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flodu77170

Non-Normal Fuel Consumption

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Hello everyone,

 

I'm doing a flight (Paris to Dakar) with the PMDG 777-200LR and when I'm checking the fuel in the Prog menu of the FMC, the fuel remaining at the arrival is decreasing constantly since the beggining of the cruise (from 9.1 ton to 1.6 ton now). I already checked the Unit (Kgs and not Lbs) and the calculated fuel and the totalizer fuel are the same. I'm using PFPX for flight planning and OPUS for weather. I though first it was the wind but it isn't (I load wind data in the FMC and there is a slight difference but it is advantageous)

 

Here is a dropbox link to a few screens and the OFP:https://www.dropbox.com/sh/gu6kjmbdn2z9a2c/AABjarSkRoMhDx7nK67JFNega?dl=0

 

I'm still in flight trying to figure out what's happening, if you need some other informations or screens

 

Thank you,

 

Florentin Guyon

 

 

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That is a significant change for a fuel remaining prediction, but the prediction is based on performance and wind. Experience has shown the fuel consumption based on performance predicted by PFPX is usually within 1.5% so that leaves wind.  Are you using optimum altitude based on the FMC or using PFPX for altitude (FMC is better)?  Is PFPX using OPUS for the flight planning, if not then that is part of the problem.

 

Try the same flight with no winds.

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Are you using optimum altitude based on the FMC or using PFPX for altitude (FMC is better)?  Is PFPX using OPUS for the flight planning, if not then that is part of the problem.

 

Try the same flight with no winds.

 

I'm using the optimum altitude of PFPX (it was 360 in the FMC and I was at 370 so that's not the problem)

PFPX is using NOAA weather server, OPUS too

 

I don't think it is the wind because I had tailwind/crosswind so it should be better

 

Florentin Guyon

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From the info you’ve posted it looks like you’re already 8000 Kg short and the prediction is pretty accurate for the remaining fuel you have onboard.  Over GALTO the flightplan says you should have  27.3 Kg yet the FMC is predicting 19.3 Kg.  Something else that’s odd to me is that you’re going direct to GALTO but you should be on UN869 which should have you passing BLN, MGA, and PIMOS from your PPOS.  This makes me wonder if your flight plan was correctly imported or entered.  

 

Were you keeping track of fuel burn and flight time up to this point?  It’s always a good  to occasionally compare your flights elapsed time and fuel usage to what the flight plan printout says, that way you can spot discrepancies as they happen.  Other good preflight habit is to check the DTG on the progress page against the flight plan distance to make sure they both agree.

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From the info you’ve posted it looks like you’re already 8000 Kg short and the prediction is pretty accurate for the remaining fuel you have onboard.  Over GALTO the flightplan says you should have  27.3 Kg yet the FMC is predicting 19.3 Kg.  Something else that’s odd to me is that you’re going direct to GALTO but you should be on UN869 which should have you passing BLN, MGA, and PIMOS from your PPOS.  This makes me wonder if your flight plan was correctly imported or entered.  

 

Were you keeping track of fuel burn and flight time up to this point?  It’s always a good  to occasionally compare your flights elapsed time and fuel usage to what the flight plan printout says, that way you can spot discrepancies as they happen.  Other good preflight habit is to check the DTG on the progress page against the flight plan distance to make sure they both agree.

 

Hello,

Indeed, I was controlled in spain so the controller gave me a direct to GALTO, otherwise, the flight plan was imported from pfpx and I didn't see any problem on it.

As you did on GALTO point, I compared the predicted fuel on the OFP and the fuel on board when I passed the point, that's how I noticed the problem when I was "only" 2000Kg short a few minutes after I arrived at cruise altitude.

Unfortunatly, I didn't compare my flighs elapsed time, only the fuel on some points.

 

 

Other good preflight habit is to check the DTG on the progress page against the flight plan distance to make sure they both agree.

 

 

I'll think about that next time I'll fly

 

Just a guess but Cost Index?

