Fine tuning fuel usage

[Ray Proudfoot 231]

I've only had PFPX a week but on several of my flights I've had less fuel remaining than PFPX forecast. I have already 'tweaked' the performance adjustments using the Evaluate tool but the disparity remains.

So today I decided to record the remaining fuel as I passed each waypoint and compare it to that calculated by PFPX.

I created a flight from LEAL (Alicante, Spain) to LIRP (Pisa, Italy). Aircraft is a PMDG737-900 from FS9 but it flies just fine in FSX. Cruise level was 410. Fuel loaded was 12,581 and it suggested 3,800 would remain at LIRP.

I'm attaching a screenshot of the fuel readings as each waypoint was passed. Climb phase is Green, Cruise is blue and descent is pink. Actual and forecast fuel readings are shown together with the difference.

You can see that during cruise the consumption matches the predicted exactly. Descent starts and ends the same. It was a long taxi in hence the extra used.

The difference seems to be during the climb where far more is being consumed than estimated. My climb, cruise and descent bias is 106.4% so why is more fuel being consumed during climb?

Any help would be appreciated.

[ajurt 3]

Ray,

Here is how I do it: instead of the evaluation, I adjust the fuel_scalar in aircraft.cfg file, in other words, I adjust the FS fuel usage to the real world. That way your plane will have the same range as the real plane. Like yourself, I fly a test route (EDDF - LIRF) with no winds until the FS fuel usage matches PFPX. I only record TOC and first waypoint after that, last waypoint before TOD, TOD and destination fuel. Once the fuel figures agreee, I do fine adjustment on climb, cruise and descent percent in PFPX so the figures match, making sure the final fuel burn figures still agrees between FS and PFPX. Many times I may have to add a few hundred kilos of fuel on the approach in PFPX, as the FS plane seems to use more fuel with flaps down than the real plane.

Finally I place the plane at the end of the runway, note exact fuel figures, then taxi for 5 minutes, figure out the difference and update the PFPX taxi fuel.

It's a bit of work, but once you got the routine down this works pretty good.

Andre

[Ray Proudfoot 231]

Thanks Andre, that's certainly something I hadn't considered as the fuel scalar has been calculated by PMDG and I would expect it to be accurate. Is it one value or several?

I will fly a test route with no wind to remove that variable. As you say, it's a fair amount of work but once done the results should match PFPX. I assume I can switch off the weather element in PFPX so it also bases fuel on zero winds?

[Ray Proudfoot 231]

Andre,

I've thought about this some more. My fuel usage during cruise is fine. The descent phase is okay too. It's only the climb where things need tweaking.

So I'm more inclined to adjust the climb bias until fuel consumption matches that of PFPX. I think adjusting fuel scalar would make things worse.

[ajurt 3]

Ray,

Adjusting the climb bias will work definitely too and it's a lot quicker.

Andre

[Pierre95 14]

Hello,

I think after tweaking the fuel burn with "Evaluator", if a difference still oocurs in a specific flight phase, you have to adjust this phase bias.

I agree with Andre.

Best Regards

Pierre

[Ray Proudfoot 231]

Andre / Pierre,

In the absence of any instructions on how to do this I am very reluctant to do it. Surely adjusting the bias in PFPX would solve the problem?

I don't even need to fly the whole route. Just to cruise.

[srcooke 422]

Hello Ray,

Which Climb Mode are you using?

Using CLB-1 or CLB-2 leads to increased fuel consumption.

By using using a de-rated TO at the required level then CLB mode my fuel figures for the -800 in FSX are accurate.

[CaptainTim 11]

200KG is a pretty good margin considering the winds may not be exactly what PFPX said compared to what the weather programme gives you as well as outside air temp.

I think on one flight I used with FOC years ago, I came within 50kg. Never did it again after but came close with 100/200kg. Also depends on what directions ATC give you.

I don't see an issue here - if it was a ton or more out of a sync then I'd see that as a support issue. I'd look for a tolerance of 5 to 10% of fuel and wind i.e. expecting what I have in the sim to be within 5- 10% of what PFPX says. I think you've got a tolerance of less than that so I see no issue.

Cheers,

Tim

[Ray Proudfoot 231]

Hello Ray,

Which Climb Mode are you using?

Using CLB-1 or CLB-2 leads to increased fuel consumption.

By using using a de-rated TO at the required level then CLB mode my fuel figures for the -800 in FSX are accurate.

Hi Stephen,

I think it was CLB-1 as the payload was light. Interesting it should use more fuel. I'll try again but switch to normal CLB mode after take-off.

[Ray Proudfoot 231]

Tim,

If you compare what PFPX calculated for the climb phase - D127G to IZA it suggested 1,900lbs but I actually burnt 2,700lbs. That's 42% more. The flight was planned and executed straight afterwards. PFPX was correct for the whole cruise phase and descent. I think Stephen's point about a de-rated take-off and climb is probably key here.

I certainly don't see it as an issue with PFPX. It's all to do with tweaking the bias in the Evaluator. But first I shall try switching to full Climb mode after a de-rated t/off.

Thanks everyone.

[srcooke 422]

I had been using CLB-1 myself generally until comparing the fuel figures to PFPX. Christian commented on a post either in this forum or over at FlightsimSoft which steered me to using CLB mode.

When you think about it you achieve a more efficient altitude quicker.

[Ray Proudfoot 231]

I had been using CLB-1 myself generally until comparing the fuel figures to PFPX. Christian commented on a post either in this forum or over at FlightsimSoft which steered me to using CLB mode.

When you think about it you achieve a more efficient altitude quicker.

Yes, but I would have thought you would burn more fuel doing so. I would expect CLB-1 to use less than CLB as it will take longer to reach cruise. In fact that was borne out as PFPX calculated I would reach CRZ before IZA but I reached it afterwards.

[Ray Proudfoot 231]

I've increased the climb bias to 110% from 106.4% and it is getting closer to the fuel used. I think a value around 113% will be very close.

[ajurt 3]

Ray,

According to your chart, you were only off by 100 lbs at TOC, so I wouldn't change anything. If you change the bias, then it probably will have an effect on the entire climb.

Andre

[Ray Proudfoot 231]

Andre,

I take your point but have a look at this chart. This was after changing Climb Bias to 110%. Looks pretty good to me. I'll need to try this with a few more flights of course. Ignore the final planned fuel at LIRP. I forgot to make a note of it.

[ajurt 3]

Ray,

Looks good indeed. The one thing you could improve though is your descent. If you look at your burn between ETAGI to UNPIV, your planed burn is 0.9, but in actual fact you burnt 1.4, that's 55% more than planned (1.4 / 0.9 = 1.55). You could just change the bias or have a combination of bias and additional fuel for the approach.

Andre

[Pierre95 14]

Hi gents,

A 3.7 reserve at landing is very correct for B739 on short trip.

Maybe as says Andre could you adjust descent bias to be in perfect accordance with planned burn.

Best Regards

Pierre

[Ray Proudfoot 231]

Andre / Pierre,

I agree the descent phase needs some tweaking. I'm getting stir-crazy flying LEAL-LIRP. But it's worth the effort to get the burn correct.

I have to say I'm loving PFPX and it's given me a whole new insight into fuel usage. Until now I hadn't appreciated just how much is burnt when taxiing or how efficient engines are at cruise.

[Pierre95 14]

Hi Ray,

PFPX is a learning tool too for me first....

Cheers

Pierre