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Old 01-31-2010, 10:08 PM   #1 (permalink)
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Default Flight Cost Index

Dear All

I have a question about using Cost Index in Airbus aircraft.

Cost Index = Cost of time/Cost of Fuel

As sper the above formula, if time cost (maintenance cost...) increases or fuel prices decreases CI goes up.
I work in the Airline Perfromace section and I advise pilots the optimum cost index (as per the above formula).

I know that pilots enter the cost index figure (optimum cost index given in the flight plan) to the FMGS before the departure.
I want to know if we (flt plan & pilots) use a Cost Index less than the optimum figure (CI-25 instead of CI-40) will it save more fuel than using the optimum cost index. (supposing the time difference is only 2-3 minutes).

And if an airline practices this for a long time is there any bad effects for the aeroplane performance.

Rgds/ULSK
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Old 02-01-2010, 12:02 PM   #2 (permalink)
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Ah, you're cranking the Cost Index numbers out?

A Cost Index of zero will save you fuel per mile flown but crew and maintenance costs will go up per mile. On a short leg it's hard to gain anything by going fast (you could cruise supersonic DTW-CLE and I doubt it it would make a minute's difference) but on a 14-15 hour leg balancing the fuel burn v. when the next "C" check comes due can matter. You might be able to squeeze some more legs in and therefore make more revenue.

Define aircraft performance. You'll go slower. That's reduced performance. Are you hurting anything or is something going to break if you put in "0"? No.
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Old 07-21-2010, 11:08 PM   #3 (permalink)
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I know I'm bringing a 6 month old thread back, but 'cost index' is a poorly understood concept amongst pilots (or at least all the ones I know). This article from Boeing AERO Magazine explains it very well.

Quote:
Originally Posted by Boeing
Cost Index Explained
by Bill Roberson,
Senior Safety Pilot, Flight Operations


Used appropriately, the cost index (CI) feature of the flight management computer (FMC) can help airlines significantly reduce operating costs. However, many operators don't take full advantage of this powerful tool.

COST INDEX DEFINED
The CI is the ratio of the time-related cost of an airplane operation and the cost of fuel. The value of the CI reflects the relative effects of fuel cost on overall trip cost as compared to time-related direct operating costs.

In equation form: CI = Time cost ~ $/hr / Fuel cost ~ cents/lb

The range of allowable cost indices is shown in Figure 1 (see next page). The flight crew enters the company-calculated CI into the control display unit (CDU) of the FMC. The FMC then uses this number and other performance parameters to calculate economy (ECON) climb, cruise, and descent speeds.

For all models, entering zero for the CI results in maximum range airspeed and minimum trip fuel. This speed schedule ignores the cost of time.

Conversely, if the maximum value for CI is entered, the FMC uses a minimum time speed schedule. This speed schedule calls for maximum flight envelope speeds, and ignores the cost of fuel (see fig. 2, on next page).

COST INDEX USAGE
In practice, neither of the extreme CI values is used; instead, many operators use values based on their specific cost structure, modified if necessary for individual route requirements. As a result, CI will typically vary among models, and may also vary for individual routes.

Clearly, a low CI should be used when fuel costs are high compared to other operating costs. The FMC calculates coordinated ECON climb (see fig. 5, on next page), cruise, and descent speeds (see fig. 6, on next page) from the entered CI. To comply with Air Traffic Control requirements, the airspeed used during descent tends to be the most restricted of the three flight phases. The descent may be planned at ECON Mach/Calibrated Air Speed (CAS) (based on the CI) or a manually entered Mach/CAS. Vertical Navigation (VNAV) limits the maximum target speed as follows:
  • 737-300/-400/-500/-600/-700/-800/-900: The maximum airspeed is velocity maximum operating/Mach maximum operating (VMO/MMO) (340 CAS/.82 Mach). The FMC-generated speed targets are limited to 330 CAS in descent to provide margins to VMO. The VMO value of 340 CAS may be entered by the pilot to eliminate this margin.
  • 747-400: 349 knots (VMO/MMO minus 16 knots) or a pilot-entered speed greater than 354 knots (VMO/MMO minus 11 knots).
  • 757: 334 knots (VMO/MMO minus 16 knots) or a pilot-entered speed greater than 339 knots (VMO/MMO minus 11 knots).
  • 767: 344 knots (VMO/MMO minus 16 knots) or a pilot-entered speed greater than 349 knots (VMO/MMO minus 11 knots).
  • 777: 314 knots (VMO/MMO minus 16 knots) or a pilot-entered speed greater than 319 knots (VMO/MMO minus 11 knots).
FMCs also limit target speeds appropriately for initial buffet and limit thrust.

