Gas Generator (N1,N2) Speeds?

[whereamI]

Does anybody know why gas generator speeds are rated at greater than 100%? I am curious why the engines on the plane I fly are rated past 100%, and have been told all turbine engines have N speeds greater than 100%. Anyone care to enlighten me?

 

[erj-145mech]

Its my understanding that on a new engine you are guaranteed to get 100%(sea level, standard day, etc). On a PT6A, for rigging purposes, 101.5% is the standard. Basically, consider the 100% rating as a datum.

Theoretically, on a less than ISA day, without automatic controls, an operator may exceed rev's, temp's or torques by not monotoring the instrumentation.

 

[mar]

A datum for normal ops

I agree with the 145 mechanic though it kind of depends on the engine.

For example on the Metroliners I flew (TPE-331) the torque guage could be used to 110 percet for wet (water-injection) power settings.

But for the vast majority of the flight the upper limit was 100 percent torque--if you could produce it, you could use it.

Other reasons would be similiar to why your speedometer in your car can be read to 85mph or more.

You're not allowed to drive that fast but you need to know when you are....

...so in the event of an overspeed (rpm) you can note the severity in the airplane logbook.

 

[avbug]

100% is a reference point, and has no particular meaning. After all, when operating at 100%, you don't think in terms of 33,000 rpm, but only in terms of percent. It's simply a translation into something you can refer to when setting power.

It was stated above that it's a datum, and that's exactly right. It's nothing more than a reference point. The actual engine limitations have nothing in particular to do with 100%; it's a reference the manufacturer has set for establishing engine parameters.

Don't think of it as absolute; it's just a reference point.