Performance question: PFPX, performance file, bias values

[Will]

I've flown dozens of long flights now (12+ hours) using flight plans from PFPX, with the CF6 performance file from Steve Bell, and the wind corridor feature.

It's not really a big problem, but I've noticed that over the course of the flight I end up slowly falling about 10 minutes behind the projected time, while I get about 4-8k lbs x 1000 under the predicted fuel consumption. I'm only bringing it up because the pattern is so consistent. Always about 10 minutes late, always burning a little less fuel than predicted.

Anyone else notice this? Is there a rule of thumb for adjusting the bias values? Would adding negative drag bias in PFPX make the time prediction more accurate?

[Bluestar]

You might want to check the Cost Index.  I've noticed he has different one's for the same aircraft. 

Although I have PFPX and use Steve's files for reference, I find that I get better results using Sim Brief where I've leaned it out to -4 which matches my flying style. 

Bode

[G-CIVA]

All my PSX Model Files have PERF Factors set correctly if that information is available from real world sources. It might now be out of date.

A quick & dirty solution is to set the PERF FACTORS in your PSX Model files to 0/0.

Continue using the PFPX model files as they are with the bias settings.

When creating a flight plan with PFPX are you adding the DEP/ARR & SID STAR?

Are you injecting the PFPX wind data into PSX to create a "Wind Corridor"?

[Will]

Hi Steve, thanks for your input.

I'm using CI 180, with SIDs and STARs, and I fly route exactly as planned, so the FMC and PFPX are in agreement from takeoff to the final approach. I am using the wind corridor feature.

I'm pretty impressed as is; I think the performance predictions by PFPX are quite usable, because the variance is small, but the variance is always in the same direction by about the same amount (more time enroute, less fuel used) so it seems I could make the predictions even better.

I could do the obvious, which is to plan exactly the same flight multiple times using different bias values in PFPX, but before doing that I was posting here in case there is a rule of thumb for this. I can imagine how fuel bias affects the burn, but I'm not really sure about drag bias.

Does drag bias just turn into a higher burn? For example, does 1% drag bias end up meaning the engines have to burn 1% more fuel to keep the same airspeed? If so, then drag bias and fuel bias seem effectively like the same thing. And I'm not how the PERF factors translate into time enroute when using ECON with a cost index. Which is why I was curious about a rule of thumb, if such a thing exists.

Also, I suppose this post should really be in the "Accessories" section, because it's not really a PSX question, but a PFPX question. Or I guess it's a question about how the two integrate with each other.

Thanks.

[Hardy Heinlin]

The FMC applies the drag factor just to the predicted TAS. (As we know, a higher drag factor decreases the TAS.)

The consequences depend on the option you choose:

Option 1 -- The flight requires more fuel because you extend the flight time (you don't increase the fuel flow per hour):
You accept the lower TAS, and you just maintain your usual FF. (A higher AOA due to the accepted lower airspeed may increase the drag even more.)

Option 2 -- The flight requires more fuel because of the higher fuel flow (you don't extend the flight time):
You regain the planned TAS and ETA by increasing the fuel flow per hour.

So either option will increase the fuel burn in general, but the exact results are different and are not directly controlled by the drag factor value you entered.

The FF factor, on the other hand, is just applied to the predicted FF in the engine per se, independent of any drag, TAS, and ETA deviations.

I don't know how exactly the dispatch professionals handle this, but if both the fuel remaining and the ETA don't agree with the plans, I would just use the drag factor, not the FF factor.

I would use the FF factor when the ETA is good and just the fuel remaining deviates (e.g. when the drag factor alone doesn't correct the fuel as well).


Regards,

|-|ardy

[Will]

Okay, changing the fuel/drag bias factors to 0.5%/1.9% adds about 6 minutes to a 13-hour flight, and burns an extra 2000 lbs, per PFPX.

Next step is to fly the flight and see how things look at the end.

[G-CIVA]

All of the PFPX Perf Files & PFPX 744 Model files I created for PFPX were created using the Boeing FPPMs.

I dont know what data set HH has used to create the complex simulation behind what PSX is & I am not here to in anyway criticize what he has created.

But if if from a performance wise point of view PSX is not using Boeing FPPM data in the same way as the PFPX files there there will be inaccuracies & differences in results between PFPX & PSX.

Its as simple as that.

No criticism intended.

[Gary Oliver]

Can I add a data point of using CI53 with a simbrief provided plan or an old BA real world cirrus was within 200kg or so of a long PSX flight.

Cheers
G

[Will]

Steve, how do things work with Cost Index in the FPPMs? Are there "benchmark" cost indexes with defined performance, with intermediate values needing to be found by the user via interpolation?

