PFPX Wind Summary

[G-CIVA]

OK, so how many hours ... would you say in total have you been monitoring this?

I'm not trying to shoot the messenger here ... its just useful to know.

The explained method I'm using doesn't care if the adjustment slider is moved or not;

Any deviation away from your PFPX created wind corridor WILL make a difference. If your system of 'excell calculus' does not take into account the actual in flight conditions (PSX vs PFPX) might be creating inaccuracies. When you used PFPX to plan your flight (assuming your PFPX Flight Plan is one of the items you are comparing your in PSX flight data too, did you set an accurate departure time in PFPX - this will drive PFPX to use the correct Wind Data that has been downloaded to create your Wind Corridor?

Is your PSX flight flying to this departure timing you set in PFPX?

Commercial Flight Planning software uses Wind Data which is normally downloaded into flight the planning software at 6hr intervals. As stated, my research & findings were gathered over an extended period of testing & data monitoring.

As I stated in my previous post ...

It is important to have the Wind Corridor & WX situation inside PSX as 'synchronised as possible' with PFPX:



And also:

[MFarhadi]

To give an example,

Flying at FL360, with a Gross Weight of 306,251 Kg, TAT at -14.96°C and TAS of 502.27 knots: Then the Fuel Flow (60 seconds average flying at straight and level resulting in the above numbers) is: 10883.89 Kg/Hr

If Mach.86 is selected in PFPX, APM results in:
Nominal Fuel Flow: 10739 Kg/Hr
Nominal Mach Number: 0.867 (don't know why PFPX puts out a different nominal Mach number if the already selected regime is constant Mach .86  )
Nominal TAS: 507 Knots
Calculated Fuel Bias: +1.3%
Calculated Drag Bias: +1.0%

and If a Cost Index of 60 is selected (Equivalent to Mach .860 and TAS 503 knots)
Nominal Fuel Flow: 10646 Kg/Hr
Nominal Mach Number: 0.860
Nominal TAS: 503 Knots
Calculated Fuel Bias: +2.2%
Calculated Drag Bias: +0.2%

[MFarhadi]

I tried monitoring using the RB211-524G2 for 2 sectors, first one 5 hours and the second for around 2 hours.
I've also monitored PW4056 (which was *partly* unsuccessful because half the route I was deviating left and right due to WX) and also the PW4062 (but for a short period; about 1.5 hours)
This is my first attempt at the CF6-80C2B1F with the method I mentioned. During the past 6 hours of the cruise, I managed to collect 7 log files, and I'm still trying to find a pattern or something that I'm missing; as your data is compared against FPPM and I'm still skeptical of my method or a numerical error I haven't noticed yet.

Furthermore, my PSX is synced with UTC, and I use the corridor function with Format "C" (NG FMC Manual page 82) out of PFPX.

[G-CIVA]

I'm still trying to find a pattern or something that I'm missing; as your data is compared against FPPM and I'm still skeptical of my method or a numerical error I haven't noticed yet.

Use the APMS facility inside each PFPX Model File which is applicable to the PSX airframe you are flying...the AMPS function compares your instantaneous recordings to the PFPX Aircraft File fuel burn data (that I entered directly from FPPMs) as a comparison.

Instantaneous data can sometimes create strange findings...thats why I took 8 months to test the data I used from the FPPMs ... to try & achieve good tight AVERAGE findings.

[MFarhadi]

How silly of me not to mention the effect of CG yet.
I don't have any figures regarding the effect of CG on fuel flow and performance factors. I believe Hardy can enlighten the subject on whether or not CG affects fuel flow in PSX; but I guess I'd be minute (less than 1% for other airplane types).

I guess I still haven't figured out why the Estimated FOB at the Destination in the CDU always shrinks over time during long-haul flights. Kind of a horror scene to be honest when it reaches the reserve value 

[G-CIVA]

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

Look at the amount of flights I had to do on just one single engine type to gain what I considered accurate data - look at the city pairs & imagine the flight durations!

How silly of me not to mention the effect of CG yet.

What do you consider a reasonable CG for your test regime?

If I tell you that an empty 400F/ERF/BCF/BDSF will have a CG value of approaching 33% AFT would that surprise you?

