DEST ETA/FUEL

[simonijs]

Hi Hardy,

Below (first image) you see numbers for Destination ETA/FUEL - for several PSX aircraft models - that were recorded at a particular departure airport seconds before setting take-off thrust, and at Top of Climb [TOC]. Two flights per tail number are listed.

As you can see, at TOC the aircraft is predicted to land with more fuel than was foreseen at the airport, just before take-off. This is already noticeable at 1500' HAA. I don't think this is correct. On KLM's 777/787 these data remain steady throughout the flight.



Some models then show changing numbers for destination fuel at cruising altitude, while others have more or less fixed numbers. See below, for instance, data for LX-WCV recorded at hourly intervals.



For now, I see this mostly happening on Cargolux RB211 models, not so much on the GE and/or PW models of Cargolux and other carriers. I am still trying to find out why and when this is happening.

Finally, and during descent, it seems that NG FMC models are showing reducing numbers for destination fuel while legacy models (like PH-CKB or PH-MPS) show relatively constant numbers. There is slightly more thrust on the NGFMC models during descent...

Can you confirm that you see this all happening as well?

Thank you, and with kind regards,
Simon



Edit HH: Image width

[Hardy Heinlin]

Hi Simon,

the descent fuel flow is higher in the NG FMC than in the legacy FMC because the NG FMC constructs a shallower, non-idle descent profile.

Sorry, I don't know the cause of the effects in your screenshots ...


Regards,

|-|ardy

[simonijs]

the descent fuel flow is higher in the NG FMC than in the legacy FMC because the NG FMC constructs a shallower, non-idle descent profile.

Well, I think the NGFMC system should take that into account. If - just before TOD - you plan to land with (slightly) more than the "Reserves" number that was inserted on the PERF INIT page and in the descent suddenly see the insufficient fuel message pop up, that would raise some questions on the real deck.

For the enroute destination fuel number decreasing on some PSX models (like LX-WCV or LX-GCL)...: opening these files with a text editor shows "CfgDragFf=0;-16" for LX-GCL and "CfgDragFf=0;6" for LX-WCV. This is shown on the Instructor/Model/Airframe page as +0.0% (Drag addition) and -1.6% (Fuel Flow addition) for LX-GCL and as +0.0% (Drag) and +0.6% (FF) for the LX-WCV.

Could it be that the FMS is calculating with +1.6% instead of -1.6% (LX-GCL) and +6,0% instead of 0.6 % for LX-WCV?

A further question on this: what will happen when I change the numbers on the Instructor/Model/Airframe page? Will that change the calculation of model specific drag and fuel flow in PSX? I tried to do this for LX-WCV, but to no avail: destination fuel numbers were still decreasing.

For the take-off phase - and occurring on all models that I tested in the past six months: if you are planning a take-off that is Performance limited by the Landing weight, then finding yourself at TOC with 1-2 tons of additional fuel would be another surprise.

Regards,
Simon

[Hardy Heinlin]

the descent fuel flow is higher in the NG FMC than in the legacy FMC because the NG FMC constructs a shallower, non-idle descent profile.

Well, I think the NGFMC system should take that into account.

I didn't write that PSX doesn't take it into account. In fact, PSX does. I wrote this line because your comment sounded to me you expected the NG FMC should compute an idle descent like the legacy FMC does.

The thing is simply this: The code in PSX is an FMC simulation. It doesn't contain the original Honeywell code. And in this example here we have again another case of a simulation not being as good as the real gear. There is a reason the real FMC costs over 30,000 EUR and PSX only 300. Performance prediction is a very, very complex, interactive can-of-worm task. Now you noticed the predicted destination fuel in PSX may vary by 1 to 2 tons during a flight of several hours. This slight variation has always been there in PS since 1996. I'm not able to make it as good as the real FMC, I'm sorry. (By the way, I doubt that the 787/777 FMC can achieve zero variation on all flights; this would require an absolutely perfect weather forecast on all flights.)

Could it be that the FMS is calculating with +1.6% instead of -1.6% (LX-GCL) and +6,0% instead of 0.6 % for LX-WCV?

