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av8a
1st Aug 2001, 11:37
Greetings,

For jet operations I understand that the crew punch in a Cost Index into the FMC which in turn considers other factors and determines optimum levels etc. Question is does it also compute the best climb speed or is this a function of weight and how does a reduced power climb affect overall trip fuel?

HELP

MasterGreen
1st Aug 2001, 12:29
OK here comes a short answer to a long question...

Cost index relates fuel costs against trip time costs. So a slightly faster climb speed will be posited against a slightly reduced rate of climb. This is - as you correctly assumed, achieved by the Cost Index. The BEST Rate of Climb is achieved at Cost Index 0. (You don't need a spoon feed here - think about it).

Derated (reduced) climb will save a little engine life, but costs about 3-5% on fuel. So if you are short on the liquids from the getgo - climb at full climb power and save 200-300 kgs (local mileage will vary depending on type).

MG

john_tullamarine
1st Aug 2001, 17:39
Not my area in detail but, I think, a bit more to it than that .... the cost index sets out to put a fence around the total cost analysis .. and then skew the present operation towards either minimum overall cost or trip time ....

Open Climb
2nd Aug 2001, 01:21
Simply put: by selecting a lower CI you'll burn less fuel on the trip but get there a little later and the other way around.

The climb, cruise and descent speeds and profiles are optimised according to the selection.

Exactly how the climb is affected I'm not sure, I've given up on trying to figure out everything the airbus does....
:D :D