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Correct for ISA deviation - experiences please!

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Hi all, particularly Mr. @srcooke

 

Does anyone find themselves having to correct for ISA deviation (negative bias for higher ISA dev and visa versa) for their aircraft?

 

I do it for the PMDG 747-400 and get very accurate results with the default RB211 profile, as well as the excellent AirlinerPerformance 744ER profile and am trying to categorically figure out this time which is it a "thing" with - PFPX or the plane?

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Yep. Just tried on the 747-8 and had the same problem.

My observation is that the planes in the flight sim don't use more fuel in higher ISA conditions and vice versa (different than in real life). However, PFPX calculates this, so the discrepancy increases with higher ISA deviations. There are two different options to correct for ISA devs:

 

a) have separate tables for ISA-20, ISA-10...and so on OR

b) have one table where you set like a "general" correction. This looks like this:

 

[CRUISE.17]
Name=[MACH]0.86

MinCruiseAlt=28000
CruiseModeBelowMinAlt=14

FuelAdj=3;-3
SpeedAdjust=10;-10
IceAdj=0;0
OptAltAdjust=0;0
MaxAltAdjust=0;0
 

There, when there is a ISA dev of +/- 10°C, PFPX will calculate a +/- 3% bias for fuel consumption and +/- 10kts for speed. a) is more accurate if our planes in the sim actually used more fuel. However, they don't so b) is the better option here.

 

On the 777, I managed to get rid of all the ISA deviations tables in order to just have tables like above with the entries like in the following example:

[CRUISE.17]
Name=[MACH]0.86

MinCruiseAlt=28000
CruiseModeBelowMinAlt=14

FuelAdj=0;-0
SpeedAdjust=10;-10
IceAdj=0;0
OptAltAdjust=0;0
MaxAltAdjust=0;0

You see, I set the fuel adjust to 0 and kept the speed adjust to 10;-10 as this is accurate. This is way more accurate than calculating for ISA deviations. Otherwise you'd always have to adjust the bias depending on the ISA deviations.

Unfortunately I can't do this for the 747-8 and the 787 since the files are encoded (.per files)...

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Thanks for your detailed reply.  I see your method, and quite like it - I am going to look into it for the ones I have.

 

I suspect this is a PMDG problem - but it is hard for me to say definitely - as I only really fly the 744 and the A320 much at the moment.  The FSL A320 doesn't seem to suffer this problem, but then it is harder to tell as it does shorter flights.

 

I see the problem you are having with your method with the per files - unfortunately you may be stuck using my method.  Figure out its burn at ISA DEV 0 (i.e nil WX), then with actual weather.  I did this with the 744ER profile just by putting plans in the FMC/loading the WX and comparing fuel burn and fiddling with bias till I got the number I needed.  Then divide that bias figure by the ISA deviation.  Downside is you have to look at ISA dev in PFPX on every flight and adjust accordingly.

 

I am wondering whether it is worth mentioning to PMDG?  I think I did once and they seemed very reluctant to consider it.

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I'm not sure if this only affects PMDG planes. I assume it may also happen when using the QW 787. But to be honest, I haven't done that much testing with other planes than I've done with PMDG planes. And as you said, on the A320 it's not that big of a problem unless you are flying more than 3hrs - that's why I didn't have a closer look here. But on long-range flights, the discrepancy is quite high.

 

My method was the following: set up the flight sim at a random location (I used KVPS or EDDF), set the clear skies weather theme without any weather tool like ActiveSky and then depart with a random load (most of the time I departed with 2/3 of the MTOW). Let it settle at the optimum FL and wait until the engines are stabilized. Then you can take the bias using the bias window in PFPX. Due to the clear skies theme, you'll always have ISA+0 and no winds, so this way there is no external influence on the performance. Write the numbers down and set the temperature to like ISA+15 (for FL330, ISA+0 equals -50°C, so in this case set it to -35°C). Wait until the engines are stable again and use the bias window again. The drag bias should be constant while the fuel bias changes..

 

I know it takes some time. I did a few tests with the 747-8 and the bias differs from -11% (ISA+15) to -6,5% (ISA+0) at CI 40.

 

Also I suspect that depending on which cost index you use, the bias will also change, regardless of the ISA deviation. On the 747-8, I estimated the bias for CI 15 to be at -6,1% while at CI 90 it's -1,1%, so another problem here.

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Good method.

 

Yep CI brings a whole new kettle of fish.

 

At least I think we now know it is not a PFPX problem?

 

 

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Indeed, that's why I fly with a constant CI most of the time because my biases were set on this cost index.

 

Exactly. PFPX does it's job right, it's more a limitation of flight simulation. We can trick PFPX to neglect any changes related to ISA deviations, but then the files need to be accessible (-> .txt files).

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Gents, from the above and some private conversations it's great to see some feedback on using the files, I think we can incorporate some changes.

 

On that note if there are any P3D users who fly PMDG and the QW787 and would like to help test some revised files drop me a PM please.

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Is there a change to the PMDG 777 I can use? Thank you

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