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Cost Index question

Posted: Mon Dec 25, 2017 9:35 pm
by wxman11
Hi folks, :wave:

I have a question regarding the cost index. If 1 airline uses pounds to construct their flight plan and uses a cost index of 40 and I wanted to know what cost index 40 (CI40) is in kilos, am I to presume that I would have to just convert the cost index number from pounds to kilos?? :scratchchin:

i.e.
pounds divide by 2.2 = kilos
40 / 2.2 = 18

Brgds
Wxman11

Re: Cost Index question

Posted: Mon Dec 25, 2017 11:13 pm
by paullam
The cost index has nothing to do with the unit you use. It doesn’t matter whether an airline uses kg or lbs, the index equals the same thing. So there’s no need to convert anything.

Re: Cost Index question

Posted: Mon Dec 25, 2017 11:30 pm
by zeke
On airbus equipment cost index is either on kg/min or 100 lb/hr. For a A330 with RR engines a cost index of 40 kg/min is equal to 53 (100 lb/hr)

Have a look online for an Airbus publication “Getting to Grips with Cost Index) for more details.

Re: Cost Index question

Posted: Tue Dec 26, 2017 7:30 am
by mmo
Here's a good review by Boeing. Cover several aircraft types.

http://www.boeing.com/commercial/aeroma ... _05_1.html

Re: Cost Index question

Posted: Tue Dec 26, 2017 1:48 pm
by zeke
Where in that article do they describe the cost index differance between normal SI units and American?

Re: Cost Index question

Posted: Tue Dec 26, 2017 2:28 pm
by kalvado
zeke wrote:
Where in that article do they describe the cost index differance between normal SI units and American?

Actually Airbus coveres that very briefly in their document, and there is a bit of contradiction:
http://ansperformance.eu/references/lib ... -index.pdf page 7 gives index units as
Units are given in kg/min or alternatively as 100 Ib/h

Strictly speaking, 1 kg/min should translate into 132 lb/hr, not 100. Not sure if that is the overall accuracy of optimization, or there is a unit switch option (that would probably be meaningless, since the way Airbus describes the system, pilots don't have the data for accurate calculation anyway, and should probably use the number provided to them by the company. Calculating proper units on the ground should eliminate some confusion)
Anyway, according to Airbus graphs, 30% difference at high values of cost index can eat up all possible optimization, especially if initial calculation is a bit off from the minimum.

Re: Cost Index question

Posted: Tue Dec 26, 2017 2:39 pm
by zeke
kalvado wrote:
Strictly speaking, 1 kg/min should translate into 132 lb/hr, not 100.


No 100 lb/hr is correct, that is why the SI CI=40 (kg/min) is equal to the American CI=53 (100 lb/hr). I think Boeing also factor theirs by around 2 orders of magnitude.

Re: Cost Index question

Posted: Tue Dec 26, 2017 3:24 pm
by kalvado
zeke wrote:
kalvado wrote:
Strictly speaking, 1 kg/min should translate into 132 lb/hr, not 100.


No 100 lb/hr is correct, that is why the SI CI=40 (kg/min) is equal to the American CI=53 (100 lb/hr). I think Boeing also factor theirs by around 2 orders of magnitude.

If you don't mind.. May I ask about the cost index assignment practices from your experience?
Thing is, the way AB and Boeing write about it, CI should really be adjusted per route, with today's fuel prices at departure and destination, frame and engine contracts (if different within the fleet), and possibly even crew seniority, plane load, anticipated delays (aka extra fuel on board) and what not. But calculation is oh so difficult.. And many decisions have a bit of granularity, e.g. flight level is assigned in increments..
The way I see it, if there is no specific CI for the trip provided by the company, that all drifts into a back of envelope estimate. If CI is not updated approximately weekly, it becomes an educated guess at best.

Re: Cost Index question

Posted: Tue Dec 26, 2017 3:33 pm
by zeke
It depends on the airline, some will even change the CI after a step climb. I reality it should be changed more often, but most airlines do not know the cost per sector.

Cost index also should change if you have two of the same airframes in the same airline with different engines installed, in reality most would use the same.