Airbus cost index and LRC
Join Date: May 2006
Location: Sydney
Age: 41
Posts: 23
I came up with 40kg/min as a fuel flow, not a cost index but evidently that’s not correct. I asked for what cost index you would use for LRC and you said 40. I can see this is getting a little frustrating for you but I appreciate you trying and your patience. To be honest, I personally am still confused. Maybe you could attach that reference you mentioned as a screenshot? The table above with the different engine types is the only one I found in the FCOM, is this the same table you are referring to?
Pineteam, maybe you can help me out and give OPEN DES a rest lol?
Pineteam, maybe you can help me out and give OPEN DES a rest lol?
Join Date: Jan 2004
Location: Here and there
Posts: 2,852
What you seem to be missing is that CI = 40kg/min is not referring to fuel flow per minute, it is referring to a CI of 40 which is telling the FMGC to work on 40 kg of fuel being the same cost as 1 flight minute.
The reason the table gives different numbers to the graph is that the table is making assumptions for the sake of simplicity, the big one being that the cruise level is within 1000’ of optimum. If you look at the higher altitudes / weights on the graph you get a CI of 30, much closer to the 40 from the table.
Also if you look at some of the CI values for various engines you’ll see that it doesn’t make sense for it to refer to a fuel flow of kg/min, for example a CFM-56-5-B8 engine would be on every A320 if LRC gave 15kg/min fuel flow!
If Airbus had just avoided specifying the CI as kg/min it would be a lot less confusing. It appears they have done this to differentiate the metric from imperial aircraft.
I hope that helps. I didn’t understand it either until OPEN DES explained it, but I think he’s on the money.
The reason the table gives different numbers to the graph is that the table is making assumptions for the sake of simplicity, the big one being that the cruise level is within 1000’ of optimum. If you look at the higher altitudes / weights on the graph you get a CI of 30, much closer to the 40 from the table.
Also if you look at some of the CI values for various engines you’ll see that it doesn’t make sense for it to refer to a fuel flow of kg/min, for example a CFM-56-5-B8 engine would be on every A320 if LRC gave 15kg/min fuel flow!
If Airbus had just avoided specifying the CI as kg/min it would be a lot less confusing. It appears they have done this to differentiate the metric from imperial aircraft.
I hope that helps. I didn’t understand it either until OPEN DES explained it, but I think he’s on the money.
Last edited by AerocatS2A; 4th Mar 2020 at 10:59.
