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pcubine

ETA and Fuel Remaining Increase

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This is a problem I just can’t understand. Use these figures as an example. At takeoff the PROGRESS 1/4 page shows an ETA of 1615 and 23.5 fuel remaining at destination. 12 hours later, the ETA has gone to 1645 and remaining fuel is 27.0. So the time in flight has increased by 30 minutes and the fuel remaining has increased by 3500 pounds. It makes no since to me. Is there an explanation?

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Is there an explanation?

 

Might be a situation where fuel burn was lower than expected up to this point, but wind ahead shows that you'll be there later than planned. The predictions are not only present point forward. The data that got you to your current point also comes into play.

 

Don't hyperfocus on numbers. However, I'm sure that if you continue to hyperfocus on the numbers, you'd likely be able to answer your own question on a later flight.

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The data that got you to your current point also comes into play.

Kyle

I typed a long narrative about the occurrences on a VHHH-EDDF flight I am doing now but I deleted that. All I can say is that one day I hope I understand.

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Kyle I typed a long narrative about the occurrences on a VHHH-EDDF flight I am doing now but I deleted that. All I can say is that one day I hope I understand.

 

haha - my scenario is a bit of a stretch, but...YAY MATH!

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I've carefully compared fuel burn and elapsed time on a number of flights with the PFPX plan and notice that given winds as predicted, the FMS fuel remaining prediction will have significant variance early in a long flight and as we get close to TOD it will start to converge on the expect values.  It's tricked me a couple of times into adding a hold in the arrival to burn off excess to stay below max landing weight LOL. Even with the fuel forecast variance, the elapsed time is usually accurate given consideration to wind variance.  If ASN is delivering winds very close to forecast then elapsed time is very close or right on plan.

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the FMS fuel remaining prediction will have significant variance early in a long flight and as we get close to TOD it will start to converge on the expect values.

So maybe it's kind of a normal situation. The flight I just did, VHHH-EDDF, indicated insufficient fuel at takeoff. But I landed with the reserves, extra fuel specified in PFPX and an additional 3500 lbs.

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the FMS fuel remaining prediction will have significant variance early in a long flight and as we get close to TOD it will start to converge on the expect values.
Dan

Do you know if PMDG is aware of this issue?

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I recall mentioning observations along this subject during testing but I never did run a set of trials to see what was actually going on because I didn't perceive a problem. It would be interesting to run a set of controlled trials but since it would take a large chunk of time it's pretty low on my list of things I want to do right away.  There were times when the PROG forecast was right on the money and others with the behavior I noted. It remains to be discovered why.

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I recall mentioning observations along this subject during testing but I never did run a set of trials to see what was actually going on because I didn't perceive a problem.
I am going to do a flight from EDDP to KLAX with no weather, traffic or time compression and see what is reported on the progress page.

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I've never been up front in a real 777 during flight, but I don't think this behaviour is particularly inaccurate. The FMS will be taking in to account a wide variety of factors in producing its forecasts: the forecast data you feed it in the WINDS page, but also the actual ambient wind and temperature. The two are weighted and intermixed dependent on various factors, primarily distance (so the forecast will have greater weight at long distances, but the actual conditions will be weighted far more heavily nearer the aircraft). So as those values fluctuate and your flight progresses, the so the calculations and predictions will update and propagate along the route -- and it stands to reason that over a long flight, the effects and fluctuations are likely to be magnified over the distance.

 

The best cross-check is always against the paper flight plan using the totaliser and actual time over the waypoint (not the FMS):

 

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Here, I've calculated my ETAs at each waypoint based on the actual time of departure (from the FMC OFF time) and the planned time between waypoints - I've then recorded the actual time over from the chronometer and fuel remaining from the totaliser to calculate a fuel prediction that is entirely independent of the FMS (and generally much more stable).

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EDDP-KLAX no weather, no time compression, and auto step climb. At T/C ETA is 1545 and fuel remaining 17900 lbs. at KLAX. At T/D ETA is 1546 and fuel remaining at KLAX 14300 lbs. Fuel at landing 13900 lbs. Would I submit a ticket to PMDG? No.

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EDDF-KIAH, no weather, no time compression, and auto step climbs. ETA at T/C 1136 and fuel remaining at KIAH 21700 lbs. per PROGRESS page. ETA at T/D 1136 and fuel remaining at KIAH 34000 lbs. per PROGRESS page. An increase of 12300 lbs. on fuel at release of 192790 lbs.

 

So in post 11 the remaining fuel decreased and in this post the remaining fuel increased. The only difference was the payload. 100000 lbs. in 11 and 195000 lbs. for this flight. The fuel calculations that the FMC does for the PROGRESS page makes no sense.

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Are you ETAs at TOC and TOD typos?

 

You are right, there is a 6% error in this case... and I'd assumed the errors would be less than 5%. The question is how this error compares to the actual.  Of course, no one is going to use the FMS for flight planning but as a guide to the pilot, given the quirks, I imagine it's still useful.

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Are you ETAs at TOC and TOD typos?

Dan

No typo. Both are 1136 Zula. Why would they be different without winds? Possibly drag.

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Michael,

 

I don't feel that you should focus on what the FMC indicates BEFORE take-off. It is making a best initial guess of fuel remaining at destination with very limited information of actuality.

 

I have found that PFPX flight plans have, if anything under-estimated fuel remaining, but given that actual weather conditions will always vary from those used in planning, I firmly believe that PFPX gives me a very good quality of flight plan. If I suspected that the plan was a little parsimonious, I can always add a little EXTRA to compensate.

 

Flight planning has, at base, a small element of 'art' to which you as an experienced pilot have to make professional judgements. This is an element contributing to my enjoyment of flight simmimg.

 

If you feel that you have a point to make about the calculations made by PFPX then you can always post in the PFPX forum.

 

Happy flying!

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