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barramundilure

Fuel at Destination prediction

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Hi everyone, I have recently noticed a problem that seems to have only just started to happen with the FMC fuel at destination prediction.

After reaching my cruising altitude, the predicted fuel at my destination drops by about 400kg/hr over the course of the flight despite the winds being as expected and the arrival time remaining unchanged.

Is anyone else seeing this?

I am using PFPX for planning and ASN for weather injection and the two seem to match up pretty well. 

Usually I even made just a little fuel but now this 400kg/hr roll back is a real pain. 

Any ideas?

Cheers

Tim Carter

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Compare the fuel remaining onboard to the predicted by PFPX at each waypoint and see how that is going. That has more significance than fuel remaining prediction by FMS, although the value is important it only serves to act as a flag in my opinion. If it flags a lower than expected level then you start looking for the reason, hence my recommendation.

 

I've done the tracking by waypoint on numerous flights and all in all the two are reasonably close.

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Is anyone else seeing this?

Yes I am having the same problem. The following information submitted to PMDG Support on a ticket.

 

Two flights using 777F EDDP-KLAX (#1) and EDDF-KIAH (#2) with no weather, no time compression and auto step climbs. About 12 hrs. and 10 hrs. in duration.

Flight #1 T/C ETA is 1545 and fuel remaining at KLAX is 17900 lbs. T/D ETA is 1546 and fuel remaining at KLAX 14300 lbs.

Flight #2 T/C ETA is 1136 and fuel remaining at KIAH is 21700 lbs. T/D ETA is 1136 and fuel remaining at KIAH is 34000 lbs.

All of these times and fuel remaining are as reported on the PROGRESS page of the FMC. So #1 the fuel remaining decreased and #2 it increased. The only difference was #1 had a cargo payload of 100000 lbs. and #2 had a cargo payload of 1950000 lbs.

 

Update 1 - It would appear that the payload figure where ETA and remaining fuel at destination move in unison is around 125000 lbs. Since the original ticket was submitted I have flown the 777F most every night on 10-14 hour flights with payloads of 165000 to 195000 lbs. with the remaining fuel increasing no matter whether the ETA decreased or increased. All of these flights were conducted using ASN.

 

Update 2 - Keeping you informed of the situation. There has been no change. Heavy payloads result in an increase of available fuel at landing. Low payloads are just the opposite. Several days ago EDDF-RJAA with payload of 95000 lbs. went from 30K to 23K. I just finished flight VHHH-EDDP with payload of 195000 lbs. Available fuel went from 25.7K to 37.6K lbs. per PROGRESS page. On this flight there was a short 39 minute span with the ETA unchanged at 1443Z and available fuel went from 36.8K to 37.6K lbs.

 

No reply as of yet.

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Hi Michael

Thanks for your info.

I did in the past make a little fuel, that is arrive with a little more than predicted but generally never much more than 1000kg even on long flights.

I did another flight today just to check that the SAT wasn't way above what had been predicted and it was not.  In fact it was a little cooler than was forecast.

I am at a loss what is causing the issue as I have not had any issues until recently.

If it was a simple case of more wind than expected then the eta would have changed as well which was not the case.  I also checked planned winds against the waypoint predictions and in nearly all cases the wind was within 10 degrees of what was expected and within 2 or 3kt in terms of speed.

Neither of these factors would explain a decrease in fuel over the destination without an increase in the expected time over the destination.

Something really screwy is going on here. 

The only thing that I can think of is that I updated ASN recently, however my SAT, predicted wind direction and speed all correlate very closely with the PFPX flight plan and ASN within the sim.  

Would love to hear from the developer in regard to this problem we are experiencing.

Regards

Tim Carter

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Micheal,

I love being a member FedEx V so I can fly the 777F but I take my hat of to you for flying them route with no time compression and the reporting you do. Great work.

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I am at a loss what is causing the issue

 

I suspect it is an issue in the FMC or a system feeding data to the FMC. I think you can rule out ASN. The first two flights in post # 3 were done without ASN. They were flow in FSX clear sky mode. Consequently there was no wind. The flights were flown on a standard day.

 

route with no time compression

 

Sometimes they get pretty boring. In particular certain routes like VHHH-KCVG or KCVG-OBBI.

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I am perplexed at the importance given the FMS fuel remaining prediction.  The important number is the fuel on board at a waypoint compared to planned fuel on board at that waypoint.  If you are at a decision point the choice to proceed or divert is based on fuel remaining. Never would you divert because the FMS prediction was not in agreement with the predicted fuel remaining.

 

If your gonna spend time on subject, I recommend you start logging your flights. I save a .txt copy of the PFPX flight plan and use my favorite text editor to enter the actual times, fuel numbers and weather at each waypoint.  I also post in the margin the time variance and FMS fuel remaining prediction so I can see how that relates to the route data. Gives me something to do on long flights.

 

What I have found, and mentioned here several time on similar threads, is that the FMS prediction is sensitive to elasped time performance.  Makes sense, the engine burn rate is predictable but time in the air is the variable that is hardest to predict.

 

I just added Imagine Sim ZSPD Shanghai Pudong to my airport inventory so maybe this weekend I'll make a trip over there and keep a log for discussion.

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I am perplexed at the importance given the FMS fuel remaining prediction.
Dan

It is important to me because it is not right. Earlier this morning I completed VHHH-KLAX in the 777F as Southern Air. Before takeoff I was receiving insufficient fuel warnings. On landing I was 13000 lbs. above the reserve. I cannot imagine Federal Express, Aerologic, Southern Air, etc. accepting a plane from Boeing in which the FMC does this kind of tricks on the PROGRESS page.

