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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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copped some stick

Tim

What does this mean?

 

Thanks

Michael

 

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

Is that the version prior to the VAS leakage fix?

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Is that the version prior to the VAS leakage fix?

 

Don't know, I never had VAS leakage with any version.

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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.

 

Keep a log of your flight. It's easy if you are already using PFPX.  Here is my KLAXZSPD flight from yesterday, show me the details for a flight where you have fuel consumption error:

DELTA AIR LINES  DL FLIGHTPLAN - IFR  DAL185  N702DN  KLAX-ZSPD
--------------------------------------------------------------------------------
ALL WEIGHTS IN POUNDS (LB)                                       STD 25SEP/2015Z
--------------------------------------------------------------------------------
OFP 1 - PREPARED 26SEP/1627Z BY DANIEL DOWNS (DD)

DL185/DAL185        N702DN/B777-2LR GE  SEL/              ROUTE:      KLAXZSPD01

DEP: KLAX/LAX 25R   ELEV   128 FT   MACH:       M.84      TTL G/C DIST:  5630 NM
ARR: ZSPD/PVG 16L   ELEV    13 FT   INIT ALT:   FL320     TTL F/P DIST:  5806 NM
                                    FUEL BIAS:  100.0%    TTL AIR DIST:  6384 NM
                                                          AVG WIND CMP: HD044 KT
ALT: ZSWX/WUX 21    ELEV    16 FT   102 NM
--------------------------------------------------------------------------------
CONFIG      DOW  PAX   CARGO   TOTAL   ULOAD LIM             ZFW     TOW     LDW
STANDARD 344244  218   50917   94553   22103 ZFW     MAX  460900  766000  492000
                                                     PLN  438797  668297  455107
                                                     ACT  438800  668800  454200
****************************** RE-DISPATCH PLANNING ****************************

INIT RELEASE KLAX TO RJNS SUBJECT TO INFLT RE-DISPATCH ONTO ZSPD OVER KAGIS

FUEL BURN FROM KLAX  TO KAGIS 179479
PLAN REMAINING OVER     KAGIS  50021          ACT REMAINING OVER KAGIS .52362..
--------------------------------------------------------------------------------
REQD OVER KAGIS TO DEST ZSPD   49966
                TO ALTN RJNS   40641          RD EXTRA FUEL      55

********************************************************************************


               FUEL    CORR    ENDUR
TRIP         213190  ........  13:09
10 PCT         3522  ........  00:16
ALTN ZSWX      5850  ........  00:20
INTL HOLD      6883  ........  00:30
MIN T/O      229445  ........  14:15
EXTRA            55  ........  00:00
TAXI           1425  ........  00:25
RELEASE      230925  ........  14:40  ACT FUEL LOADED 230,958 LBS

KLAX/LAX   STD 20:15   ETD 20:15  ACT OFBL 20+16    EST T/O 20:40  ACT T/O 20+25
ZSPD/PVG   STA 09:45   ETA 10:04  ACT ONBL 09+42    EST LDG 09:49  ACT LDG 09+38
                                  TTL BLCK 13+26                   TTL FLT 13+13
PLANNED FREM 15.5 ACT 15.2

********************************************************************************

ATC ROUTE: N0502F320 VTU5 RZS DCT LIBBO DCT BRINY DCT BOARS DCT AMAKR DCT REDWD
           DCT UNVER DCT 42N130W 45N140W/N0489F340 47N150W 48N160W
           49N170W/N0485F360 49N180E 47N170E 43N160E/N0488F380 DCT EMRON OTR9
           AVBET OTR11 KAGIS Y51 INUBO Y16 CHINO Y88 CUE Y28 ISAKY Y60 ONIKU
           A593 DUMET DUM21A

--------------------------------------------------------------------------------
AWY      WAYPOINT   MT    ALT  WND/VEL   TAS  DIST  FUEL REM / USED   LEG   ACC
         NAME             TMP     FREQ    GS   REM  POSITION          ETO / ATO
--------------------------------------------------------------------------------
         KLAX/25R         128                          229.5 /   1.4 FMS FREM 22.7 PREFLT
         LOS ANGELES INTL                     5806  N3356.4 W11823.0 ...../.....
                                                                                  DEST  ACT
                                                                                  FREM  FREM
                                                                                  PRED  DELTA
                                         ACTUAL>>      214.8 /  16.1       00+28  20.8  + 2.0
DCT      LIBBO     299  FL320  225/047   501    88     212.8 /  18.1   10  00:30
-KZOA                     -38            495  5593  N3600.0 W12150.0 ...../.....

