The condition isn't normal, I think. The clutch is the first suspect.
Despite what Brother Lappos says, this is
completely normal. It is the sure sign of an inexperienced turbine pilot when he complains that he doesn't get any "needle-split" during a practice auto. (Here's a hint: it's called a free-turbine.)
vaqueroaero:
Initially if NR increases then N2 will follow it with an increase, but then slowly it will start to decrease, but N2 never gets to the idle position of around 60-65%.
And it won't. The N1 will surely fall to it's idle range, the but even at "idle," the 250 engine is putting out somewhere around 18 horsepower- enough to keep the unloaded N2 wheels spinning at their governed rpm.
Governed?
Yup. Just because the fuel control lever (that "throttle" thingee) is in "Idle" doesn't mean that the governor isn't still working. And yes, it will keep the N2 from overspeeding if the NR increases above 100%.
Many new turbine pilots are confused by this. They do not understand that the N2 needles WILL NOT necessarily split like a recip in a practice auto. One fatal accident occurred because during the practice auto the two pilots were more concerned about the lack of a needle-split than what they were doing. The Rolls Royce engine manual even states that some "N2 instability" is normal. Yeah, once in awhile I've seen a little "hunting" of the N2 needle in a practice auto (especially with a C-30), but most of the time the N2 stays right up there at 100%. In fact, in practice, there is very little difference between flat-pitch "full throttle" and flat-pitch "flight-idle." Even the N1 readings may be similar.
But just for argument's sake let's rule out the freewheeling unit! When you shut the engine down, do you get a needle-split then? (Go to cut-off from 70 or 80% NR if you have to.) When the ship is shut down and you turn the rotor in the direction of rotation, can you hear the N2 wheels turning?
If the answer to those two questions is "yes," then there is no reason to ground the ship. Fly on, hombre.