Hi,

 

I just noticed something that puzzled me. I am flying from Bangkok to Sydney, currently cruising at the optimum altitude of FL310 and using a Cost Index of 200. I decided to play around with the Cost Index and see what outcomes I got with EFOB and ETA on arriving in Sydney.

 

I lowered the Cost Index to 50 and the PROG page told me I would use more fuel and arrive later. I raised the Cost Index to 500 and the PROG page told me I would arrive earlier and use slightly less fuel.

 

The part I found strange and don't understand is when I raised the Cost Index to 500, my optimum cruise altitude dropped to FL275. When I lowered my Cost Index the optimum cruise altitude remained at FL310.

 

Why the altitude drop on a higher Cost Index?

 

Also, what is the Cost Index sweet spot, or is it too variable depending on weight and trip length?

Why the altitude drop on a higher Cost Index?

 

I don't know.

 

Also, what is the Cost Index sweet spot, or is it too variable depending on weight and trip length?

 

Cost index is dependent on standard operating procedure at an airline. United (I think) has a cost index of 100 on their 777 (which is an M11 competitor so yeah...). I read that FedEx has a CI of 200 unless noted on the paperwork. So I think that 50-200 would be in the ball park with a long-haul bird. I think 500+ would be used if you were late and needed to red-line her a little.

CI 500 lowers your OPT altitude because you'll get a higher TAS. Of course, your fuel flow will skyrocket, but at 500, you've told the FMC that time is WAY more important tan fuel.

 

Paul

So the bird can get along faster at a slightly lower altitude because there is more air for the engines to use?

 

When I was punching CI 500 in at FL310, the fuel consumption hardly changed. But obviously there is no point going to CI 500 if your not going to let the aircraft speed up.

So the bird can get along faster at a slightly lower altitude because there is more air for the engines to use?

 

 

yes, that is a way to explain it.

Your TAS (or mach speed indirectly) is what will be affected by your CI value.

When you tell the FMC 500CI that means you want the highest TAS possible (I dunno the max for the MD-11 by heart but that must be something like mach 0.86)

In order to be able to get to that speed the FL has to be lowered. In other words, past FL300 the plane does not have the power to keep a speed of mach 0.86 (because indeed the air is not dense enough)

 

(If you are using real weather) are you filling in the winds aloft data for every waypoint? This will make a difference because those calculations are based on what the FMC effectively knows.

Facing a headwind all the way surely will make a difference in fuel-burn.

Maybe the calculations didn't change that much but at 500CI you will definitely run into fuel problems if not planned properly.

Maybe a small tip. Martinair doesn't use CI really, they will have a max spd at .83. For a short trip it will be around 90, long trips about 125. The last one you have to adjust (if you want to use CI) during the flight (to maintain .83) as you will burn fuel and becomes lighter. Not sure about KLM.

 

John