                                                       194.4 /  38.7       01+35  20.6  + 2.1
DCT      42N30     293  FL320  226/078   497   170     192.3 /  38.7   21  01:37
         42N130W          -43            477  5050  N4200.0 W13000.0 ...../.....

                                                       175.8 /  55.1       02+35  20.4  + 1.9
DCT      45N40     278  FL320  265/048   494   471     173.9 /  57.0 0102  02:39
         45N140W          -45            450  4579  N4500.0 W14000.0 ...../.....               FMS
                                                                                               ETA
                                                       159.4 /  71.6       03+31  20.5  + 3.0  0917
DCT      47N50     272  FL340  296/070   489   326     156.4 /  74.5   45  03:38
         47N150W          -49            421  4145  N4700.0 W15000.0 ...../.....

                                                       144.3 /  86.7       04+24  20.4  + 3.9  0917
DCT      48N60     267  FL340  286/049   491   410     140.4 /  90.5   58  04:36
         48N160W          -47            443  3734  N4800.0 W16000.0 ...../.....

                                                       129.9 / 101.1       05+16  18.9  + 4.0  0926 selected STAR
DCT      49N70     268  FL340  224/043   490   402     125.9 / 105.0   53  05:29
         49N170W          -48            461  3332  N4900.0 W17000.0 ...../.....

                                                       115.9 / 115.1       06+07  18.8  + 4.0  0926
DCT      49E80     263  FL360  220/030   486   144     111.9 / 119.0   19  06:20
         49N180E          -52            464  2938  N4900.0 E18000.0 ...../.....

                                                       101.9 / 129.0       06+59  17.3  + 4.0  0932 updated WINDS
DCT      47E70     253  FL360  241/025   486   419      97.9 / 133.1   55  07:15
         47N170E          -52            461  2518  N4700.0 E17000.0 ...../.....

                                                        57.5 / 173.5       09+56  17.2  + 2.6  0932
OTR9     AVBET     248  FL380  260/074   490   247      54.9 / 176.0   35  10:08
                          -49            420  1241  N3608.5 E14528.0 ...../.....

******************************** RE-DISPATCH FIX *******************************
                                                        52.4 / 178.6       10+18  17.1  + 2.4  0932
OTR11    KAGIS     273  FL380  264/074   490    64      50.0 / 180.9   09  10:28
                          -49            416  1098  N3549.2 E14233.8 ...../.....
********************************************************************************

                                                        36.4 / 194.6       11+27  16.9  + 2.2  0933
Y28      WASYU     256  FL380  261/078   491    12      34.2 / 196.7   02  11:36
                          -48            414   633  N3428.3 E13323.0 ...../.....

My last logging was close to 100 nm before descent.

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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.

Dan

Below is some information on a flight I completed this morning. As you can see I started the flight with less than the reserve fuel according to the PROGRESS page so there were insufficient fuel in the FMC. I changed the reserves on the INIT page to 5000 lbs. to get rid of the warning. As the flight progressed to Houston you can see that the remaining fuel at landing increased until it was right at required reserves.

 

As for the 13000 lbs. above reserves at KLAX - that included 10000 lbs. extra fuel carried from VHHH-KLAX. BTW the 10000 lbs. was included in the insufficient fuel warning at VHHH prior to takeoff. Without it, I would have really been in the hole.

 

I think the plane always has enough fuel as long as PFPX is used for planning and the winds don't go crazy enroute. During the flight your method shows that it does. My method shows that it does at the end of the flight. Why can't the two agree? Because the FMC PROGRESS page fuel remaining is either getting incorrect data from somewhere or it can't calculate the correct data.

 

----Flight Referred to Above----

EDDF-KIAH 205000 lbs. cargo, EU-OPS fuel policy, fuel bias 95%, release fuel 184499, reserve required 22418, PROGRESS page prior to takeoff fuel remaining at KIAH 9900 lbs, headwind comp 4 kts.

PROGRESS page at TOC - ETA 1304 fuel remaining at KIAH 9100 lbs.

PROGRESS page at 1 hour - ETA 1305 fuel remaining at KIAH 9800 lbs.

PROGRESS page at 2 hours - ETA 1304 fuel remaining at KIAH 11600 lbs.