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Dan

It is important to me because it is not right. Earlier this morning I completed VHHH-KLAX in the 777F as Southern Air. Before takeoff I was receiving insufficient fuel warnings. On landing I was 13000 lbs. above the reserve. I cannot imagine Federal Express, Aerologic, Southern Air, etc. accepting a plane from Boeing in which the FMC does this kind of tricks on the PROGRESS page.

 

 

Michael,

 

From personal experience, like you I have many hours in the sim and doing countless ULRs out of OMDB. Never have I had a 13,000lbs difference! I plan with PFPX servers as wx source and fly with ASN. Even with the expected minor differences between forecasted and actual weather the biggest change in fuel at destination is 2000lbs. Typically in the 1000lbs range.

 

However, I just realized that you are exclusively flying the -200LR/F whereas I am only flying the -300ER. Which raises the question if there may be something in the -200LR/F code that is causing this!

 

I will operate the return SBGR-OMDB later tonight in the -300ER and I will make sure to screenshot my fuel at destination before pushback, at TOC, and before TOD. 

 

Note: I uplink wind data during pre-flight and I always manually enter the 3-4 flight levels ill be flying at and then re-request the winds so that the uplink populates every waypoint with the winds for the 3-4 flight levels ill be flying at. Gives by far the best data for the FMC to work with. For example, by default ASN will uplink flight levels such as FL330/310/290/270, which isnt ideal when youll be cruising at FL310/330/350/370. So better go to the first waypoint in the RTE data page, and enter your cruise flight levels on the wind page for that waypoint, and then re-request winds. All following waypoints will now also include these flight you entered and their associated wind/temp.

 

I re-request at TOC, and up to 2-3 times more up to before TOD depending on stage length.

 

 

 

Any ideas?

 

With which model are you seeing this?

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I fly the 77L a lot more than the 77W and I have never had an insufficient fuel warning before takeoff. This is a big red flag that your flight planning has a problem.

 

As for remaining fuel being 13,000 above reserve, I'm not sure how to interpret that. Pilot can enter any value in the FMS reserve field. What was the PFPX fuel remaining prediction? What was the fuel released and did how close to that were you in setting the fuel in the tanks?

 

That trip probably has a typical trip fuel prediction of (I am guessing) 180,000 lbs, plus all the adders your fuel on release was maybe 220,000 lbs? A variance of 13,000 is a 6% variance, which in my experience is way too much. I rarely see actual performance vary by more than 2-3% on long flights.

 

Let's dig deeper into your problem... how does your actual remaining compare to planned remaining fuel on board at waypoints along the route, maybe 2 hr spacing?  Just as important, how does the ETE at each fix compare with actual elasped time?  I like to emphasize that time enroute is the most important variable, it is the key to fuel prediction.

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Completed a B77L KLAX ZSPD DAL185 trip.

PFPX predicted 15.5 klbs fuel remaining at destination, FMS indicated 22.7 klbs after preflight before engine start and after engine shutdown at ZSPD gate there was 15.2 klbs remaining.  This was a 13 hr flight flown with ZFW 438.8 klbs fuel on release 230.9 klbs and a redispatch DP was selected. Details available on request.

 

If there is a problem it's probably not with the product. Lets see some data justifying the claims.

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Dan

As an example I flew the B77F from YPPH - WMKK (2300nm) yesterday basically to confirm nothing was screwy with the SAT which I had not made a point of checking too hard in the past.  Conditions were basically ISA+10 most of the way and a little cooler than the ISA+13 conditions expected.  Flight plan had my initial cruise at FL340 stepping to FL360 up to TOD and a CI of 70 giving a cruise around Mach 0.83. 

I believe I have a reasonable grip on flight planning having done my ATPL including high altitude flight planning here in Australia in the early 1990's.

My PFPX flightplan had an estimated landing fuel of 13,200kg.  This included 30min holding and diversion fuel to WSSS 181nm away.

While I didn't pay a heap of attention to my predicted landing fuel in the fmc on taxi out, I did note my predicted landing fuel at my top of climb at around 14100kg.

Also taking careful note of my wind speed and direction during the flight at the waypoints and they were very accurate - as I have stated before.  Within 10 degrees of forecast and with 3kt in terms of speed.

On arrival over WMKK my fuel on board was 12200kg, consistent with the 400kg/hr reduction I am reporting here.  My fuel on landing was less than 12200 as the star I was on overflew the airports and vectored downwind for an ils approach.

My arrival time at WMKK was 0634GMT and arrived overhead at 0632GMT.

There is no fixation of my fuel remaining except that until recently fuel over destination generally remained very stable throughout the flight unless major deviations in wind speed or direction were encountered and this is simply not the case now. 

There appears to be a 400kg/hr reduction in fuel on arrival or increased fuel flow if you like that I cannot account for.  It was not a problem before and now it is.

Any additional comments?

Sincerely

Tim Carter

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I did not start this thread. I was only replying to Tim Carter that I had a similar issue and I gave him some examples of flights that I had done. I appreciate the comments of Dan and Leo but for me this issue has been turned over to PMDG via a ticket to their Tech Support. I am out of here.

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Sorry you copped some stick Michael.  I think something is definitely wrong. 

Currently doing another flight and logging all the info.

Cheers

Tim Carter

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I had this problem too with V1.10.6492.

If you downgrade to V1.10.6370 the problem is gone.

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