[G-CIVA]

Cost Index Cruise Tables at 0, 100, 500, .84, .85, LRC & .86 are included in Boeing GE & RR 744 FPPMs.

They are sufficient for PFPX conduct the necessary calculations to cover the majority of scenarios.

Only Cruise Tables at .85, LRC & .86 are included in the Boeing PW 744 FPPM.

Slightly more limited.

I have set the last known real CI that the actual operator used in each individual PFPX Aircraft Model File.

I suggest you leave your PSX PERF Factors at 0/0.

I did extensive PFPX APMS testing of the PFPX data (using the proprietary Boeing book numbers) against a vanilla PSX Engine PSX PERF Factors at set at 0/0 with of each type with wind data corridor conditions injected from the PFPX OFP.

Thus I extrapolated a bias value inside the PFPX Model Files to match each PSX engine type with PSX PERF Factors at 0/0.

To summarise:

After extensive PFPX APMS (set at PERF FACTOR 0/0) comparison with PSX Burn data (set to PERF FACTOR 0/0)

PSX GE Engines burn an average of + 0.2% more than Boeing FPPM Book figures across all flight regimes.
PSX RR Engines burn an average of - 0.6% less than Boeing FPPM Book figures across all flight regimes.
PSX PW Engines burn an average of - 1.1% less than Boeing FPPM Book figures across all flight regimes.

Its as simple as that (but it took me a year of testing & data entry in notepad from circa 3000 pages of FPPM Books).

When you download my PSX Model Files you might find some of them do have different PERF Factor set; that might be a legacy issue because once upon a time I may have been lucky enough to have had access to RW OFP data to test against - sadly no more.

Change them back to 0/0 & you will be good.

Use a realistic CI - right now 50-90 is a realistic & economical CI in real-world use.

[Will]

Thanks Steve, that's helpful. I'll give it a try.

[G-CIVA]

Thanks Steve, that's helpful. I'll give it a try.

My pleasure.

I suppose I should get round to altering the PERF Factors numbers in all those PSX Model Files I have uploaded back to 0/0 at some stage.

It does make things confusing.

[simonijs]

Hi Steve,

With respect to this topic: would you allow me one email to the address that you used for providing me with PFPX data?

Kind regards,
Simon

[G-CIVA]

Hi Steve,

With respect to this topic: would you allow me one email to the address that you used for providing me with PFPX data?

Kind regards,
Simon

Yep ... send it via the usual way - email me through this forum

Steve

[simonijs]

I can see the mail icon below my name and photo, don't see it below yours ... (nor do I see it in the members list). I sent it to the one you used for sharing PFPX.

Regards, Simon

[Mariano]

Just a quick point of interest. In the company for which I work, replete with older Boeing aircraft, Performance Engineering only manipulates F-F values in the FMCs, (drag values are always zero).

I assume that this method varies by airline.

Regards,

Mariano

[Will]

Just a point of clarification here, I wasn't tweaking the bias values in PSX; I was tweaking the bias values in PFPX, so that the PFPX predictions to more accurately match what performance PSX gives. The 1.9% drag bias that I spoke of, which added 6 minutes to a 13-hour flight--that was PFPX's drag bias.

Mariano, I don't suppose you'd know this, but with your company, when they do change a fuel flow number in the aircraft's performance, do you think that follows the engines or the airframe? I would imagine it's an engine value, so  that if they took the engines off one airframe and hung them on another, the bias would be expected to travel with the engines. Of course, if they move engines around, they probably start from scratch and recalculate the bias, since the fuel bias refers to all 4 engines collectively.

[Mariano]

I will certainly ask that question.

When a new frame is introduced to an ETOPS fleet (767/777), they have to spend six months operating with a mandatory F-F factor of 5.0, before Performance Engineering can assign a new, more realistic F-F value.

I assume that the way my company does it, is by tracking and comparing real performance vs. FPPM/PEM values and then tweaking the F-F value as an overall combination of fuel flow and drag, leaving the drag value at zero.

I have seen one airplane with a negative F-F value. The only way I see this being accurate is by swapping a engine with a higher idle value (the value behind the decimal in the IDENT page under aircraft type) with an engine with a lower idle value. This is also what leads me to believe that, at this company, F-F values might be tweaked to reflect a combination of F-F and drag values.

Best regards,

Mariano

[simonijs]

Yep ... send it via the usual way - email me through this forum

Steve

I can see the mail icon below my name and photo, don't see it below yours ... (nor do I see it in the members list). I sent it to the one you used for sharing PFPX.

Regards, Simon

Hi Steve,

I'd still like to ask you some questions with regard to the PFPX Data files, preferably outside of the forum. Could you please send me an e-mail that I can reply to, at s.ijspeert@upcmail.nl ?

Kind regards,
Simon