Some airlines pay a lot of money for consultants to study their overall costs and give them a single CI to be flown system-wide for all fleets. This seems questionable to me, since these flights are using many different fuel costs and have different economics of being late vs. early vs. on-time based on how delayed their departure is, how early/late in the day they are, whether they're inbound/outbound from hub, etc.
Some airlines use fixed hourly costs and the actual fuel costs of the departure station. This is better than a single value fleet-wide, but not clear how much better.
My experience from working with many airlines is that most of them don't really have a very good idea of what CI they should be using on any given flight. Airline costs are very complicated, and the costs based on time are usually non-linear given crew contracts, schedules to make, connections, next flights for the aircraft/crew, etc., not to mention slots vs. holding in some parts of the world. So maybe what they're doing isn't much worse than trying to do something better from flight to flight, and it's certainly a lot easier.
I guess that doesn't exactly answer your question.
I think usually the CI value is standard either fleet-wide or by a combination of fleet and airport-pair or even individual flight, and not determined dynamically so it's just published as a standard operating procedure or a note to crew. There are generally stored notes given to the crew as part of their briefing, classified by the flight or the fleet or the departure airport or the airport pair, and the CI value to use is sometimes just one of those. When it is dynamically determined as part of the flight planning process, it is usually printed directly on the flight plan.
[Edited 2006-05-21 01:32:15]