Fuel/Load Planner - Cost Index influence

[PaladinX 11]

Dear Aerosoft Support Team,

 

a fuel question:

Is there a thumbrule, how to increase the amount of fuel, when i fly with a higher cost index? (E.g. when i want to fly faster, i fly with an index of 50, 70 or 90).

How much more fuel do i need then compared to the results of the fuel planner?

What cost index is taken in the background?

[Fragged^2 40]

This would be in the OFP. You can use a tool like Simbrief to generate one and it will show how flight levels and cost index change the flight time and fuel burn. Below is a paste from my A319 flight from LOWI to EGKK. The fuel planner that comes with the plane is really simple and I only use it to plug in the numbers from Simbrief (ZFW and Block fuel).

 

 OPERATIONAL IMPACTS
                         -------------------
WEIGHT CHANGE UP 1.0          TRIP  P 0037 KGS   TIME P 0000
WEIGHT CHANGE DN 1.0          TRIP  M 0033 KGS   TIME M 0000
FL CHANGE     UP FL1                 NOT AVAILABLE
FL CHANGE     DN FL1          TRIP  P 0018 KGS   TIME P 0000
FL CHANGE     DN FL2          TRIP  P 0094 KGS   TIME P 0000
SPD CHANGE    CI 0            TRIP  M 0064 KGS   TIME P 0002
SPD CHANGE    CI 50           TRIP  P 0017 KGS   TIME M 0000

[PaladinX 11]

Hmm, interesting.

Why does SPD Change CI 0 (which would be slower) use 64 KGS, while CI 50 (which would be faster) use only 17 KGS more? *confused*

 

*Edit: Ok, now i understood: "M" 0064 KGS "minor"? And "P" 0017 KGS "more"??

That would be nearly neclectable, seen on the whole trip.

 

Or do i interprete wrong?

[Fragged^2 40]

58 minutes ago, PaladinX said:

Hmm, interesting.

Why does SPD Change CI 0 (which would be slower) use 64 KGS, while CI 50 (which would be faster) use only 17 KGS more? *confused*

 

*Edit: Ok, now i understood: "M" 0064 KGS "minor"? And "P" 0017 KGS "more"??

That would be nearly neclectable, seen on the whole trip.

 

Or do i interprete wrong?

 

M = Minus / less, P = Plus / more. This is vs the calculated CI of 21 for the flight.

 

Simbrief has a interactive tutorial/guide for what all the info in an OFP mean: https://www.simbrief.com/system/guide.php#ofpsample

[Secondator 643]

On 8/13/2019 at 4:27 PM, PaladinX said:

Hmm, interesting.

Why does SPD Change CI 0 (which would be slower) use 64 KGS, while CI 50 (which would be faster) use only 17 KGS more? *confused*

 

*Edit: Ok, now i understood: "M" 0064 KGS "minor"? And "P" 0017 KGS "more"??

That would be nearly neclectable, seen on the whole trip.

 

Or do i interprete wrong?

 

To simplify Cost Index: CI = cost of time / cost of fuel

With low cost index you are using least possible amount of fuel to complete the flight but at the same time you are also using the most time to complete the flight. With higher cost index you are using less time to complete the flight but in exchange you are using more fuel to complete the flight. This is the very basics of it. Depending on how much different areas cost to operate the Cost Index can be adjusted accordingly.

So since CI=0 means that you are using least possible fuel to complete the flight that's why the M for Minus in front. Means that you will save that fuel from trip fuel and therefore can subtract it from the trip fuel is the flight is completed with CI=0 instead of the planned cost index in the flightplan.

[PaladinX 11]

Thank you for the additional infos.

I am aware about the function of the cost index. But while the speed changes more than expected (e.g. 290 kts with CI=30 to 340 kts with CI=90), i have no feeling about the fuel usage.

Thats why i ask for the CI that is used with the Fuel Planner by Aerosoft. Its still unclear for me, because it contains no information.

[Secondator 643]

Especially on shorter flights Cost Index doesn't really play that big of a factor on fuel usage. On Longer flights it becomes more important but still the difference isn't that big on single flight. But the cumulative effect when you apply this to whole fleet over the span of a year is already significant.

So the difference is so small that when doing fuel calculations the cost index doesn't really matter that much. But with software like simbrief the cost index affects several other things than just fuel usage and that's probably why it asks it to make better predictions (on top of estimating the fuel usage).