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Cruise fuel bias and drag

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Hi,

 

I've just tried fuel PFPX bias calculator (during a stable flight - NGX + Active Sky P3Dv4 + Vatsim).

 

My CDU/MFD shows:

ISA +9C
TAS 389 kts
FF 1250 kg/hr
GS 403 kts
GW 66100 kg
SAT -22C
FL230

CI 15 / M.63

 

When I put all the data to fuel bias calculator I get following result:

Cruise fuel bias: -49,2%
Add drag: 0,3%

 

Is it possible to get -49,2% fuel bias value? My PFPX calulates now ~1600 kgs fuel less comparing to my previous settings - fuel bias +101% and drag 0,2%.

 

Cheers!

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I never use the bias calculator, but I wouldn't think FL230 is a typical cruise altitude for the 737 either.

 

Personally I use the profile available from AirlinerPerformance , adjust the weights and capacities to suit the PMDG and it is very accurate without any other adjustment.

 

My fleet is setup with profiles from here, if really necessary adjusting bias manually by a percentage based on historic flight data.

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9 hours ago, srcooke said:

I never use the bias calculator, but I wouldn't think FL230 is a typical cruise altitude for the 737 either.

 

Personally I use the profile available from AirlinerPerformance , adjust the weights and capacities to suit the PMDG and it is very accurate without any other adjustment.

 

My fleet is setup with profiles from here, if really necessary adjusting bias manually by a percentage based on historic flight data.

( I know FL230 is not a typical cruise altitude but I made a flight with FL restrictions so my cruise level had to be "not above FL245" according to RAD ;-) )

Stephen, what do you thing about FlyPrecisely profile for NGX? Is this one from AirlinerPerformance more accurate?

Cheers!

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Just a quick question - was the fuel flow readout for one engine or two?  I think you may need to double it?  1250kg/hour seems very low.  Looking at a real 738 flight plan, (this includes takeoff and climb as well), I can see a flight fuel of 12,200kg for just over 4.5 hours.  This equates to around 2,700kg an hour (including climb).  

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On 7/22/2018 at 9:01 PM, srcooke said:

I never use the bias calculator, but I wouldn't think FL230 is a typical cruise altitude for the 737 either.

 

Personally I use the profile available from AirlinerPerformance , adjust the weights and capacities to suit the PMDG and it is very accurate without any other adjustment.

 

My fleet is setup with profiles from here, if really necessary adjusting bias manually by a percentage based on historic flight data.

 

Also, Stephen, do you fly the PMDG 744 at all?  No matter which profile I use (have tried Airlinerperofmance 744ER profile - which is good!) and Doug Snow's 744 RB211 profile.  I have to adjust for ISA with bias each time I do a flight (I find a rough figure to adjust per ISA dev - negative bias for positive ISA dev, and positive bias for negative ISA dev).  I have not figured out if I have a PFPX or PMDG problem.  I read from years ago PFPX was having a bias problem with ISA dev?  I am on 1.28.8

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1 hour ago, VHOJT said:

Also, Stephen, do you fly the PMDG 744 at all?

 

I have the 747 but have had little time to run flights for any reasonable statistics.

 

As posted elsewhere by others I see the same issue regarding max altitude been assigned by PFPX rather than optimal, this would affect the outcome. Something I don't generally see on other airframes.

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Hmm.  I've found the following with the PMDG 747, in case anyone else stumbles or searches this one day:

 

Boeing 747-400 RB211-524G (PFPX default profile) @ CI50: -4.7% bias, +0.3% drag.  Then -3% bias per 10 degrees above ISA, +3% bias per 10 degrees below ISA

 

Boeing 747-400ER CF6-80C2B5F (Airlinerperformance profile):

 

image.png.9bb42f0b3efead51bb6b14302fa88c42.png

 

*CI50 is using the CI100 speed in PFPX, with bias.  APP = approach fuel.

Then -4.6% bias per 10 degrees above ISA, +4.6% per 10 degrees below ISA.

 

 

 

 

 

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On 7/23/2018 at 1:52 PM, VHOJT said:

Just a quick question - was the fuel flow readout for one engine or two?  I think you may need to double it?  1250kg/hour seems very low.  Looking at a real 738 flight plan, (this includes takeoff and climb as well), I can see a flight fuel of 12,200kg for just over 4.5 hours.  This equates to around 2,700kg an hour (including climb).  

 The FF readout was for one engine only, so that's why I got these unrealistic values. Will make more calculations during next flights. Cheers!

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