Empty Ferry situations in freighters do encounter high AFT CG situations & fuel is added to achieve the Minimum In Flight Weight (MIFW) - which you will see I have accounted for in every PFPX individual tail number.   Fuel is often added to achieve & maintain the MIFW as unusable or possibly ballast fuel. This data is taken from RW sources. Additional Fuel carried attracts a higher fuel burn penalty .... possibly up to 300-500kgs for every 1000kgs carried over the arrival threshold.

That CG value can then vary FWD towards circa 20-25% when the aircraft is as MTOW with the MAX payload & fuel right up to MTOW.

If your PSX aircraft is flying with a silly or unrealistic CG value in cruise it will affect data.

[MFarhadi]

Flying at FL360, with a Gross Weight of 306,251 Kg, TAT at -14.96°C and TAS of 502.27 knots: Then the Fuel Flow (60 seconds average flying at straight and level resulting in the above numbers) is: 10883.89 Kg/Hr

If Mach.86 is selected in PFPX, APM results in:
Nominal Fuel Flow: 10739 Kg/Hr
Nominal Mach Number: 0.867 (don't know why PFPX puts out a different nominal Mach number if the already selected regime is constant Mach .86  )
Nominal TAS: 507 Knots
Calculated Fuel Bias: +1.3%
Calculated Drag Bias: +1.0%

and If a Cost Index of 60 is selected (Equivalent to Mach .860 and TAS 503 knots)
Nominal Fuel Flow: 10646 Kg/Hr
Nominal Mach Number: 0.860
Nominal TAS: 503 Knots
Calculated Fuel Bias: +2.2%
Calculated Drag Bias: +0.2%

This one was taken with a CG value of approximately 22%.
As I'm completely alien to 747, I didn't have a clue about nominal CG values. Had previously seen a 747-400F Weight and Balance Report but never really bothered to take a look at their Load & Trim Sheets.
I'll take another shot with a CG of 28%. I'll let you know if it was drastically different than the other observations.

[G-CIVA]

As I'm completely alien to 747, I didn't have a clue about nominal CG values. Had previously seen a 747-400F Weight and Balance Report but never really bothered to take a look at their Load & Trim Sheets.
I'll take another shot with a CG of 28%. I'll let you know if it was drastically different than the other observations.

The Weight/Balance operating limitations of a 747 are vast.

Here is one example in lbs:



Here is an ER Pax example:

[Hardy Heinlin]

In PSX, the CG doesn't influence the airspeed directly. The CG affects the pitch control rate and range. The CG also affects the MAX ALT. The more downward force is added at the elevator and stabilizer, the lower the total lift, thus the lower the MAX ALT.


|-|

[simonijs]

But when the weight is reduced down (say around 320 tons), FF is about 2.2 to 4.2% higher than the PFPX's nominal (0.0%) value. Why the variation? While keeping a constant Mach of .85, PFPX's calculated fuel bias is 4.2%, while if CI is adjusted to give the exact .850 and though TAS and TAT remain the same, FF bias is calculated at 2.2%.

Hi,

I may be misunderstanding your point here, but...: under normal circumstances and using ECON, your TAS will decrease as aircraft mass decreases. And so does the Mach number. At some point aircraft mass has decreased far enough to allow a step climb. At that higher altitude, TAS has increased again, temperature has decreased and your Mach-number might be back to - say - M0,85. This is all very nicely modelled in PSX.
However, when you are keeping the speed fixed at M0,85 then more thrust is required as aircraft mass decreases. ECON speed is flying at a constant ratio of CD over CL. Flying at a fixed Mach number will change that ratio, influencing drag, and hence thrust required which is proportional to FF/hr.

For PH-BFU, LRC at OPT ALT would be M0,859; for PH-CKA (400ERF) this would be M0,852. I can't speak for models other than CF6.

If a Cost Index of 60 is selected (Equivalent to Mach .860 and TAS 503 knots)

According to Boeing, LRC for the 747-400 is at approximately CI-230. CI-60 is well below CI-230. My guess would be that at CI-60 TAS and Mach are lower numbers than the quoted numbers.