PROGRESS page at 3 hours - ETA 1307 fuel remaining at KIAH 12800 lbs.

PROGRESS page at 4 hours - not at PC.

PROGRESS page at 5 hours - not at PC.

PROGRESS page at 6 hours - not at PC.

PROGRESS page at 7 hours - ETA 1310 fuel remaining at KIAH 17800 lbs.

PROGRESS page at 8 hours - ETA 1308 fuel remaining at KIAH 19300 lbs.

PROGRESS page at 9 hours - ETA 1308 fuel remaining at KIAH 21000 lbs.

PROGRESS page at T/D - ETA 1307 fuel remaining at KIAH 22300 lbs.

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Your fuel bias adjustment of 95%? How did you arrive at that? I am looking for a reason why your fuel remain predicted is much less than plan.

 

I have never see a fuel warning when using PFPX fuel loads. Gotta be something in the way you have things configured.  EDIT:  I've tried the fuel adjustments in PFPX and usually only tweaked 3% or less and over the long run I just leave it at 100% now.

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Your fuel bias adjustment of 95%? How did you arrive at that?

Trial and error. Trying to come up with a bias and free fuel that work in the FMC on both ends of the flight.

 

Gotta be something in the way you have things configured.

So what is configured wrong with the flight to Houston. I arrived with required reserves. It's just that I had to takeoff with insufficient fuel according to the FMC.

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I don't understand a couple of things that you do. The fuel bias already mentioned is one, because my experience is leaving fuel bias at 100% consistently works best. Then there is the planning.  I have just finished preflight with a PMDG B77L in UAL colors at gate A20 EDDF getting ready for UA47. This is flown by the company in a shiny new B789 but simming with the B77L works for me.

 

You said you have a payload of 205000 lbs, I max out the ZFW with a payload of 116,656 lbs. Maybe that's the diff between 777F and -200LR? You are using EU Ops fuel policy, I am using US Flag, no biggy.

 

So, at max ZFW of 460.9 klbs and a route that varies from great circle distance by 2% (Using NAT A) there will be differences between our preflight but I do not have a problem with FMS prediction.

 

My release fuel is higher at 194,679 lbs but that could be route and weather variations (I am using historical weather with an off block time of 271150 UTC. The planned fuel remaining is 27.6 lbs and the FMS Progress page indicates 27.3 klbs at KIAH. That is a perfectly reasonable variance.

 

Now the trick is to figure out why your configuration is giving you fits with FMS predicted remaining fuel.  Right off the bat there are a couple of things that will vary the FMS guess from the PFPX plan first of all being winds. I've noticed that the FMS uploads winds up to FL350 so if you are higher you may want to adjust the legs wind levels. Also, if the flight is > 6 hr then the forecast period for the winds you've loaded has expired. Selection of an arrival while enroute instead of during preflight is another source of variance. However, taken all the things I know will cause the FMS fuel reminaing forecast to vary does not come close to the error you are seeing. Since we are both using the same PMDG product, on the same route, I must presume the variance is caused by something different in your setup that we haven't discussed.

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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.

 

Dan, what text editor do you like to use? I would always print out flightplans, but I like to keep a paperless environment. The problem I've had with PFPX is in its export function- the flightlog doesn't line up in an organized manner in any text editors I've used, so I simply eyeball everything to make sure fuel is still in check for the flight. 

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Hi Dan

My problem is a little different in that I am landing with less fuel than originally predicted.

I did a flight OTHH-VTBS yesterday in the B77W and kept a detailed log of Altitude, Wind Speed and direction estimated fuel at destination etc.

Prior to Taxi my landing estimated fuel was 14,600Kg.  On Take Off it was 14,000kg.  At top of climb the estimate on landing was 13,600kg.  My PFPX flight plan estimated a landing fuel of 12,200kg.

I logged ever waypoint and watched the predicted fuel at destination roll down at the 400Kg/hr mentioned until at top of descent my estimated fuel on landing was 12000kg.  By the time I landed and the STAR was pretty direct. I landed with 10,400kg.

The flight time was 2 minutes longer than expected at 6hr 02min.  Temperature and wind speeds and direction all very close to the flight plan forecast.  Can photograph the log sector by sector if it helps. 

This is the problem I am facing. 

It was never a problem until recently and don't know the cause or solution.

Regards

Tim Carter

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