thanks for your answer,
for cost index i will try to resume t be sur i anderstand it in the right way.
for Fixed cost: the more your fly ( as many rotation as possible), the cheapest it will be
for variable cost: the les airborne time you have the best it is.
CI= fixed + vairable costs
--> .... i don't anderstand very well your conclusion so may be if you can explane me it in an other way i would understand.
thank you very much
elpilote