Right. Now you have induced a follow-up question!
When I talked about 'legality', I assumed that the CI was fixed at take-off, provided on the various documents given at the gate and plugged into the FMC before pushback along with all the other palava, and that your company would be a little miffed if you changed it en-route since they fixed it to adhere to their own fuel policies/prices for that day of weather... the route planning chaps, etc.
So it seems that you don't have an official CI for a flight, so my question is: How do you know what CI to put in when you want to modify your ETA/Cruise FL? Is there a graph to look at? A data sheet of some kind? Or do you guess and say "Oh well CI80 is giving us FL360 and ETA of 1645, let's try, erm, 45 and see what that does"? I'd be surprised if that were the case? Hence my original reference to legality - having crews sodding around with CIs without the company knowing, costing more fuel perhaps, flying too fast, arriving to early and messing up slot/gate times...
I have seen online some 'pilots' saying that "Southwest use CIXX for their 737's" or "Oh, we used to use CIXX but now we use XX for our triple 7's" - I mean, how can a company 'USE' a CI when it should be different for every routing, weather forecast, weight, etc... and now you're telling me you can even play around with it when cruising for various reasons?
Hope you understand my quest for clarification.
Thanks again, gentlemen
TP