The cost index (CI) is a ratio of the cost of flying time, to the cost of fuel.
To calculate the CI, divide the operating expense of the aircraft, excluding fuel, by the cost of fuel in cents per pound/kg.
The cost index for an aircraft that is configured in pounds for ZFW/fuel etc will be different for a metric aircraft to give the same performance.
Low CI = slower speed
High CI = higher speed
Operators can bump up the CI for items not directly "seen" as being direct operating expenses, i.e. the cost of curfew charges, hotel accommodation for all the passengers if the aircraft cannot get out before a curfew on a later flight, flights going into a known predictable headwind/jet stream etc. A company may offset the cost of burning a little more fuel than the cost and inconvenience of delaying passengers.
They can be stored in the flight management computer with the company route and automatically inserted when the company route is selected.
The CI is only one of many factors the flight management computers use, other factors it uses includes, ZFW, ZFWCG, speed constraints/limits, ECON speeds, Temps/Altitudes of the tropopause, performance data for the airframe and engine, cruise flight level, current CG, current and forecast winds climb/enroute/descent/destination, OAT and more....
Cheers
Z
O\ZON
Do you have the Airbus/Honeywell reference to the recommendation of Airbus not to change the CI in flight ?