getting to grips with the cost index(21)

9. PRACTICAL USE OF THE CI - OPERATIONAL RECOMMENDATIONS

9.1 Cost index revisions
Correct use of the cost index requires a dedicated estimation for each route considering both time-and fuel-related costs involved on outbound as well as inbound sectors. Periodic revisions by means of monthly reviews should keep track of fluctuations if the airline wants to be really cost-conscious.
After analysis, adapted cost index values should be rounded values possibly aggregated in a small matrix of values corresponding to several routes with similar cost structure or cost combinations (fuel- and time-related). The low, medium, high assortments proposed in Section 3 may be a good start to setting up such a system.
Airbus has proposed already such an approach in the course of many fuel burn audits and operational liaison visits.
In this context and for consistent fuel predictions, the correct performance factor should also be inserted in the FMS and in the computerized flight plan (CFP). This factor takes into account specific range deterioration figures of individual aircraft by periodically running the performance monitoring program or resulting from dedicated performance audits.
The importance of using the same performance factor in pre-flight planning (CFP) and in the FMS cannot be over emphasized. In the past updating the FMS Performance Factor was restricted to maintenance staff, but now some of our customers have adapted this policy. Some airlines have defined company policy to allow the crew to check and enter the Performance Factor. This factor is communicated to the crew via the flight planning document for the specific aircraft tail number concerned.

9.2 Changing the cost index at departure / on ground
The cost index can, if necessary, be changed on ground to avoid a delay at arrival in case of late departure and in order to prevent important cost repercussions such as passenger dissatisfaction, missed connections, diversions due to curfews, etc.
The tables in Appendix 1 respectively provide default to new cost index repercussions with regard to .
 time and .
 fuel for the A300/A310, A320, A330 and A340 (delta values with regard to time and fuel are largely the same whatever the temperature and wind conditions). Trading fuel for time as tabulated is what really matters here.

9.3 Changing the cost index in flight
(a) Changing the cost index in the case of different en-route winds is irrelevant.
Indeed, it is not even necessary to vary cost indices with seasonal wind fluctuations. This is because the FMS integrates ground speed (i.e. wind) when computing ECON Mach corresponding to a given cost index. This has already been reviewed in Section 2.
As a reminder of Section 7.5, the Mach correction referred to hereabove, for all Airbus models, is of the order of :
Mach + 0.005 MN for 50kt headwind
Mach - 0.005 MN for 50kt tailwind.
In addition, the wind model accounted for by the FMS in its ECON Mach calculation results from :
　
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