Figure 3 illustrates the values for a typical 757 flight.

FACTORS AFFECTING COST INDEX
As stated earlier, entering a CI of zero in the FMC and flying that profile would result in a minimum fuel flight and entering a maximum CI in the FMC and flying that profile would result in a minimum time flight. However, in practice, the CI used by an operator for a particular flight falls within these two extremes. Factors affecting the CI include time-related direct operating costs and fuel costs.

TIME COST
The numerator of the CI is often called time-related direct operating cost (minus the cost of fuel). Items such as flight crew wages can have an hourly cost associated with them, or they may be a fixed cost and have no variation with flying time. Engines, auxiliary power units, and airplanes can be leased by the hour or owned, and maintenance costs can be accounted for on airplanes by the hour, by the calendar, or by cycles. As a result, each of these items may have a direct hourly cost or a fixed cost over a calendar period with limited or no correlation to flying time.

In the case of high direct time costs, the airline may choose to use a larger CI to minimize time and thus cost. In the case where most costs are fixed, the CI is potentially very low because the airline is primarily trying to minimize fuel cost. Pilots can easily understand minimizing fuel consumption, but it is more difficult to understand minimizing cost when something other than fuel dominates.

FUEL COST
The cost of fuel is the denominator of the CI ratio. Although this seems straightforward, issues such as highly variable fuel prices among the operating locations, fuel tankering, and fuel hedging can make this calculation complicated.

A recent evaluation at an airline yielded some very interesting results, some of which are summarized in Figure 4 (see previous page). A rigorous study was made of the optimal CI for the 737 and MD-80 fleets for this par ticular operator. The optimal CI was determined to be 12 for all 737 models, and 22 for the MD-80.

The table (see fig. 4, on previous page) shows the impact on trip time and potential savings over the course of a year of changing the CI for a typical 1,000-mile trip. The potential annual savings to the airline of changing the CI is between US$4 million and $5 million a year with a negligible effect on schedule.

SUMMARY
CI can be an extremely useful way to manage operating costs. Because CI is a function of both fuel and nonfuel costs, it is important to use it appropriately to gain the greatest benefit. Appropriate use varies with each airline, and perhaps for each flight. Boeing Flight Operations Engineering assists airlines' flight operations departments in computing an accurate CI that will enable them to minimize costs on their routes. For more information, please contact FlightOps.Engineering@boeing.com.
Frankly, there is some AMAZING stuff in this magazine. For those who care about technical issues relating to commercial aircraft and airlines, it's certainly a good read. The topics are relevant to all aircraft and airlines, not just Boeing operators.
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Old 09-11-2010, 06:15 PM   #4 (permalink)
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Quote:
Originally Posted by ULSK View Post
Dear All

I know that pilots enter the cost index figure (optimum cost index given in the flight plan) to the FMGS before the departure.
I want to know if we (flt plan & pilots) use a Cost Index less than the optimum figure (CI-25 instead of CI-40) will it save more fuel than using the optimum cost index. (supposing the time difference is only 2-3 minutes).

And if an airline practices this for a long time is there any bad effects for the aeroplane performance.

Rgds/ULSK
Well, by definition, I would say that using a lower CI than optimum on the bus would result in an increase is fuel savings. However, this would result in higher operating costs due to an increase in time. Optimum is optimum for a reason.
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Old 09-12-2010, 05:23 AM   #5 (permalink)
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Word to the wise: use the CI as assigned in the DX release. Remember that the A320 series down links everything so if you input something other than the assigned CI they know about it in Dispatch(and possibly higher in the food chain). If you want to fly fast or slow do it off the mode selector panel..
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Old 09-12-2010, 01:15 PM   #6 (permalink)
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My airline uses CI 6 all the time. Only if you are really late and the return flight needs to be on-time do they bump it up. I heard that as long as you return the cost index to the original value before you activate the approach "they" will not know about it.
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