I don't think so. The Model page shows the physical, actual addition factors: A positive FF value means this aircraft model uses more fuel. A positive drag value means this aircraft model arrives later. If these values are also entered in the FMC, the FMC will predict a higher FF (re FF factor), and a lower TAS (re drag factor). Of course, the drag factor itself implies even more FF, regardless of the entered FF factor.

A further question on this: what will happen when I change the numbers on the Instructor/Model/Airframe page? Will that change the calculation of model specific drag and fuel flow in PSX?

Yes. See page 372 in the PSX manual, the part in italic font.

When I load ...

Basic 004 - Cleared for takeoff.situ

... and enter 9.0% drag addition on the Model page, the EHAM ETA display in the FMC will increase by 3 minutes.

When I enter 9.0% FF addition on the Model page, the EHAM fuel display in the FMC will decrease by 8 tons.

This is the positive/negative logic that I expect.


Regards,

|-|ardy

[Jeroen Hoppenbrouwers]

I think you missed a zero in that FMC price.

[Hardy Heinlin]

Hi all,

for the next PSX update I'm considering a little fine-tuning of the FMC FF prediction using a fixed, general extra factor for CLB, and another one for DES. These factors will be hard-coded in the FMC model and not visible in the drag/FF edit fields (which will remain independent).

Any suggestions for two average values (CLB and DES)?

Maybe 5% for CLB, 10% for DES? E.g. when the route from T/D to DEST will take 1 hour, and at T/D the FMC predicts 10 tons remaining at DEST, that extra factor will make it 9 tons remaining.


Regards,

|-|ardy

[G-CIVA]

I'm considering a little fine-tuning of the FMC FF prediction using a fixed, general extra factor for CLB, and another one for DES. These factors will be hard-coded in the FMC model and not visible in the drag/FF edit fields (which will remain independent).

Any suggestions for two average values (CLB and DES)?

Maybe 5% for CLB, 10% for DES? E.g. when the route from T/D to DEST will take 1 hour, and at T/D the FMC predicts 10 tons remaining at DEST, that extra factor will make it 9 tons remaining.

Hardy,

I can give you (in .txt format) the Proprietary Boeing FPPM fuel burn numbers for CLB & DES for various CLB/DES Scenarios for all the PSX engine types simulated.

Surely this is more accurate than an arbitrary average guess?

Here is an example of one Descent Scenario from the PW4056 FPPM ... its used by the aircraft engine type file in PFPX, firstly the FPPM entry:



As I have transposed (KGS) it into .txt format for the PFPX Aircraft File:

Code Select Expand

[DESCENT.1]
Name=.84/290/250

[DATA]
ISA-20;  160000          ;  400000         
  1500; 4.00;  400.0; 12.0; 4.00;  400.0; 12.0
  5000; 6.00;  500.0; 24.0; 6.00;  500.0; 24.0
 10000;10.00;  620.0; 40.0;10.00;  620.0; 40.0
 15000;13.00;  710.0; 60.0;13.00;  710.0; 60.0
 17000;14.00;  730.0; 66.0;14.00;  730.0; 66.0
 19000;15.00;  750.0; 71.0;15.00;  750.0; 71.0
 21000;16.00;  770.0; 77.0;16.00;  770.0; 77.0
 23000;17.00;  780.0; 83.0;17.00;  780.0; 83.0
 25000;17.00;  800.0; 89.0;17.00;  800.0; 89.0
 27000;18.00;  810.0; 95.0;18.00;  810.0; 95.0
 29000;19.00;  830.0;102.0;19.00;  830.0;102.0
 31000;20.00;  840.0;108.0;20.00;  840.0;108.0
 33000;20.00;  860.0;112.0;20.00;  860.0;112.0
 35000;21.00;  870.0;117.0;21.00;  870.0;117.0
 37000;22.00;  890.0;122.0;22.00;  890.0;122.0
 39000;22.00;  910.0;128.0;22.00;  910.0;127.0
 41000;23.00;  930.0;134.0;23.00;  920.0;133.0
 43000;24.00;  950.0;139.0;24.00;  950.0;139.0
 45000;24.00;  970.0;144.0;24.00;  970.0;144.0
[/DATA]

