Originally posted by Staga
IIRC turbines are in their most efficient use when running close to the max rpm (fuel usage vs thrust) or at least they were 10 years ago.
They're definitely not running at 'very low' power settings, but closer to max, although not _really_ close to max.
Most planes with max thrust could easily overspeed beyond approved structucal speed limit.
However theres another thing to it also: maintenance.
Higher RPM means higher maintenance cost, which is one reason why pilots can choose derated power settings for takeoff and climb.
The thing with jets is the temperature - higher altitude means colder outside temperature and that makes jet engines more efficient.
The colder it is, the less fuel is needed for a distance.
Which again means less power.
