I just found out that Lufthansa, for example, uses CI 160 on longhaul flights.However, pilots do change speeds without changing CI.I guess the CI gives speed for a certain altitude. Not an altitude for the given CI.Teeloo

Unfortunately my real-life experience with CIs is restricted to the 777, but I can tell you that for Varig's 777 flights the company policy is to input 180 under Cost Index on the FMC. On a recent flight from SBGR to MMMX where I flew the jump seat the captain decided it was best to lower the CI to 150 during the flight because he thought the aircraft was using up too much fuel at M0.845 and the FMC immediatly lowered the cruise speed to M.838 (and believe it or not there was quite a difference in fuel consumption)So what I know is that for the 777 (and probably the other Boeing aircraft) lowering the CI lowers your climb/cruise and descent speeds while saving fuel and vice versa. Now I'd like somebody to answer my question about whether or not the CI is actually modeled for those of us unfortunate enough to not own the 800/900 upgrades?

Cost Index = 0 results in Max Specific Rangewhere SR = TAS/Fuel Flow; result in NM/lb of fuelCost Index = 500 results in Minimum TimeCI Range is 0-500 in NG and 0-150 in 737 Classic w/ FMSHere's an example from Bulfer's B737NG FMC Guide: FL370/OPT ALT FL360, GR 149.6, -800:CI=500, climb 332/.798, cruise at .799, down at .786/328CI=50, 298/.789, .790, .786/296CI=0, 250/.750, .753, .764/250As indicated, CI=500 results in highest speeds, with the buffet boundary being the only constraint. A lower, but non-zero CI results in weighting trip time versus fuel burn. A zero CI results in minimum fuel consumption, so climb is at max rate to get to optimum altitudes faster (versus max speed over ground), cruise is just below Long Range Cruise speed (LRC), and descents are close to L/D max. From what I've seen, CI's over 150 on med/long legs should converge on the numbers close to CI=500.If you want to invest $45 in really wringing out your FMC, get Bulfer's guide, which explains the above, plus covers dozens of techniques for a whole range of climb, cruise, and descent situations. No affiliation, etc. www.fmcguide.com

That confirms what I saw then. Changing the CI doesn't effect the cruise Flight Level, but the cruise speed. A higher CI means a faster speed at a given level.And, once again, each airline's CI is different because it is based on the company's baseline fuel costs. There is no "hard and fast, use this CI" number.

Not that it is anyhing new under the sun, but this is the forum to go when you want to learn something new ;-)Thanks for the info!

>I seem to think that the cost index is not working in the>FMC. I read it in one of the tutorials.>>It wasn't working in earlier releases, but it got fixed in the 800/900 release. I've got the 800/900 and it DOES work, and since I fly for SimSouthwest, I generally set mine to 36. When I'm flying a BBJ, it's in the upper 200s.On long trips, it will not affect altitudes, but only speeds. On shorter trips, it may affect altitudes depending on your climb profile, as spending more fuel may allow you to reach a higher altitude in a shorter time.

What a great thread. Thanks folks. I always wondered what the impact of differnt CI numbers might be.