KC10 FATboy

Most airplanes I've flown had engines that were de-rated. In other words, the engines were able to produce more thrust than the aircraft was able to handle with one engine inoperative, thus, VMCG or VMCA would become a big problem above a certain engine RPM. Therefore, the engines are de-rated. And of course, as others have said about being able to know how much above a limit an engine went.

Twin Wasp

Normally a derated engine is a maintenance issue. You have an engine with the structure to produce X amount of power but you've limited it to say 85% of X. So it's not working as hard and there is less wear and tear.

It also helps on performance. Where a "normal" engine makes X amount of power on a standard day at sea level, it'll start dropping off on hot and high days. A derated engine has the ability to make it's derated power over a wider environmental envelope.

ERJF15

IIRC, the F100PW-100/220/229 motors (mostly F-15's) only go to 94% and N1 is only read. That's the maximum output of the motor at mil power. The turbine will not turn any faster unless....the "VMAX" switch is activated -100/220 only.

bcrosier

This has always been my understanding, with a slight twist:

The original -1 engine was designed to produce (to make up numbers) 2500# thrust at 100%. Airframe manufacturer "X" wants to build an aircraft which this powerplant would be appropriate the appropriate class for, but wants a slightly different TO thrust, or MCT, or Cruise thrust, or TBO. The engine manufacturer then looks at what changes might be required to obtain this performance.

Let's say the desired performance improvements are higher takeoff, climb, and cruise thrust, with a flat rating allowing standard day performance to 90 degrees F.

They may install a turbine with improved cooling characteristics to permit the flat rating for TO, and uprate the N1 RPM to obtain the desired MCT and cruise performance. As a result, the N1 redline would be increased.

MoZak18

Thanks for all the answers!