tripjet320

Hi guys,

have a question in my mind I can't find any answer...
We know on the Airbus (320s) with managed speed and fixed COST INDEX (CI=20 for exemple), the aircraft calculate his optimum speed according to the wind component its has in the computer you inserted.
We also know that if there is tail wind for exemple, the FMGS computes a lower Mach number as the tail wind component increase (to let the wind work and save fuel) ...
Can this managed Mach number come up to the one corresponding to the one you would have if you had inserted CI=0 ?
If yes, do you know from how much tail wind in knots, the Mach number is actually the one corresponding of CI=0 ?

Thanks a lot !!

OBK!

Interesting question. I would have thought ci0 would always produce the slowest speed so even a 200kt tailwind with ci20, the managed speed will still be a nanomach faster than ci0, most likely unnoticeable. Ci0 is max range, ci20 isn't so there must be a tiny difference in speed. Don't forget the ci also affects optimum cruise alt information. Ci100 will offer a lower optimum level, or higher tas.

tripjet320

Thank you !
Well, apparently it would always be saving fuel puting CI=0 as soon as you have a lot of tail wind ... but how much ?
At least, enough to not affect the normal flight time (lower speed but tail wind) especially when there is no need to arrive early at destination as the next departure time is not going to change anyway.
I'm thinking about this from a couple of days, and I might suggest my airline to change our fuel policy about this thread.

737Jock

If your airline works out the CI in a proper way than going CI 0 will not save money if the calculated ci is 20.

CI is determined by fuel cost vs time related cost. When calculating the CI wind component will have already been taken into account.
This offcourse means that the CI will vary from day to day on the same route and between routes.

If fuel is the overriding factor you should always fly ci 0.

dmacnz

I think its in the FCTM it states that the managed speed target per 50kts or head/tail wind component will change the speed target 0.01M.
ie. 50kt of headwind speed target will increase 0.01M