Originally Posted by
befree
Done selectively this is not a bad plan, even for airlines that have money left. The same applies to driving. If you are late then some may race down the M1 at 85 mph. If however you do not need to be at luton so soon then keeping it down to 70 will save a fourtune. It is very non-linear with a large saving on fuel for only a small reduction in speed.
It doesn't quite work that way. Every aircraft type has a most efficient cruise speed and a most efficient cruise altitude. You may file a flight plan which includes both but it is up to air traffic control (specifically Eurocontrol Central Flow Management Unit) whether you still have that flight plan when you get your departure slot, and it's up to the controllers enroute whether you're allowed to stick to it once you're airborne.