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Richard McDonald Woods

B747-400X fuel leaks?

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Recently I have been getting the FMC message FUEL DISAGREE PROG 2/2 on longer flights. When I watch the totaliser and calculated figures on PROGRESS 2/3, it appears that I have a fuel leak because the totaliser quantity (sum of contents of tanks) gradually drops, whereas the calculated figures (the total of fuel loaded minus engine burns) seems to keep falling steadily, in line with expected engine consumption. I believe that the message is supposed to appear when the difference is greater than 9000 lbs/4080 kgs, but I believe that PMDG have modelled this with a higher value. To play safe, I chose a USE prompt that accepts the totaliser quantity, but then my fuel remaing at landing is very low.I have no faults set, so don't understand why this is happening to me. Does anyone have any ideas as to why?Cheers, Richard

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A picture of your overhead and fuel synoptic would help. The PMDG 744 doesn't simulate fuel leaks so it has to be something else. CheersRob

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Hi Richard,I believe this is due to higher than expected fuel burn due to Head Winds. (The FMC assumes 0 Wind unless you tell it otherwise). As you progress, you take slightly longer to reach each waypoint, and thus burn more fuel. When you cross the waypoint, the FMC assumes the fuel level that it predicted was correct, regardless of the qty in the tanks, but of course the overhead is still indicating the correct amount.To check for a fuel leak - add all the FUEL USED from each engine, and subtract from your block fuel. This should be equal to your totaliser (less a little bit for APU use). To solve the disagree, either enter wind data prior to departure, or update your block fuel each time that message appears.Hope this helpsChris

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The 'fuel leak' is clearly not caused by head winds. Un-forecast head winds would, I agree, cause a potential embarrasement of low fuel, but this is a quite different situation. And the fuel synoptic does not support fuel loss configurations except for the Jettison switches which were certainly not used.On my next flight, I kept better fuel records so that I could try to more clearly identify when 'the leak' was happening.Shortly after take-off on a OMDB-EGLL trip, my expected landing fuel at EGLL was 16 tonnes. Throughout the trip, the totaliser amount fell below the calculated fuel remaining by very nearly the amount the expected landing fuel reduced. This indicates to me a 'fuel leak', which I didn't know was modelled in the B744X.Again I eventually get the FUEL DISAGREE - PROG 2 FMC message. This time, the totaliser amount was 22.3 tonnes and the calculated amount was 26.4 tonnes at 814 nms from EGLL. The difference, 4.1 tonnes, accounts for the expected landing fuel now reading 11.9 tonnes.In this siuation, I could only press the USE key for totaliser, being the lower amount, but this left only 4.6 tonnes of fuel at EGLL.Does anyone else have a similar experience? What do you believe is the cause of the message on your flights?Any further ideas most welcome,Cheers, Richard

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Guest chrut
The 'fuel leak' is clearly not caused by head winds. Un-forecast head winds would, I agree, cause a potential embarrasement of low fuel, but this is a quite different situation. And the fuel synoptic does not support fuel loss configurations except for the Jettison switches which were certainly not used.On my next flight, I kept better fuel records so that I could try to more clearly identify when 'the leak' was happening.Shortly after take-off on a OMDB-EGLL trip, my expected landing fuel at EGLL was 16 tonnes. Throughout the trip, the totaliser amount fell below the calculated fuel remaining by very nearly the amount the expected landing fuel reduced. This indicates to me a 'fuel leak', which I didn't know was modelled in the B744X.Again I eventually get the FUEL DISAGREE - PROG 2 FMC message. This time, the totaliser amount was 22.3 tonnes and the calculated amount was 26.4 tonnes at 814 nms from EGLL. The difference, 4.1 tonnes, accounts for the expected landing fuel now reading 11.9 tonnes.In this siuation, I could only press the USE key for totaliser, being the lower amount, but this left only 4.6 tonnes of fuel at EGLL.Does anyone else have a similar experience? What do you believe is the cause of the message on your flights?Any further ideas most welcome,Cheers, Richard
Use of step climbs? Perhaps you could post your fuel planning calculations? Since you use FSX (B744X), maybe the culprit is the FSX wind shift?

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Guest chrut

Richard, how is it going?

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Guest chrut
I doubt it's a wind problem. You probably left the APU running.Paul
Nice suggestion, I did not think about that.

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If tank 2 was low, I'd bet my next paycheck on it...Paul
My words exactly...

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I have since discarded my fuel stats but I am sure that the fuel levels were roughly equal, so can discount the idea of APU being left on (but quite an imaginative idea, all the same!).Will have to record more on my next flight.Thanks so far,Cheers, Richard

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Yes, the Fuel Disagree message has nothing to do with expected fuel burn/winds/flight planning, etc.It is a simple comparison test between the Totaliser value (i.e currently sensed fuel quantity in tanks) and the FMC-calculated fuel burn up to that point in time. The FMC measures the fuel usage (using fuel flow sensors on the engines) and then subtracts this value from the fuel level from the very start of the flight. i.e., roughly, start fuel - (fuel flow rate x time). The FMC calculates current fuel level a bit like the IRS computes current position. From a known starting point, they compute the changes (accelerations, velocities, flow) over time. Yes, the APU is a possibility.Cheers.Q>P.S. I've just seen Richard's last post: I seem to recall that PMDG added a tank levelling algorithm. A real world pilot on the Beta team commented that the pumps in tanks with lower fuel levels ouput less pressure, so less fuel is used in that tank (and more in the other tanks), and this has a tendency to balance out the tank levels. Whilst this is true for large discrepancies in levels, I can't comment on small discrepancies. Some pumps are stronger than others, so there is lot of randomness involved.

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