Mutt is better placed to talk on this one as he routinely works with derate situations on shiny aeroplanes .. while I only get to play with dinosaurs ....
I think of it along these lines ... and if I have it not quite right, then I am sure Mutt will correct me ...
Many turbine engines are available with different rated thrust levels depending on where the fuel limits are set ... the same, or very nearly so, engine ... but different maximum thrust outputs ... and the certifications are built around the maximum output for the particular spec engine. Structurally much the same engine but, from a certification viewpoint, different engines.
Now turn the idea around ... if you have a big engine with lots of grunt, there is no reason why it cannot be operated to mimic an engine set to a lower maximum output. If this is certificated as an optional way to operate the engine then, in effect, it is a bit like being able to unbolt an engine of one thrust rating and bolt on another of a different rating. The certification data (ie AFM stuff for pilots) will differ depending on the rating adopted. The sorts of reasons why this might be desirable could include the Vmcg discussion here (as the certification Vmcg is a fixed speed depending principally on maximum thrust output), maintenance implications generally, and so on.
With an engine of a given rating, however, it is common to operate at reduced thrust (generally using the familiar Assumed Temperature approach), where practicable, to save on hot end maintenance costs. While we might do this quite routinely, the engine still remains able to operate at the rated thrust output and the certification (and AFM) data are based on that maximum level.
While I haven't any direct experience with derate operations using the same engine on the wing .... I guess that one could schedule both a derate, and a reduced thrust takeoff for that particular derate, at the same time ... Mutt will be able to comment further on this aspect of the matter ...
Hence two rather different animals when you look at the detail of the thing ...
Last edited by john_tullamarine; 25th Jun 2002 at 00:25.