Regards,
Simon

[MFarhadi]

Dear Simon,

I highly appreciate your reply and the invaluable data points you mentioned.
Basically, the reason I initiated such tests was, as my flights progress to the destination, the estimated fuel figure (no matter what engine type I had chosen) over destination decreases. I use the PFPX winds for the corridor function and the built-in ACARS network for Wind downlinks, thus if randomized variations of Wind Speed/Direction and OAT were not used, I should be at each point with the exact predicted fuel and time figures (if the FMC FF/DRAG and the model Fuel and Drag values are identical which I set all of them back to 0.0% before each flight as per Steve's recommendation).
As a result, I started using PFPX' APM function to get a grasp on what's going on with PSX during later stages of flight when the aircraft has become less heavy. The sole reason I mentioned CI of 60 was to get two redundant M.860 readings from PFPX, one with a constant Mach segment and another with ECON CI xx, just to crosscheck whether flying at the same Mach would result different Fuel Figures, which it does. PFPX spits out different fuel figures for the exact same Mach and TAS that were derived from different flight regimes.



Best,
Reza

[Hardy Heinlin]

Hello Reza,

now a completely different question:

For your fixed-Mach tests, what FMC speed mode do you use? SEL SPD or MCP SPD?

On the VNAV page title you see the active speed mode for the current aircraft control,
while in 2L you see the speed mode used for the ETA/fuel predictions.

Even when the page title says "MCP SPD", 2L will never display "MCP SPD". That is, the FMC considers the MCP SPD mode a temporary intervention that affects just the next 50 to 200 nm. In that short range the FMC uses actual sensor data which will be gradually mixed with calculated data based on SEL SPD, ECON, LRC or whatever mode is displayed in 2L.

In other words, for a long-term constant speed you should use SEL SPD, not MCP SPD. You probably use SEL SPD already. I'm just mentioning it to be sure that this point is checked.

On the ACT CRZ page in 4R you see the predicted ETA/fuel. When you are in MCP SPD mode and you change the MCP speed, you will see the data in 4R will not change. To get a different ETA/fuel in 4R you need to change the SEL SPD value in 2L, not the speed on the MCP.


Regards,

|-|ardy

[MFarhadi]

Dear Hardy,

I normally fly using Cost Index but during the constant Mach tests, I use the SEL SPD function (e.g. ACT .850M CRZ).

[MFarhadi]

One other thing,

Correct me if I'm wrong.
The following conditions:
747-400F using GE CF6-80C2B1F Engines, at FL360 - CI=100 - TAT=Minus 18C (ISA+7) - GW=278,000Kg and 22 knots Headwind
Should give:
Mach .866 , TAS=505 knots , FF 9,730 Kg/Hr

Because the FMC is maintaining Mach .841 with the said Cost Index.
However, strangely, maintaining the VNAV CRZ page depicted N1% (92.1% in this case) roughly maintains Mach .866~.868 and not the .841 which it had calculated.

Edit: Delta ISA has been corrected.


Using the Mach .866 (MCP SPD) data, PSX burns around 2.7% more than what PFPX (Steve's profiles) calculates.

[Hardy Heinlin]

The CI algorithm in PSX is not exactly equal to the CI algorithm of Boeing. The details in their algorithm are sort of a company secret. Known are just some corner stones for interpolation.

In PSX, CI 0 uses the minimum cruise speed performance table, and the highest CI uses the high speed performance table (LH CF6). Between these two tables there is a certain non-linear interpolation which is factored so that CI 230 is approximately equal to the LRC performance table.


|-|ardy

[MFarhadi]

Thanks for the clarification, Hardy.
So, using LRC should give the most consistent results when compared to PFPX and FPPM, right?

[Hardy Heinlin]

I don't know PFPX, I'm sorry.

[simonijs]

By the way: It is ISA + 7... (and then TAS would be 490 kts; CAS 281 kts; M0,841; TAT -17,7; & OAT -49,3).

[MFarhadi]

It is ISA + 7

Corrected and Edit notes added

[MFarhadi]

I believe I finally found the culprit.
Off-block fuel (after APU usage) was 100.0 tons.
Seemingly, some of the on board gas goes missing during the cruise (it was either after TANK to ENGINE or after Reserve 2&3 Transfer; I couldn't recall) resulting in the gradual diminishing of the EFOB over the Destination.