C
index = C
time/C
fuel
To get a better picture of the whole thing read:
http://www.iata.org/NR/ContentConnec...x_Material.pdf
SOP = Standard Operating Procedures (I think). So whilst most phases of flight(s) are standard, some might not be and thus a modified descent speed might have to be used (i.e. selected).
Or, looking at it from a different angle, going to certain airports, it is SOP to fly such and such a speed in the descent to comply with ATC requirements. I'm sure that complying with (good) ATC is part of most of our SOP's.
Obviously if your descent profile (descent arrow) is based on CI 0 (low) i.e. early descent and you initiate your descent based on that and fly the managed speeds associated with that CI and are
then asked to fly high speed, you will burn more fuel than originally computed by the FMGC.
The way I understand it, for a given CI, there will always be one (1) optimum point of descent (for a given profile on a perfect day). For that specific CI, descending before
OR after that point you'll burn more fuel.
BUT, we don't live in a perfect world and thus I think we should try and be as close to SOP's as possible, but have enough flexibility (in our minds) to accept deviations from these same SOP's, knowing that we will return to SOP when appropriate. Part of SA is not only knowing where we were 1 minute ago, where we are now and where we will be in 1 minute, but also knowing who is around us and how we can contribute to a most efficient way of operating to assist all parties involved (aircraft & ATC). (SA involves much more obviously...)