[DATA]
ISA+20;  160000          ;  400000         
  1500; 4.00;  400.0; 12.0; 4.00;  400.0; 12.0
  5000; 6.00;  500.0; 24.0; 6.00;  500.0; 24.0
 10000;10.00;  620.0; 40.0;10.00;  620.0; 40.0
 15000;13.00;  710.0; 60.0;13.00;  710.0; 60.0
 17000;14.00;  730.0; 66.0;14.00;  730.0; 66.0
 19000;15.00;  750.0; 71.0;15.00;  750.0; 71.0
 21000;16.00;  770.0; 77.0;16.00;  770.0; 77.0
 23000;17.00;  780.0; 83.0;17.00;  780.0; 83.0
 25000;17.00;  800.0; 89.0;17.00;  800.0; 89.0
 27000;18.00;  810.0; 95.0;18.00;  810.0; 95.0
 29000;19.00;  830.0;102.0;19.00;  830.0;102.0
 31000;20.00;  840.0;108.0;20.00;  840.0;108.0
 33000;20.00;  860.0;112.0;20.00;  860.0;112.0
 35000;21.00;  870.0;117.0;21.00;  870.0;117.0
 37000;22.00;  890.0;122.0;22.00;  890.0;122.0
 39000;22.00;  910.0;128.0;22.00;  910.0;127.0
 41000;23.00;  930.0;134.0;23.00;  920.0;133.0
 43000;24.00;  950.0;139.0;24.00;  950.0;139.0
 45000;24.00;  970.0;144.0;24.00;  970.0;144.0
[/DATA]
[/DESCENT.1]

By the look of things my numbers here need a quick tweak anyway since I have been lucky to acquire a more up to date FPPM than the one I originally used to create the table illustrated.

[G-CIVA]

Hardy,

While I am thinking about it ...

I have very detailed Aircraft Perf data in .txt format for each PSX engine type in all flight regimes CLB/CRZ/DES (in tabulated formats with ISA DEV including 1EO & 2EO CRZ & EMER DES scenarios).

Updating the numbers to satisfy any new FPPM in my possession will not be a big job since the only new FPPM I now have are PW series documents.

[Hardy Heinlin]

Steve, that kind of work would take another year. I can only spend a day. My tables used in PSX are from LH (GE) and Boeing systems (RR, PW). There are always slight disagreements with performance database updates and empirically measured average fleet data; I will not chase the one or the other. The beans just roll forever in this and that direction and back and forth. I'm too old for that :-)

The only additional factor I would accept for the general CLB and DES factors is the gross weight.


Regards,

|-|ardy

[G-CIVA]

Hardy,

Totally understood, the copying of all this data took me about a month for each Aircraft File & ENG type!

Send me an email & outlining the ballpark numbers for Gross Weight information than you require & I am sure I can help.

You will note that the numbers in the FPPM (for DES) I used did not take into account Gross Weight.  The PFPX files I created require minmax weight tables & thus to make things spit out the correct data I had to arbitrarily make 2 identical tables at 160000kgs & 400000kgs - as you say 'chasing beans' could otherwise go on for eternity

Some of my FPPM do include additional DES tables for different Landing Weights which give differences in fuel burn & track mileage.

The CLB tables for all engine types do take into account gross weight.

[Hardy Heinlin]

Thanks. I would prefer an open discussion here on the forum.

[G-CIVA]

Hardy,

Sure...I just did not want to clutter up the thread, how can I help? What data would be useful?

[Hardy Heinlin]

I'm just asking for suggestions for the two fixed CLB/DES factor values :-)

When multiple users suggest some values based on their experience, I might see a trend and make a compromise.

It would be good if the suggestions come from users that have done many flights in PSX, so that the final picture won't be too much distorted by single anecdotal data.

