How do they "derate" jet engines?

[ptwaugh]

I know that the Allison 250-C20J engine (used in the B206) is 420 SHP which has been derated to 317 SHP for use in the helo.

Now, I note that the transient torque limit is 110% (348 HP), and I'm wondering if the engine is capable of 420 out of the box, then how come there must be a 5 second limit on it at 348 SHP?

What have they done to it that results in this limit?

[ahramin]

I do not know the exact reason but in general the engine has been derated for safety and to provide full performance over larger range of altitudes.

However, you cannot assume that because an engine is derated that you have larger safety margins. For example the 420 shp will be at a certain RPM. If you are turning a slower RPM then the same torque limit applies but you will not have full shp available. If you torque the engine to get to the rated 420 shp at a lower RPM you may damage the gear box or other torque limited components. Just one way you can go wrong not following limits.

[ptwaugh]

Ok, you answered my real question that I didn't know I had.

Basically, the limit on torque is due not to the lack of the engines capability of producing/handling it, but rather to protect the rest of the transmission as it has only been designed to handle so much. So, the engine had a max set to protect the helicopter while allowing the pilot some extra emergency power for safety.

Thanks.

[ahramin]

There is no "Extra power for safety". Many of these engines will have fuel topping governors which will limit the output regardless of what the pilot wants. The safety in derating an engine comes from not running it so hard, so it is less likely to fail.