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B777Heavy
1st Jul 2007, 00:39
This is a question for B777 pilots. We fly B777-200ERs and the cost index initially started off roughly at around 120. Then some "expert" came in and decided to drop the index to about 18. Then he decided that, that 18 was not working so he took it back to 120.

I do understand that the cost index is variable and has a lot of things involved.....

Im wondering just what rough cost index do most of you fly on the B777-200ER?

mutt
1st Jul 2007, 04:52
120 should give you an approximation of M.84.
18 should give you an approximation of MRC.
We presently use 75, this is reviewed every quarter and changed accordingly, we dont use route specific values. If we want to avail of cheap homebase fuel, we can use 450, this gives an approximation of M86.

I would presume that slowing down to CI=18 cost more in flight time than the fuel saving gained, so it makes sense to return to a higher CI value.

Mutt

pstaney
1st Jul 2007, 12:59
Mutt, would your company adjust it for wind? For eg 100 kt H/W versus 100 kt T/W. I'm thinking the cost savings would be significant over flying at CI 75 at all times. Any calculations on how much?

B777Heavy
1st Jul 2007, 15:09
Well interestingly ...try this out on your next flight.. ...say you are flying at 75 C.I. look at the progress page(in crz) check out fuel at destination, then change to cost index of 18 and u get an extra like .5-1ton of fuel at destination.

I guess with a low CI, u fly nose up...causing more drag, u fly lower speeds and therefore you are in the air longer but Fuel at Destination is higher....Thats the confusing part...

mutt
1st Jul 2007, 16:14
pstaney,

The ECON mode of the FMS uses the inputted CI value together with the wind values to calculate the cruise speed. That really is the advantage of using Cost Index over a selected cruise speed such as LRC/MRC.

B777Heavy,

The lower CI value is very close to Maximum Range Cruise, so we would expect to have more fuel, however the trade off is between fuel cost and aircraft costs, so just because you saved fuel,it doesnt mean that you made money !

Mutt

pstaney
1st Jul 2007, 16:29
"The ECON mode of the FMS uses the inputted CI value together with the wind values to calculate the cruise speed."

Mutt, so using your suggested CI of 75, what calculated cruise speed spread would you typically expect for a T/W of 100 kts vice a H/W of 100 kts?

zlin77
2nd Jul 2007, 03:50
Used C.I. 60 in the 200ER & 50 in the 300. Equates to an approx cruise of M.83.

Old Smokey
3rd Jul 2007, 13:02
pstaney,

Some "ball-park" figures for CI=75 on B777-200ER

At Optimum Level, cruise = M0.835, plus or minus M0.001 per 20 knots of Headwind / Tailwind, i.e. for 100 knots Headwind, expect about M0.84, and for 100 kts Tailwind expect about M0.83.

Regards,

Old Smokey

pstaney
3rd Jul 2007, 15:47
Old Smokey, For 100 kts headwind I get 100/20 = 5 times .01 = .05 mach. plus .835 = .885 mach.

Would that be .001 mach per 20 kts wind? But even that seems awfully small increment for a headwind.

Old Smokey
4th Jul 2007, 06:18
pstaney,

Arrrrgh!:eek: I never could count, yes, I did mean .001, and not .01!:ok:

For the sake of clarity, I'll go back and amend my previous post (which should keep future thread readers wondering what I said):E

Yes, the increment is quite small as a comparison against the wind component, it's all about tangents to graphs with a sliding base-line for speed as wind component varies, which does not necessarily lend itself to simple corrections for wind component, hence my qualification that they were "ball park" figures.

Thanks for keeping me honest, constructive criticism always welcome!:D

Regards,

Old Smokey