Cost index?
 

I'm finding PPRuNe to be a great source for help with my atpl studies so here I go again with a question I cant figure out with the help of my books only...

How does cost index work and how does airspeed change with increasing cost index?

Kind Regards
Tim

To the best of my knowledge, CI is a figure that dispatch will put on a flight plan which you input into the fms. It is a calculation for the most efficient routing. Takes into account Aircraft weight/crew costs/fuel burn/trip time/route.

Values are
CI=0 = Max Range
CI=999 = Max Speed

Hope it helps.

Hi F,

Cost index is an input into the Flight Management Computer that balances cost of fuel against cost of time. Time includes crew costs, maintainance costs and all other costs associated with hull ownership. It is simply a ratio of cost of fuel over other costs. Therefore a cost index of zero input into the FMC will give the least fuel required for a flight. At this point it is necessary to realise that the resulting speed demand will be dependant upon weight and wind inputs - real time wind for a while and flight planned winds for projections. To minimise flight time regardless of fuel usage on Boeing aircraft with Honeywell FMCs a maximum cost index of 999 could be utilised. I'm not sure of the Airbus equivalent figure. The average cost index in commercial use is about 90 but it does vary depending on the airline and their view of the balance of costs, including timetable robustness.

Thanx very much for teaching me this =) great help and much appreciated =)

It’s a rather complicated subject but basically it’s a method of working out a balance of time related costs Vs fuel related costs.

I'll try to explain. If an aircraft requires maintenance after 100 hours of flight and it costs £10,000 for the check then maintenance cost £100 per hour.

If the aircraft is flown on the same route which is 1 hour per flight at normal cruse speeds then you can see that maintenance costs would be £100 per flight. Now what happens if we fly faster? We could fly the route in 48 mins that would mean for every 5 flights we get 1 free.

So if we charged the passengers a total £1000 per flight, normally we would make £900 with £100 going towards maintenance so for our 100 hours we would make £90,000. However, if we have flown faster we have had an extra 20 flights with no maintenance fee so that’s pure profit of £20,000!

So by increasing our cruise speed we have increased profits by more than 20% BUT we haven’t considered the dreaded FUEL COSTS!

If our aircraft burns £10 per minute, our one hour flight costs £600 in fuel. So now we have a total cost of £700 and a profit of £300 per flight. Our aircraft flying our route at standard speed earns us £30,000 per maintenance cycle.

Now if we fly faster we know that we get an extra 20 flights BUT if we want to fly faster, we need more thrust hence we need to burn more fuel. If we want to do our route in 48 mins we accept we are going to burn more fuel so we need to know if it’s economical to fly as fast as possible or if we should accept the maintenance increase because we will save fuel costs.

This is a cost index.

If we fly our aircraft at the 48 min speed our fuel burn increases to £15 per min so fuel burn per flight is now £720 per flight and the maintence cost (now spread over the 120 flights) is approx £84 per flight. Our profit is £196 X 120 flights. Total £23520 per maintenance cycle

So you can now see that flying faster because of the extra fuel burn would cost us an extra £6480.

There maybe a speed that will get us there in under an hour but burn less than flying at the 48min leg speed. This is where you need a cost index.

If we give the standard speed a cost index (CI) number of 0 and the 48 min leg speed a CI of 200 we will then have alot of numbers in between.

So an airlines operations department will feed in the route to a computer which will work out what number is appropriate to the flight. The pilots then get this number to put into the flight computer.

There are ALOT of variables to take into consideration and I have only really considered 2 of them. Crew pay, wind speed direction, Nautical air miles etc. are all things that impact a cost index.

I've tried to explain as simply as I can so I hope it makes sense (or hopefully someone can elaborate better than me ;))

So to sum up a CI (on the Boeing 737 anyway) is a number between 0-200 which the pilot feeds into the computer. The higher the number, the faster you fly. :ok:

Mercenary Pilot,
As an ancient, I'd never heard about "Cost Index". Thanks to your write-up it's perfectly clear.
I hope Founder got it as well!
Cheers.

A womderful way of putting it Merc! :D

Thanks. :)

Some operators I have spoken to recently are now using CIs between 30-50 due fuel costs .....

Excellently explained Merc! I'm sure Founder will have many more questions for you :ok:

Our standard CI is as low as 12 for standard flights and 13 or 14 for economic tankering. On longer charter flights (longer > 2 hours) we might get a CI as low as 10. A year ago it was around 15, 3 years ago it was 35. All values for 733/5 and mostly domestic operation. Our standard descent speed with CI 12 is 252 kias, at least we dont have to reduce much approaching FL100.

This is a very interesting subject. Is there a standard piece of industry software that all airlines/dispatches use to spit out the bottom line figure?

I've talked to a captain at MyTravel and they are running at a cost index of 32 with their A320's... Is there a difference in the way the CI is calculated at different companies, for example a CI of 32 might mean something at MyTravel but something completley different at BA or is it just a difference between aircrafts like Airbus and Boeing?

Thanx for all your help

/Tim

For the moment, we fly CI=80, on our scheduled network (B777-200 LR/5200 nms leg). We get M0,83 at the begining.....:8

So in summary CI is something for the accountants and planners to worry about not the pilot other than to input into the FMS what he has been told to input?

Exactly. Tho' once underway if the pilot wants to put saving fuel as a priority he can lower the CI and stay in ECON-or got to LRC etc. Some airlines put a lot of resources (cost accountants) into calculating CI for sepcific routes, times of day etc. And that repays itself handsomely. Others use rules of thumb. On all the figures I've seen the better the level of analysis of variable and non-variable costs, the better the CI and the better the result.

No. You get the CI before the flight. However that CI is a planned figure. It is all theoretical. So if you are, say running late, you can fiddle with the CI so that you arrive closer to scheduled arrival time. ie use a higher CI. What happens on the day is that it becomes a trade between time and fuel.

You decide what fuel you want to land with, then up the CI till you get the arrival time you seek with the fuel you also want. However if the headwinds are greater than anticipated you have to reduce the CI to get the arrival time that is dictated by the fuel you require to be remaining.

Alternatively. Bin the cost index method, and put in the speed you want that gives you the arrival time with the fuel you require.

So, as the Captain what you decide to do on the day is not necessarily dependant on the required CI.

Is that clear? I think I just confused myself!

Ltt