[G-CIVA]

It would be good if the suggestions come from users that have done many flights in PSX, so that the final picture won't be too much distorted by single anecdotal data.

That's me then...I fly PSX several times a week; simulating some very diverse real routes & schedules across the entire performance capability of the 744 & 744 ER models.

After extensive testing the PSX engines burn at the following rates in comparison to real world FPPM data used within PFPX model files which is already accounted for in the PFPX data I offer:

PSX GE engines burn +0.2% MORE fuel than the FPPM Book figures

PSX PW engines burn -1.1% LESS fuel than the FPPM Book figures

PSX RR engines burn -0.6% LESS fuel than the FPPM Book figures

I see it is a topic for discussion in another thread. I have done my best to provide an explanation that I hope provides more clarity here:

https://aerowinx.com/board/index.php/topic,6969.0.html

[Hardy Heinlin]

When I see these beautiful, low tolerances I think I discard the idea of adding any new factors.

Instead, I'll think about checking the sim's internal FF prediction calculation specifically for the approach segments, so that the simulated FMC's FF prediction matches the simulated engine's FF (it's about the link between FMC simulation and engine simulation within the PSX world; not about the link between the PSX world and the real world).


|-|ardy

[G-CIVA]

When I see these beautiful, low tolerances I think I discard the idea of adding any new factors.

Glad to be of assistance!

[simonijs]

Any suggestions for two average values (CLB and DES)?

Hi,

Although I am still copying "DEST ETA/FUEL" data into a spreadsheet on every flight, they are not that reliable for the average values that you are looking for: quite often in the climb the destination fuel number changes, but so does the ETA (to a later or to an earlier arrival time). Since a month or so I am now also making notes of the fuel remaining numbers as shown on the "Print" (RTE page 1) and CALC Fuel (PROGRESS page 1).

On a number of these flights, I am seeing Fuel to Climb as:
- 11,7 (print)/10,6 (CALC) - LX-JCV; factor: 1,1
- 8,9 (print)/8,1 (CALC) - LX-UCV; factor: 1,1

In general - at TOC - I am seeing CALC fuel numbers to be between 1,0 and 1,5 tons less than print values. But same tail numbers sometimes show larger differences (up to 2 tons, so that's not very helpful...). 10 % addition here perhaps?

For fuel to descend, I am seeing:
- 0,7 (print)/1,3 (CALC) - LX-UCV - Fuel at TOD: 18,8 and close to the airport 18,1 (print) / 17,1 and 15,8 (CALC)
- 0,6 (print)/1,4 (CALC) - LX-UCV - Fuel at TOD: 16,6 and 16,0 (print) / 12,8 and 11,4 (CALC). *

Descent fuel for the NG FMC models in general is 0,6-0,7 tons more than for the legacy models, irrespective of the model used (GE, RR, PW). Perhaps factor for the descent set at 0,92 - 0,93 of the fuel at TOD? Hence: landing fuel equals 0,925 * (fuel at TOD) 17,1 = 15,8.

Regards,
Simon

* The large difference between CALC and print TOD fuel numbers has evolved during flight and is specific for RB211 models, Cargolux anyways... Some 0,8 tons per hour 

[Bluestar]

I know I'm not as serious about this as some folks, but I don't have any problems with the current data used by PSX.  What I have done is adjust my fuel factor in Simbrief to match the PSX data.  On a 8 to 10 hour flight at MGLW I am within 2-3 thousand pounds at shutdown.  I think the fuel flow will also vary based on pilot technique. 

[MFarhadi]

Since a month or so I am now also making notes of the fuel remaining numbers as shown on the "Print" (RTE page 1) and CALC Fuel (PROGRESS page 1).

Dear Simon,

Sorry to sound novice here, but by the "Print" you mean the initial Time and Fuel estimate values calculated by the FMC prior to departure?

[simonijs]

Hi,

From memory: on RTE Page 1, LSK 4L or 5L (away from home and my computer) you can print a complete overview of your flight as calculated by the FMC/FMS (NG FMC only). I usually press this LSK first, then TOGA. Hope this helps.

Regards,
Simon