 

 

64, as I had in PFPX

 

Thank you,

 

Florentin Guyon 

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64, as I had in PFPX

On the PFPX OFP I noticed a fuel bias of 4% when it is usually around a 100%. This might be your problem. Maybe you didn't start with right amount of fuel. With a 99.5% bias your release fuel should be 40228 and trip fuel should be 32754. 

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On the PFPX OFP I noticed a fuel bias of 4% when it is usually around a 100%. This might be your problem. Maybe you didn't start with right amount of fuel. With a 99.5% bias your release fuel should be 40228 and trip fuel should be 32754. 

 

I don't understand what fuel bias is, but the release fuel I had was 43992Kg. And before take-off, the fuel remaining at the destination was correct, it just decreased with time

 

Florentin Guyon

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it just decreased with time

 

It is normal for the remaining fuel prediction to change during a flight, increase or decrease, I've seen both.  I rarely see the FMS fuel remaining prediction on the PROG page be the same as PFPX remaining, in fact this might be a clue to your problem.  Michael and I have gone back and forth on the fuel bias idea... he likes to use it to adjust fuel remaining and I only use it to adjust no wind performance fuel burn and that's okay. 

 

Honest, if you fly the same trip with no winds... using no winds in the PFPX plan and no winds in the simulator, you'll see how accurate PFPX actually is and in this case the fuel remaining value in the FMS will remain relatively constant. Give it a try.

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I don't understand what fuel bias is

 

Fuel bias is a corrective factor for the fuel consumption. If your aircraft consumes exactly the same amount of fuel that the performance model in pfpx, then the bias should be 100%.
If your aircaft consumes more, then the bias will be above 100% and vice versa. E.g.: if you have a bias of 4% it means your aircraft consumption would be 4% of the PFPX performance model.

But it also means that if you enter a bias of 4% in PFPX, then it will calculate the amount of fuel required based on a consumption that 4% of the performance model... hence giving you much less fuel than required if your aircraft consumption corresponds to the model.

 

In the aircraft database of pfpx, there is a tool that allows you to assess your real fuel consumption in cruise and calculate the corresponding bias.

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Fuel bias is a corrective factor for the fuel consumption. If your aircraft consumes exactly the same amount of fuel that the performance model in pfpx, then the bias should be 100%.

If your aircaft consumes more, then the bias will be above 100% and vice versa. E.g.: if you have a bias of 4% it means your aircraft consumption would be 4% of the PFPX performance model.

But it also means that if you enter a bias of 4% in PFPX, then it will calculate the amount of fuel required based on a consumption that 4% of the performance model... hence giving you much less fuel than required if your aircraft consumption corresponds to the model.

 

In the aircraft database of pfpx, there is a tool that allows you to assess your real fuel consumption in cruise and calculate the corresponding bias.

 

Thank you for the explanation, I checked the fuel bias of the 777 I used in pfpx and it is 104% I think the OFP just removed the '10' (It is the template provided by PFPX for the 777-200 PMDG)

 

Here if the aircraft config used by pfpx:

mini_635290aircraft.png

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It makes sense, 4% more than the model.

 

You may try to run the tool while you cruise in FSX (Bias button with the gears icon) to check if it matches the 104%.

Also, check whether you use the anti-ice in the preparation / flight the 5% increase in consumption may also explain partly your discrepancy.

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I think the OFP just removed the '10' (It is the template provided by PFPX for the 777-200 PMDG)
I have never seen that happen even one time and I have been using PFPX since the Fall of 2013.

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In the aircraft database of pfpx, there is a tool that allows you to assess your real fuel consumption in cruise and calculate the corresponding bias.

 

I tried that and the it gave me a fuel bias of 105% (104% was the default value) so the difference should be much more smaller than the difference I have

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Ok.

Have you try to do what Dan suggests in his post:

 

 


Honest, if you fly the same trip with no winds... using no winds in the PFPX plan and no winds in the simulator, you'll see how accurate PFPX actually is and in this case the fuel remaining value in the FMS will remain relatively constant. Give it a try.

 

I don't know Opus, but if the weather you encountered during your flight (specifically winds aloft and temperature) was different that was was used for the fuel calculation, it could explaint the issue.

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