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fliegzeuger

PMDG 777-300ER fuel burn

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Hello,

 

after a couple of flights I noticed that die 300ER burns significantly too less fuel. I've compared that with EK OFPs (10%+) including a 1.5-2.5 BIAS as well as with PFPX OFPs (5-10%) with 0 BIAS. Both show a huge difference in fuel burn compared to the sim. EET was similar. In most cases the flight was even longer than planned!

 

For example I'm right now cruising over South China Sea at FL360 and having a fuel burn of appr. 6,7t/h. PFPX has an evaluation tool included. This is what comes out:

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Looking at the latest 777-300ER FPPM (Flight Planning and Performance Manual), at FL360 and that weight, the fuel burn should be ~7100kg/hr.

 

So the aircraft is definitely burning significantly less (~10%) in the sim. 

 

Can you please submit a support ticket to PMDG so they can look into the numbers?

 

Anyone else care to share some weight vs. fuel burn figures. (Include FL, TAT, Mach)

 

Leo Cal

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I have reported this issue back in the 200LR days and even included the option to PMDG to activate the FMS's fuel/drag bias adjust field. That way you could adjust your burn scalar in the cfg files and match it on the FMC. Of course it is easier said than done and even so PMDG would run into millions of unneeded tickets due to people not knowing exactly what they are doing with regards to cfg files. As it is now I have my 200LR scalar adjusted up slightly and will probably be doing the same on the -300ER. One thing though. When doing the PFPX evaluate tool, make sure you are flying over the ocean, or in any case make sure the winds and temps are constant. Next engage altitude hold and disengage the A/T and spend no less than 15 minutes finetuning the N1 and speed. Give the 777 about 5 minutes to stabilize its speed before taking readings. Finally when taking fuel flow I personally always add 0.1 to the sum of the 2 flows to make sure hidden decimals are covered. Doing this I comeout to a constant 97% to 98% of PFPX data. I then added 2% to the value of the default fuel burn scalar for my personal use.

 

 

Cheers,

 

Xander

 

EDIT: PMDG's reply back then was that they would look into the feasability of activating this feature..

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Hi Xander,

 

Glad to hear fliegzeuger and I are not the only ones to have noticed this. 

 

I have a question regarding the scalar, I would assume that this does not affect the FMC fuel prediction. So even if by adjusting the scalar, and yes before the usual trolls chime in, I know PMDG does not like when we do our own tuning (what other choice do we have?) does the FMC fuel prediction remain accurate when compared to PFPX flight plans?

 

In simple terms, when I adjust/correct the scalar, what happens to the FMC predictions?

 

Leo Cal

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I can confirm I have noticed the same discrepancy while testing the 77W with SimBrief. Reference the following fuel burns for the cruise portion (between TOC and TOD only, since climb/descent/approach phases can skew the figures):

 

Cruise fuel burn and time according to real 77W OFP: 40.3 / 5:32

Cruise fuel burn and time according to SimBrief: 40.1 / 5:30

Cruise fuel burn and time observed using PMDG: 38.4 / 5:33

 

This was at CI22 giving Mach 0.83 for the cruise portion. Average ISA was +4. ZFW was 225 tons and TOW was 283-284 tons. Initial cruise at F330 with steps to F350 and F370, weather parameters were closely matched in the sim using ASN.

 

This gives about a 5-6% discrepancy when you crunch the numbers, PMDG themselves advertise the 777 as being "within 5% of the actual Boeing aircraft performance charts", so one could argue this is pretty close to 5%. You also can't base this sort of thing on one flight, several tests have to be made and averaged out, but at first glance it does appear to be a little "lean".

 

As Xander said, you can tweak the fuel flow using the "fuel_flow_scalar = " parameter. I haven't experimented yet with the 77W but might do so in a few days, I suspect it doesn't correct the FMC predictions unless PMDG specifically programmed it to base calculations on the Fuel Flow Scalar.

 

 

 


I have reported this issue back in the 200LR days

 

I have done testing on the 77L as well and have actually found it to be very accurate to both SimBrief and real OFPs. It should be noted that the PFPX template for the PMDG 77L includes a fuel bias of 1.04, aka +4%. Did you take that into account during your testing, Xander? One could argue that was an error on PFPX's part rather than PMDG's, when using a fuel bias of 1.00 the numbers match up nicely for me. Granted, I only did 2 flights with it so I suppose I could be wrong.

 

 - Derek

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Guys,

 

in the 200LR, I ran at 100% scalar for some time. And noticed that fuel was still off a bit. Always around 2% for me. So I added the 2% to the fuel burn scalar.

In other words, if it was at 1.0000, I set it to 1.02. I have not had time to really test the -300ER. I did the PFPX test only once and again would up at 97-98

percent. Now please, all readers bear in mind that this is only noticeable for very long range flights. And as soon as you get a direct routing etc the difference

becomes really small and differences measurable. This is only for personal taste and use. And it is still within PMDG's advertised 5%. Furthermore even though

PFPX informed me that their data comes straight from the official sources, I don't know to within how many % they are either.

So again, for my personal taste, I make a one time adjustment to the scalar and I am happy. With both types I see that a 2% increase is good.

 

Also this will only adjust the burn %, and will not affect the FMC's calculations (99.9% sure). In real life, this is what the flow/drag adjust in the FMC is for.

As engines get overhauled, and planes go through life, they become less efficient, (Metal wear, wider tolerances, small dents on planes etc etc) you 

need to tell the FMC this. On the MD80's I fly, it measures actual thrust required for a given speed, together with the actual fuel burn, and crunches out

the difference in flow and thrust required as flow and drag percentages. We have them set at +3% flow and +6% drag on the MD80's that have this feature

in our limited fleet.

 

Xander

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I can confirm that.

 

Plan says 67.2t Trip Fuel. So with BIAS calculated out still 65.8t Trip Fuel.

 

However my 77W only needed 64.5t Fuel, and that was Gate to Gate with the second half of the flight flown at FL300 and Mach .87 selected in order to burn more fuel to get under the MLAW + 1 round in the usual BUBIN hold at OMDB. So that's quite off the calculation.

 

Robert

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Confirmed.

 

At least in the 10 flights (for me) it shows 10-11 % less fuel consumption compared to PFPX 100% fuel bias and FPPM.

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