Zero Cost Index

[SuperRanger]

gents,

i know that enter zero as cost index will theoretically give you MRC (essentially most fuel upon arrival, right?). this don't seem to be the case on our b777-200. cost index of anything below 50 don't seem to 'gain' any fuel except 'lose' time. does this have anything to do with the fact that the b777 have a supercritical wing? can you point me to any site regarding this subject? have tried google. lots of reference to supercritical airfoil but nothing with relation to cost index. thanks in advance.

SR

[Alex Whittingham]

The FMS should modify still air MRC according to the cruise wind and cost index entered. If the cost index is zero then MRC will be faster in a headwind and slower in a tailwind to give you the lowest fuel consumption in terms of kgs burnt for each ground nautical mile.

In the 'big picture' the cost of fuel burnt is not the only thing to be considered. The cost index, calculated by the operator, feeds a second input into the speed calculation to allow for leasing costs, maintenance costs, getting an extra sector in to the day etc. Thus a correctly calculated cost index of 50 will have you flying faster than MRC for the wind, and therefore burning more fuel, but the increased fuel costs will be more than offset by the savings related to airborne time.

The lack of visible fuel savings below a certain point on the B777 might be a result of a very flat drag curve so, yes, it might be your wing.

[Intruder]

Since the max cost index allowed is 5,000 (or is it 50,000?), a CI of 50 would show very little change in fuel burn, cruise speed, or time of flight. They may exist, but smaller than the number of decimal points showing on the CDU.

[Alex Whittingham]

Many FMSs have a maximum cost index of 100 or 200.

[18-Wheeler]

Many FMSs have a maximum cost index of 100 or 200.

FWIW ours is 999. (tried it one night)

[Intruder]

Correction: Ours goes to 9999 (747-400, GE engines); confirmed in the FHB and Honeywell FMS manual. I thought I had read 5000 in a Boeing manual somewhere...

[SuperRanger]

The thing is with a cost index of say Zero, there's practically no increase in fuel gain but a time lost of almost 6-7% ie. 4-5mins every 1 hour. for a 10-hour flight, you will be talking bout 40-50 mins of later arrival!

unfortunately, boeing's text clearly states Zero cost index gives you MRC. clearly this doesn't seem to be the case in real life? does this happen to others or is it just me?

SR