If you're on time, or early, it looks like it's 18 or 19. Gives you an .84 cruise.

Late? Much higher. CI-300 gives you a .86 cruise.

Most of the flying is done at the slower, lower cost index, cruise speeds.

You might see minor adjustments but the on time Cost Indexes don't change much. But the FMC uses the tail wind or head wind and adjusts the cruise speed but even then it tends to be minor changes. A low cost index (CI range 30-39) might have a cruise speed on the 777 of .829. Tailwind might drop it to .828 if at all while a significant headwind might increase it to .836.

And all speeds adjust via the CI if you're at your optimum altitude. If you're at optimum altitude a standard CI of 30-39 gives you .83 (+/- for headwinds). If you're 4000' below OPT ALT your ECON cruise mach will lower, perhaps .815 (+/-) and decreasing to .77 at the end of a 10 hr flight.

So CI is fairly constant if you're flying at OPT ALT. It makes some adjustments for headwinds and a smaller adjustment for tailwinds. If you're at a low CI reductions for tailwinds don't occur since you're already at LRC (long range cruise).

Flying 4000' below OPT ALT using the planned cruise mach had an over burn of 5,000 lbs. Using ECON CRZ (adjusted by CI and off altitude cruise) reduced the projected over burn by 50%.


Overwater/non radar we typically fly an assigned mach number. When that happens the CI, and headwind/tailwind adjustment from the FMC, is meaningless.