CI for a Certain Speed

[DFWFlightOps 0]

Recently I came across a document for the T7 and it gave this bit of info on CI

 

Eastbound cost index: 300ER (70), 200LR (62), 200E

R (76). Westbound: Fly on M 0.83

 

with a CI 62 you will have a good mach # of .83 your whole flt.  Im trying to find a CI# for a .84M.  Can anyone help me?  I am also looking for a CI for a full flt set Mach# for .83, .835, .840, & .845, without putting in the FMC CRZ or MCP.  Any Ideas on how to figure that out?  Thanks for you help.

 

 

 

 

[willywonka 36]

I assume this is purely academic? Otherwise, to fly at a certain mach it's best (and most convenient) to explicitly input the required mach into the LEGS or CRZ page, or use speed intervene.

 

If you're just gathering data, just go into cruise and mess with the CI on PERF INIT until you get the speeds you want.

[Fabo 213]

You don't do a CI to fly a set speed, as speed in ECON mode is not just based on CI, but also on other factors. (atmospheric conditions)

 

When you need to fly a set speed, use a speed cruise mode.

[scandinavian13 5,192]

Any Ideas on how to figure that out?

 

Hey Scott,

 

The whole purpose of CI is to be flexible.  The CI is a value that drives the aircraft to behave a certain way to achieve a certain end goal, which is (grossly simplifying here) to balance the cost of time versus the costs of fuel.  Because these inputs are variable, the CI drives the aircraft to behave in a similarly variable manner.  If there is more of a tailwind, the CI will drive the aircraft to respond to that tailwind by reducing cruise speed (under the assumption of: if there was no wind, I could achieve a ground speed of X; with a tail wind, I can reduce my speed, save fuel, and still hit X).

 

If you want to fly a particular speed, then you should set that speed either on the MCP or on the FMC CRZ page.  There's honestly no reason not to, and attempting to do it otherwise is essentially (and unnecessarily) turning the FMC into a Rube Goldberg device.

 

Simplify  :wink:

 

For further reading (or, more accurately, viewing):

[bjratchf 80]

The cost index is meaningless if you edit the speeds for climb/cruise/descent in the VNAV pages. For oceanic operations, this is usually the case and is called "mach number technique"- you program your filed speed into the FMS and fly only that speed- otherwise you'd run into spacing issues on the tracks with other aircraft. In ECON, the speed will change based on your cost index (influenced by weight, etc). 

[Richard McDonald Woods 168]

Kyle (or PMDG),

 

I have just watched your video again. Excellent!

 

What you are saying is that each operator has a varying set of variables as input to their CI calculations. Each of these then results in a CI number.

 

What you don't address is how the resultant CI values are then used by the FMC. This must surely be set, just for this operator (PMDG B77L), and is expressed in the FMC programming logic.

 

So, for this operator (PMDG B77L), what are the commanded speeds resulting from the CIs?

 

(I hope that I am not making a fool of myself :( )

 

Cheers, Richard

[scandinavian13 5,192]

 

 

I have just watched your video again. Excellent!

 

Thanks!

 

 

 

What you are saying is that each operator has a varying set of variables as input to their CI calculations. Each of these then results in a CI number.

 

Correct.  Most operators (that take advantage of the concept of CIs anyway - some of them just use a single CI by default, and never adjust it) have different operational needs that drive their different CI values, based on the metrics they've come up with.

 

 

 

What you don't address is how the resultant CI values are then used by the FMC. This must surely be set, just for this operator (PMDG B77L), and is expressed in the FMC programming logic.

 

Correct.  I don't know the specifics, so I couldn't speak to it, directly.  The FMC does obviously crunch the numbers, but I don't know how, and I'm betting the data is somehow protected/proprietary.

 

 

 

So, for this operator (PMDG B77L), what are the commanded speeds resulting from the CIs?

 

See my original post in this thread.  :wink:

 

Again: CIs do not drive speeds.  CIs drive a set of performance factors that the FMC then employs to adapt to the existing situation.  This is illustrated by the fact that a set CI may show varying cruise speeds, based on aircraft weight, altitude, wind, and several other factors.

[Richard McDonald Woods 168]

OK, Kyle, now I understand. Thanks for your clear answer.

Cheers, Richard