Reading the books on A320 the Cost Index made me feel that cost index number is a good factor to manage the fuel consumption in relation to the cost of time (airplane, crew, etc..)
however on line operation I'm finding that flying high level (FL330-390) on anything less than .78 increases cost of time with no fuel saving. the other day at TOC I tried the FMGS predictions on .74 through .80.
I got predictions that confirmed my suspicions
.74 through .78 gave me fuel estimate of 2.6T at destination while costing me 15 less minutes to destination while .79 and .80 cost considerably more fuel.
Same thing happened with CI anything less than 35 cost no more but plenty of time incurred, while anything more than 40 cost considerable amounts of fuel.
Talking to the friendly folks
at dispatch said their calculation gave me an allowance of CI 3-5 and no more.
Two question to fellow bus drivers out there especially A320 guys.
1-Do you find that there is an optimum speed or CI? or is it that I am missing something?
2-What CI are you dispatched with (if any) and does it matter which operation you fly for? I hear folks on the radio flying 320s at .80 or .81 so why do you do it?
Thanks for reading and I appreciate your replys