A320 Cost Index

[raraa]

Reading the books on A320 the Cost Index made me feel that cost index number is a good factor to manage the fuel consumption in relation to the cost of time (airplane, crew, etc..)

however on line operation I'm finding that flying high level (FL330-390) on anything less than .78 increases cost of time with no fuel saving. the other day at TOC I tried the FMGS predictions on .74 through .80.

I got predictions that confirmed my suspicions:D .74 through .78 gave me fuel estimate of 2.6T at destination while costing me 15 less minutes to destination while .79 and .80 cost considerably more fuel.

Same thing happened with CI anything less than 35 cost no more but plenty of time incurred, while anything more than 40 cost considerable amounts of fuel.

Talking to the friendly folks :ugh: at dispatch said their calculation gave me an allowance of CI 3-5 and no more. :=

Two question to fellow bus drivers out there especially A320 guys.

1-Do you find that there is an optimum speed or CI? or is it that I am missing something?

2-What CI are you dispatched with (if any) and does it matter which operation you fly for? I hear folks on the radio flying 320s at .80 or .81 so why do you do it?

Thanks for reading and I appreciate your replys

[Microburst2002]

the CI is the price of one minute of flight (fuel cost excluded) in terms of kilograms of fuel.

If your airline has very very well paid pilots, cabin crew and employees, and has very high costs for maintenance and others, and fuel price in your home base is very very cheap, then your CI will be high.

If your airline pays peanuts to the employees, and is low cost, and maitenance is cheap, and fuel is very very expensive, then the CI will be low.

The guys in the first airline "we few, we lucky few..." will cruise at .79 or .80. The unlucky ones will cruise at .77, 76...

But don't worry too much about that. Knowing what is the real CI of an airline for each particular route is so so difficult, that most of them just stick to a CI for years until nobody dares to change it, as if it was some ancient god.

[Microburst2002]

As for the changes in predictions when you change MN:

first, the time change will only be in minutes and the fuel in hundreds of kilos, so using ETA and EFOB for calculations is not accurate.

Anyway:

If your CI is 40 kg/min means that it is worth to accelerate and arrive one minute earlier only if the extra fuel burnt incurred is not more than 40 kg.

this is very difficult to see accurately, specially in short legs. In long flights, you can see that the econ mach is good for your CI, because increasing mmach in 0.01 increases your fuel burnt in more than the CI value per each minute saved.

If you had exact fuel and time figures in the predictions, to the kg and to the second, you could increase your mach until the increased fuel burn was higher than the CI. That MN would be your ECON MACH.

[Superpilot]

See how scientific boredom can make you? :E

[Young Paul]

Changing CI actually makes most difference at lower levels.

At CI 10, your cruise mach at FL370-390 is likely to be around .77. At CI 50, about .79. That's a change of less than 2 minutes per hour - for a three hour cruise, about 5 minutes better. Your climb speed might change from 270-300, and similarly descent speed, which at a guess would net you another 4-5 minutes.

However, the difference between cruise speeds at FL290/310 (as you may end up on an ATC restricted route) is likely to be significantly different. CI 10 at FL270 is painful - with anything like a tailwind, you may end up at an IAS of about 240 kts, which is just WRONG (but might unfortunately be company policy).

[raraa]

Thanks for the replies, my point is on 9:40 hrs recent flight which is two sectors (a.k.a legs) I ditched my CI and went up to .78, that jump from .74 to .78 is around 4 minutes per hour or around 36 minutes total.

The fuel prediction to the nearest 100 showed no loss which in my head meant we couldn't used up more than 100 kilos extra and at the current prices (I checked) meant we couldn't have used more than $130 extra while saving the company 35 minutes of allowances for a two man crew and 7 F/As of around $340.

I ran the numbers through my a friend and then through various departments in the company and they checked out. I went to the Chief Pilot and he was bewildered. I told him I will fly .78 until he can tell me why not :D . His only reply was that my allowances will take a dive. :eek:

My allowances are around 60% of my total pay, for those in different parts of the world (different terminology :confused: ) it consists of:

1-per sector allowance
2-per flight hour
3-per duty hour

[Young Paul]

There are various aspects of the CI/flight economy debate that I'm unsure about. If you were in an A320 and had that as a viable range of Machs, could you not have increased your flight level? Would that not have improved speed (or at least resulted in an Econ of .78) and improved fuel performance?

We are told to use the flight levels on our flight plan (and not ask to change them) - but often these are performance levels. If we are a tonne lighter, the next level up is often available early - and I still reckon that most times (other than with unusual winds), this gives a fuel saving.

[Bula]

If you do it in cruise you will see very little in the way of cost/fuel management when switching between CI 10 and CI 30 - 40. For these CI's, the optimum level does not change, and your cruise mach number only decreases significantly in the event of strong tailwinds otherwise there is only a couple of minutes in it.

What you will find though is that CI makes a considerable difference to ECON climb and ECON Des speeds. What you will also find is that any CI over 60 - 80 will only cause a decrease in you Opt level once ECON cruise mach is .80.

Just remember CI 0 it the stated minimal ECON fuel burn i.e. LRC, while descent is similar. The saving are such that a CI 10 climb speed will save 20 - 35 kgs of fuel on the climb to the optimum level as opposed to CI 30. The same can be said for descent. If you descend earlier i.e. at a lower speed, your TOD is sooner. Now if your burning 38 kg/min in cruise, and 10 - 14 kg/min in descent, a 1 min delay in your TOD theoretically will cost you an extra 24 kg in Fuel, realistically this is closer to 15 - 20 kg's as you can add an extra minute to your flight time from TOD.

So to this end, actual difference in fuel burn is between 35 kgs and 55 kgs which is why you wont find a change in you FOD on all occasions as the decimal point is not rounded.

