Airbus cost index and LRC
 

Can someone confirm that setting LRC in the A320 /321 requires reference to the LRC tables and manual setting of a specific Mach number that will vary as the weight decreases, rather than setting of a specific Cost Index.

I understand cost index 0 is max range and cost index 999 is min time but we never refer to LRC in our company documents except for some non normal ops.

The 2008 “Getting to grips with Cost Index” doesn’t appear to discuss LRC at all.

Thanks in advance.

Does this help? PRO-NOR-SRP-01-50 P 15/16

COST INDEX FOR LONG-RANGE CRUISE
The flight crew can use the table below to find an approximate Cost Index value that is calculated for cruise at long-range cruise speed. This value is valid for CRZ FL = OPT ALT ± 1 000 ft.

ENGINE / CILRC kg/min / CILRC 100 lb/h
CFM 56-5-A1/A3 45 60
CFM 56-5-A4/A5 40 55
CFM 56-5-B1/B2/B3 65 85
CFM 56-5-B4 (A321) 65 85
CFM 56-5-B4 (A320) 55 75
CFM 56-5-B4/P 25 35
CFM 56-5-B5/B6/B7 25 35
CFM 56-5-B5/2P CFM 56-5-B6/2P CFM 56-5-B7/2P 30 40
CFM 56-5-B8/B9 15 20
CFM LEAP-1A 40 55
PW6122/PW6124 20 30
PW1100G-JM 40 55
V2500-A1 45 60
V2522-A5/V2524-A5/ V2527M-A5 50 70
V2530-A5/V2533-A5 50 70
V2527-A5/V2527E-A5 40 55

You can also try out the numbers on Planestats https://www.planestats.com/bhsn_2018dec

Wow. There’s something else the archaic 737 does with just a button push........

Ive never understood this table even though I have seen it. Tthey're not saying, as far as I can understand, that we set a cost index 40 for LRC because from what I can see in my case using the IAE 2527 engines the value of 40 is representative of 40 kg/hour not a cost index of 40. I’m obviously missing something.

Seems like nobody else understands it either.

You can use a cost index, but it will depend on flight level and weight.
Airbus‘ document „Getting to grips with the cost index“, appendix 6, has some diagrams for LRC cost indices.

So I've looked at that graph but it still doesn't make sense to me (I'm looking at the version II 1998 appendix 6) it shows at the bottom of that graph that "Cost Index = KG per minute" and that we would potentially have 8 kg per minute burn for LRC which is clearly not correct aws that would only be 480 kgs per hour. Obviously I am missing something in the interpetation of this chart but I don't know what it is.

CI is price of kg of fuel devided by price of minute of flight. Setting CI 25 you say to an aircraft "Hay man, price of 1 minute is the same as price of 25 kg of fuel. Set such speed to save as mach money as you can". CI is not about fuel, it is about money.

I believe the QRH in the A320 specifies LRC CI as 40Kg/min. So we would adjust the cost index till we could see 40kg/min on the FUEL SD page.

No. CI have no direct connection with fuel burn. It is well discribed in GTG with cost index.

No.
What you see in the table is the approximate CI that equates to LRC.
The unit of CI being kg/hr (metric) or 100 lb/hr (imperial).
Agreed that adding the unit only leads to confusion.

https://cimg4.ibsrv.net/gimg/pprune....6cb95ba1d.jpeg
ref: GTG a/c perf

Ok so how do we set our selves up for LRC, not interested in what it costs?

Simple: just insert the CI of the LRC.

Most airlines fly a CI lower than the one associated to LRC, somewhere between LRC and MRC. LRC is actually a speed increase and range reduction in many cases.
Otherwise LRC continues to be a concept used in planning stage for driftdown, decompression, ETOPS/EDTO.

Ok and how do we know what cost index to insert for LRC?

scroll up. CI is given in PRO NOR SRP

Ok cool, our company’s A320’s have the VA2527-5A engines and from my understanding of that table, as long as we are 1000 feet +/- from the optimum level we can adjust and find an approximate cost index value for LRC which in turn will give us 40kg/min for this engine type, correct?

No. read my post again.
CI=40
The unit of CI is kg/min but this does not resemble a Fuel Flow!!

https://cimg4.ibsrv.net/gimg/pprune....99058791a9.png
Ok I get the fuel flow bit but how do we explain the cost index being 17 and not 40 as you have suggested given in this example using these graphs?

40 was just an example, a number you came up with.
Note that CI for LRC is to be extracted from FCOM/PRO/NOR/SRP/FMS/Prep for descent and approach

Thank you guys, especially OPEN DES for the explanations. I finally understand it now haha.

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?

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.

To answer your specific question as to how to use the table. For an A320 with V2527-A5 engines within 1000 feet of optimum FL and metric fuel gauges, set CI40 for LRC.

Ok thank you all for your input and patience. So out of all that I can take away that we insert a CI of 40 in the FMGC to get LRC. I hope that’s right otherwise shoot me now lol.

Sorry, Just read your post now! xD Happy other members explained it. =)

Is this applicable for single engine scenarios as well? If we set CI 40 in a single engine situation, does the the FMGS still give us LRC for one engine inoperative?

As far as I remember, Single Eng FMGS calculations are by default at LRC. They don't use Cost Index