Use of Cost Index with winds

[Cedar Tree]

Can someone explain if we would change the CI while enroute for certain reasons? Would a head-wind require an increase in CI, or do we allow the FMC to determine this?

[Phil Squares]

On the 744, and for most FMS aircraft IIRC, if you are flying with ECON Cruise the FMC takes into account the winds. If you flying with a substantial tailwind the FMC will reduce the ECON MN to take advantage of the winds; conversly, if you are going into a headwind the FMC will then increase the ECON MN. So, there is no reason to adjust the CI. However, some airlines have a SOP if you're extremely early then you can adjust the CI.

[T6NL]

Why would you want to change the cost index if you are running early/late? Surely the easiest course of action is to override the ECON speed and select the slowest/fastest permissible speed in the FMC, provided this won't run you out of fuel if you accelerate, of course.

[Cedar Tree]

Thanks for the reply!! :ok:

[Dehavillanddriver]

On the 737 the FMC increases the mach number by M.01 per 100 kts of headwind and decreases it by M.06 per 100 kts of tailwind.

One problem of using a CI to determine your cruise speed though is the reduction in speed with a reduction in cruising altitude.i.e. the lower you go the slower you go.

If you are trying to stay out of a strong headwind you can loose a lot of the advantage because the speed drops off considerably.

[Intruder]

If you normally fly with a high CI (e.g., 250) and run into significantly higher headwinds than planned, you may burn much more fuel than planned, eating away all your reserves. In that case reducing CI will help. ECON with CI=0 yields the best fuel burn, including wind considerations.

[411A]

Indeed.

With MANY jet transport aircraft, cruising at a faster mach number into a headwind, makes perfect sense, likewise, with a strong tailwind, a much slower mach number (with regard to lift/drag issues) is more suitable.

Yes, Lockheed provided this info in their AFM and in the FMS, superbly presented.

And yes, before anyone says it (especially for Capt Claret)...Lockheed did it BEST.

Bar none!!

Of course, 'tis an old design, but a good one, as those who flew the tri-motor will certainly agree...:D

[Capn Bloggs]

And yes, before anyone says it (especially for Capt Claret)...Lockheed did it BEST.
Given Claret's background, I doubt that he would know what a Lockheed is, let alone be able to recognise one! :} :ok:

[Soga]

Airbus 320
the cost index is a strategic parameter in cruise.it is to be changed only for strategic purposes such xtra fuel getting close to 0 or flight time considerations ( in this case i prefer select a given fixed cruise mach number on FCU and all predictions will be update accordingly)
Be in mind tha any time we changed the cost index the FMS will be computed for a optimum flight level, optimun step level
i never won more time in speed up the mach in cruise but if you decide for a high speed approch and quick rotation on ground you can get some more minutes or be on time .
Nice high speed app