Hello all, this is basically a copy and paste from a topic i made at the PFPX forum which nobody seems to have an answer for, so i figure why not give it a shot here.

 

There is an issue which has plagued me for some months now.  Something seems to be up with PMDG 777-300ER and the same performance data in PFPX. When i use the bias tool and I enter the values i see real time in the sim, the calculated TAS value is off.

For example, when i enter these conditions

 

GW - 635,000 (lbs)

Alt = FL340

Cruise/Cost Index = 200

SAT = -49

Act Fuel Flow = 18000

Act TAS 487

And leave the Ground speed blank, the TAS calculated is 496kts which is almost 10 knots higher than i am seeing in the sim, however when i change the Cruise/Cost Index field from 200 to M.83, while the 777 in the sim is crusing at M.835, the TAS calculated by the bias tool (485) is closer to what i am seeing in the sim (487).

 

Can anyone help me figure out what is going on? It's almost as if the Cost Index data is off in either PMDG or PFPX. I understand that there can be discrepancies between flight planning software and the aircraft but the only reason i bring this up, is that it used to line up almost perfectly (within 0-3 knots) on every flight, and somewhere along the lines, when updates for both the 777 and pfpx were released, this discrepancy shows up. So i'm asking here to see if anyone else that uses this combo notices this and what they think could be the issue.

 

 

 

 

 

Can anyone help me figure out what is going on? It's almost as if the Cost Index data is off in either PMDG or PFPX. I understand that there can be discrepancies between flight planning software and the aircraft but the only reason i bring this up, is that it used to line up almost perfectly (within 0-3 knots) on every flight, and somewhere along the lines, when updates for both the 777 and pfpx were released, this discrepancy shows up. So i'm asking here to see if anyone else that uses this combo notices this and what they think could be the issue.

 

Our data comes from Boeing. I've never been able to track down where their data is from, though all I usually get is people saying "well, I'd assumed it was from Boeing."

 

Verified versus assumed - I'll let you be the judge.

PFPX uses a very simplified table look up algorithm for aircraft performance. I do not used PFPX for determining CI.

 

I works very close to actual simulation using .84M instead of a CI, and my PMDG 777 is set up with a CI of 200.  I've had predictions come in surprisingly close for long flights.

 

 

Our data comes from Boeing. I've never been able to track down where their data is from, though all I usually get is people saying "well, I'd assumed it was from Boeing."
 
Verified versus assumed - I'll let you be the judge.

 

I figure, i just thought i'd cover whatever angles i can.

 

 

 

I works very close to actual simulation using .84M instead of a CI, and my PMDG 777 is set up with a CI of 200.  I've had predictions come in surprisingly close for long flights.

 

Yeah when using fixed cruise speed vs cost index for planning it is spot on. So since no one has responded over at the other forum, ill just have to edit the performance file myself, oh the pain 

 

Thanks for your responses guys.

I've found PFPX to be an excellent fuel planner with the B777F.  I did EDDF-KSPS with PFPX showing 09:43 with the actual time being 09:42.  Fuel planned by PFPX was 184,927 with actual trip fuel being 182,919.  The TOW was 733,545 and LDW was 548,618.  I do two things 1) I use a cost index of 80 and 2) set the altitude in PFPX to MAX.  I also set the step climbs as shown on the PFPX flight plan.  PMDG did an excellent job with the aircraft and PFPX has built a good fuel planner for it. 

 

 

I use a cost index of 80

 

Yeah thats the cost index i used to use myself, until i discovered that simbrief appears to sort of calculate a cost index for you. It seems to be accurate with a cost index of 80, but the further you deviate from that, is the less accurate it seems to become, at least the TAS that is.