jtr,
just quoting/unquoting, call it sharing information.
"""CI gives the Mach number that minimizes direct costs per flight hour.
Direct cost = Fc .c + Pt.t + Fc
Fc : fuel cost (/Kg)
c : fuel consumption (Kg)
Pt: maintenance + crew costs /flight hour
t : flight time
Fc: fixed costs ( maintenance costs/cycles, handling, fixed salaries, pax fixed costs, depreciation)
If we want to minimize this direct cost, we have to minimize Fc.c + Pt.t... After a small mathematic development, we find that the Mach number that minimizes the direct costs per flight hour depends on the following ratio:
CI = Pt/Fc (CI: cost index)
This shows that it doesn't make any sense to fly a CI if it is not specifically computed for each leg.... CI is really related to price of fuel and the amount of fuel that you take at a certain price ( economical tankage & through tankage considerations).
If your company doesn't perform this calculation for each leg, it makes more sense to fly a fixed Mach number. CI only affects ECON speeds. If you played a little bit with your FMS, you realized that if you change the CI, OPT FL will change (but not MAX FL, of course): the higher the CI is, the lower the OPT FL is. Also tail/head wind has an important effect. These 2 examples prove that price of fuel has to be considered if you use the CI.""""
apologies to the 'chappy' who wrote this.
and thank you!