I know this has been highlighted already that the predicted fuel figure in the fmc tends to drop significantly as soon as one or both engines are started. i don't know if I need to ticket this or if its already been done but in addition to this initial drop I am finding that the predicted amount also slowly creeps down through out a long haul flight. and this happens in every flight ive done. the initial amount slowly drops. almost as if it over estimated to begin with .

 

 

 

the initial figure seems to be fairly accurate until the winds are added then it gives a few thousand higher figure always then I land with as if the fmc is underestimating the fuel burn per hour or not able to properly calculate the NEW in air distance the wind entries will give. I have made an extremely accurate exel fuel planner for the 777 that takes in to account the wind component precisely for each leg of a flight and I tend to land with nearly exactly the amount the sheet calculates. and as I get closer and closer to destination the higher figure in the fmc reduces and reduces to match the spread sheet as we land. so I now don't trust the fmc arithmetic as it has displayed this same thing every single flight. I ignore the fmc figure to some degree knowing its higher than reality.

 

 

 

 

 

 

 

 

 

 

 

 

 

has anyone else experienced this ?

 

 

 

 

 

 

 

 

 

 

 

 

 

tks

 

kav

I have seen this as well, and it happens with or without weather. I can fly in FSX with the weather cleared, and fuel prediction still drops. I believe they are looking into this.

Yes - understand this too, but just curious if your seeing any variation in ZFW? Asking as the most accurate I've seen the forecast was when ZFW was 185 for a 10 hour flight and then it was very accurate. When my predicted value has been off, I've checked that planned flight time was similar to actual, and indeed it is.

Last example was for around 10 hours - ZFW was 193. Forecast landing fuel was 11 tons (as I know for now to take more fuel. Landing fuel was 7 tons and the time flown was 9:50.

Regards

yes the prediction still drops weather or not but what I noticed about wind is that the fmc just doesn't seem to be able to make accurate calculations using the wind data on top of the fuel drop so making it even worse.as soon as you add winds to the fmc it miss calculates and the figure jumps up excessively.

three issues here as far as im concerned.

 

 

1. predicted fuel altering as soon as engine started. should not change at all after engine started as all calculations at the gate should assume the engines will be running between then and point it lands!

 

 

 

2.fmc not able to accurately calculate fuel burn using the wind data entered. result unrealistic predicted fuel figure in fmc initially after entering winds. initial prediction seems ok before winds added.

 

 

 

3.predicted fuel figure slowly dropping throughout the flight irrespective of winds entered or presence of any weather. this figure should remain the same unless there are changes to the flight plan after takeoff

. even if exact plan is followed as per the fmc initial calculated plan the figure will drop. predicted fuel upon take off is never matched by landing amount . the latter is always less significantly the longer the flight the less you end up with miss proportionally

 

 

 

 

 

 

kav

_{I use PFPX as my flight planner and fuel load predictor, but having experienced what you have observed in your thread, I now invaribaly always add a 1000-2000kgs on top just to make sure I have enough EFOB at the destination.}