Page 24 - October 2022

“ ... watching ITT very closely when the feathering check is conducted, you will observe that it actually decreases a few degrees.”
In fact, watching ITT very closely when the feathering check is conducted, you will observe that it actually decreases a few degrees. Amazing! Why? The theory is that when the power turbine slows down so dramatically it is not as easy for the airflow to exit the gas generator and the “flame front” of the fire in the combustion chamber retreats a bit upstream ... moving it further away from the ITT probes located at engine station #5, between the compressor turbine and power turbine.
About now I will wager that a few readers are thinking, “Why is this Clements guy considered a King Air expert?! The ITT in my King Air goes up a lot when I feather! He’s full of it!”
Slow down, amigo. I am sure that what you are observing is true in your airplane. However, it is caused by the fact that your engine (or engines) has a rigging problem and you need to have this addressed and remedied.
The only device sharing information between the front (power section) and back (compressor section) of the PT6 is the Py air-filled line that connects the fuel control unit in the back to the fuel topping governor in the front. (And also to the overtorque limiter in the 200-series.)
What is happening is that some air is being vented from this line when feathering and this, in turn, is slowing down N1, leading to less cooling airflow and therefore higher ITT. The fix usually involves repositioning the Reverse Reset arm into a more proper location.
Reverse Reset arm? What’s that?
You may recall that the fuel topping governor (FTG) has two operating speed points. Most of the time, it should function if every propeller speed goes 6% faster than the setting of the primary propeller governor (PPG). For example, at takeoff in a C90GT, with a takeoff RPM of 1,900, the FTG will activate only if the speed reaches about 2,014 (1,900 x 1.06). In cruise, if we pulled the props back to 1,700, now the FTG would be waiting at 1,802 (1,700 x 1.06).
However, when the power levers are pulled all the way back to Maximum Reverse after landing, mechanical linkage to this Reverse Reset arm resets the FTG’s operating speed to a value that is about 95% of the normal governor speed. Of course, it is our job to push
22 • KING AIR MAGAZINE
OCTOBER 2022