When conducting the run-up propeller feathering check, have you noticed what happens to ITT? Of course, most of our attention is directed to the propeller tachometers, as it should be, making sure they decrease rapidly and significantly. Some pilots conduct the check just as they were taught in the Seneca or Travel Air or Duchess they used for their multiengine training: Making sure they observe the rapid onset of RPM decrease but then pushing the propeller levers fully forward before the speed drops so far as to labor the engine. Others, realizing that the propeller is not mechanically connected to the compressor in the “free turbine” PT6, go ahead and allow the speed to stabilize as the propellers go to their largest blade angle. This speed will almost always be below 500 RPM, even if the condition levers were set for High Idle.
A clever and common procedure is to conduct the PT6 feathering check while taxiing down a long taxiway on your way to the run-up pad or takeoff runway. With the propellers in feather, forward thrust is eliminated and it is very quiet for the passengers. Remember that it’s a no-no to have the power levers over the Idle gate into Beta while the propellers are feathered, so make certain they are at Idle first, then pull both prop levers fully back into feather. You will feel an initial acceleration surge as the blades take a more positive bite of air followed by no thrust at all as they become feathered. It’s quiet, taxi speed is nicely controlled, and the brakes can be spared any usage.
Although the propellers feather rapidly, the reverse is not true. They typically require 20 seconds or more to reach normal idle speed after the prop levers are pushed forward. This means you need to plan ahead so that you will have thrust available if you’ll need it to make a turn at the end of your taxiing “coast.” Do not lift the power levers to reenter Beta until the propeller RPM is back up to its normal idle value.
Now back to our pending question: What happens to ITT during this time? In fact, how about Torque, N1 and Fuel Flow?
Since the compressor (or gas generator) section is separate from the propeller and its power section, the fuel control unit should continue dutifully governing N1, meaning we should see no change in N1 when we feather. Yes, torque will increase due to the propeller blades slapping the air with the largest bite, but at Idle power its small rise will not be an issue. With the same compressor speed being maintained, the same fuel flow will be required. With the same compressor speed, the same amount of air flow is being moved through the engine.
Hence, ITT changes very, very, little. Almost all newcomers, when asked this very question, immediately presume that ITT will increase very noticeably since airflow will be reduced during feather. In a fixed-shaft turboprop like the TPE331, that would be correct, since whatever percentage speed loss the propeller experienced, the same would be true of the compressor! (In the King Air B100, with that powerplant, the same lever that does the feathering also manually shuts off the fuel flow … first!)
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 the propeller levers fully forward before selecting reverse so, in our C90GT example, the PPG should again be set for 1,900 RPM. The FTG is now waiting to operate at 1,805 (1,900 x 0.95). Perhaps I should remind the readers here that Maximum Reverse power should not cause propeller speed to ever get this high. On the other hand, if Maximum Reverse is selected while the prop levers are not fully forward, remaining in their descent or cruise position by mistake, there is a good chance the FTG will be activated, power will be limited, and Reverse will not be nearly as powerful as you were expecting. That’s why we have that “Reverse Not Ready” annunciator light that doesn’t extinguish until the propeller levers are forward.
Here’s a rhetorical question that does not really have an answer: At what speed is the primary propeller governor operating when in feather?
Like I said, there really is no answer for that question. But would you buy the fact that it is very, very, low, well below the lowest governor speed setting? In fact, in feather with a shutdown engine the propeller should stop, right? So maybe we’re kinda, somewhat, sorta setting a PPG speed of 0 RPM, eh?
Do you see where I am going with this? What is 6% faster than zero? What is 5% slower than zero? Zero times anything is still zero, right?
So perhaps that leads to a bit of insight of why sometimes the FTG gets involved in feather even when it should not … all those speeds come rather close together down there at the bottom end of the governor speed range.
Having read this article, the next time you do your run-up feather check, let the propeller speed stabilize as low as it will go and carefully check all of the other gauges, too. If you see a decrease in N1, leading to an increase in ITT, add that to your squawk list and have your PT6 guru set the rigging properly to eliminate that fuel topping governor interference.