The fundamental rationale of the cost index concept is yo achieve minimum trip cost by means of trade-off between operating cost per hour and incremental fuel burn with time related cost , the faster the aircraft is flown the more is saved. THis is cos the faser you fly , the more milestime related components can be used and the more miles can be flown. However if the aircraft is flown faster to reduce time , fuelburn increases and mony will be lost in turn. The CI is used to take into account the relationship between fuel and time related cost. The FMS is able to counterbalance these cost factor in order to generate the min cost speed ( ECON)which takes into account only A/C and environmental factors. The CFP will select the min cost CI considering fuel,maintenance and overflying cost for given sector and a/c type. the overflying cost is included on routes where have more than one route selection is available. The CI value appears for CRZ as CI XXX, where XXX is cost index value.
For cross checking fuel figures, utilize FMS as primary tool or the flight planning and performance manual.The cost index for LRC would be approximately 60 . By definition this would be 1% higher burn than can be achieved by flying max rang cruise . For cost index higher or lower you can estimate within 1% what different from LRC would be i.e for cost index of 30 the fuel required will be 0.5% less than LRC or index 120 fuel will be 1% more than LRC fuel. Cost index value change on a given sector within day dependant on the change to the time dependant cost ( fligft time ) , overflying cost and fuel price .
