A quick explanation for you.
The basic engine is built for a certain max thrust.
The airframe manufacturer designs and will certify the aircraft for a range of thrust setting. For example you could have a B735 with 18.5K or 20K however the B734 may be 24K or 26K. You cannot get a 26K B735 or a 18.5K B734 as the are not certified.
The Aircraft Manufacturer will limit the max thrust due to perfromance limits such as the size of the rudder, length of the rudder arm. I.e. More thrust needs bigger rudders. The minimum thrust is limited by runway length for operation and climb requirements.
The owner/operator choose which thrust option from the range available that they need based on a study of cost v performance required. They then buy the Aircraft Flight Manual supplement for the performance needed and then a "derate plug" from the engine manufacture. If the operator buys too much thrust they pay more for the engine, if they don't buy enough they may have to leave payload behind. It is a pure cost/benefit exercise.
The "derate plug" is fitted to the engine. This tells the engine it's operating power. This is an operational and certification limit and cannot be changed without the aircraft AFM and certification change.
The pilots may also have "fixed derate" available via the FMC which cannot be higher than the "derate plug". A B733 may have a 22K engine but the option for pilot selectable down to 20K via the FMC. Either "rating" thrust may be further reduced but the assumed temp method (ATM) if allowed by your regulator.
A large number of airlines operate an engine lease called "power by the hour". The airline and the engine owner negotiate how they will operate the engine, derate, ATM etc and then derive a price that the airline will pay for the thrust they use. Some times it is a mix of low derate to max thrust within the engine life that is agreed and if the airline does more max thrust then a penalty applies. Likewise at overhaul time if the engine wear is beyond the expected then the airline has to pay a penalty.
Good engine management will have lower costs due to increased engine life. The CFM56 has had on wing life of over 30,000 hours with some operators.
Last edited by c100driver; 4th Sep 2014 at 05:35.