emergency landing leading to a lot of delays in arrival.

Interesting situation, I was taking off so I didn't suffer from the situation but several pilots where rerouted to holds during the emergency to give way to the aircraft in emergency.

Thanks for the replys. I suppose its all down to route knowledge and the dispatchers will know this and load extra fuel. Now i know for next time and for future flights i'll be adding 300kg extra for these eventualities.