[HundredPercentPlease]

raraa,

I take you have read the "Getting to grips with Airbus performance" books. They are large and boring, but mostly quite good. Googling around for the cost index one and fuel economy one:

http://www.smartcockpit.com/data/pdfs/flightops/aerodynamics/Getting_To_Grips_With_The_Cost_Index.pdf

SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/flightops/aerodynamics/21)

In summary,

Calculate your CI. As the cost index drops below 20 or so, the fuel savings get much smaller but the time costs get much larger. The display on the MCDU might not be able to resolve the fuel saving.

Give the machine as much wind data as you can, ideally with a good wind profile at plenty of cruise points.

Fly managed speed and as close to the optimum level as possible.

[Microburst2002]

It is curious, regarding the CI, that most of the airlines have CIs below the LRC CI, which is about 40 kg/min in the 320.
If we had to fly at LRC, we would have to accelerate!

[Bula]

Micro, curios comment as I would have to say LRC is CI 0 on any aircraft.

What makes you say CI 40 is the holy grail for fuel consumption?

[Canuckbirdstrike]

Thoughts to consider.....

1. LRC and CI values have no direct relationship and many LRC values in QRH's are poorly or not corrected for wind.

2. The most accurate CI values are very airline and city pair specific.

3. The FMS is an enabling tool for the autopilot to fly CI profiles and its predictive and analytical capabilities are limited by the data entered.

4. Modern flight planning systems are far more capable of accurate analysis of flight profiles than an FMS.

5. When comparing the Fuel/Time for a different CI while in cruise using the FMS you are only doing a comparison of the remaining flight time and phases and are not comparing the total time/fuel changes. This is a fundamentally flawed comparison methodology.

FMS's are wonderful tools, but like any tool the understanding of its limitations and when and how it should be used is critical.

Lastly, the Airbus "Getting to Grips With Cost Index" publication has lots of good information, but it is outdated because all the assumptions are based on much lower fuel prices. The information contained in it must be tempered with that in mind.

[capt. solipsist]

The LRC Cost Index is neither 0 nor CI 40. It is "a cost index that gives you a fuel consumption of 40kgs/min."

Therefore, it's determined by a sort of trial and error method until you get a fuel flow of 1200kgs/hr on each engine.

[FlightDetent]

curios comment as I would have to say LRC is CI 0 on any aircraft. :) You said it twice already, but that doesn't make it correct. See QRH, most likely p 4.11.

Our CI is also less than LRC.

Yours,
FD (the un-real)

[9.G]

CI very muh depends on the route and equipment flown as well as engine specs. It could be 200 going to and 30 coming back. It's a very specific airline tailored figure if implemented correctly.:ok:

[Microburst2002]

The CI dimensions are Kg of fuel per minute, but it is not fuel flow. It is a coincidence that 320s cruise fuel flows are in the range of 40 kg per minute.

Airbus says that in the 320s I fly, a 40 CI inserted in the FMGC will give us cruise LRC, approximaly.

Canuckbirdstrike

I think it doesn't make any difference when you are comparing the results of changing a CI or mach number. If from then on the result is better in fuel and time, then flying at that MN is better.

agree in the other things, specially the second point. I think it is really difficult for many airlines to come up with the real CI for each city pair. The money they would have to spend in calculating those CIs and updating them frequently could render the savings useless. Ormaybe they just have idiotic management.

[HundredPercentPlease]

You said it twice already, but that doesn't make it correct. See QRH, most likely p 4.11.

Our CI is also less than LRC.

Yours,
FD (the un-real)

Indeed - that is an approximation of an approximation (which is a weight/FL/CI graph).

But surely when most people say LRC they mean what Airbus call MRC. Which is, of course, CI 0.

[Bula]

:{.. Terminology faux pas on my behalf.

[Microburst2002]

First thing, I was wrong in airbus 320 approximate CI. It is 45, not 40.

LRC is one thing and MRC is quite another

MRC: CI=0

LRC: go to the performance tables to find cruise MN or just consider inserting CI 45 because it will give, approximately, LRC.

If I recall correctly, LRC whas that cruising speed such that it gives a 99% of the MRC, which gives a considerable time saving (in the order of 10% or more, maybe?). It makes it more confortable to fly, too.

As a rule, it is not a big deal if you are not flying exactly at LRC, or slightly faster. So I guess I would just insert 45 in the CI field if i wanted to fly at LRC. If I want to be very very exact, then I will reach the FCOM 3. Even though, we can't select mach numbers to the thousandths.

Anyway, it still puzzles me that fact, ECON speed being slower than LRC. So if we divert to the alternate after a pull up in destination airport, we have to accelerate to LRC??

Another thought. Shouldn't the CI be different from A to B than from B to A if fuel prices are different in A and B?

[Bula]

True, but how much does it save and how much does it cost to employ someone to monitor and change it for every plan?

Interesting concept though

[9.G]

Bula, it very much depends on the scale of the operation. A correct implementation of a variable CI on each sector requires a complex and more importantly integrated flight operation solution using updated APMS data and accurate flight planning software. All available on the market. :ok:

[Microburst2002]

Many companies don't know themselves the costs they have. Finding those is the difficult part. Then it should be very easy to calculate the CI for each city pair, up and down.

[Canuckbirdstrike]

9G I am curious what is the requirement for APMS data to calculate the CI. APMS is a valuable tool to calculate the Performance Degradation Factor, but this is not a component of the CI calculations.

As for cost determination that presents some challenges, but it just requires some research and a clear understanding of what time costs must be mapped. Not all costs are required only those that are affected if the operating time increases or decreases.

[Microburst2002]

In some companies it would take an Economy Nobel Prize:p