...deunitnoc
The airplane I was flying this summer used a Garrett TPE331-10/11U engine. (we had airplanes with either the -10 or -11, depending on how recently it was converted). It was set up to produce a significant amount of drag at idle...enough that retarding the power lever to idle in cruise could throw you forward against the shoulder straps and harnes, and even with the nose pointed very steeply down hill, the airplane would or could stall. LOTS of drag. We had them set up that way because the drag was a big asset when making steep downhill runs, especially ones with long descents.
In a very steep descent, the aircraft would pulse in and out of NTS, sometimes in a very pronounced manner, like a surging. A little disconcerting at first, it was also reassuring because it meant everything was working properly. When my engine failed it occured due to failure of the rear turbine bearing seal, which allowed all the oil save two pints to be ported overboard through the exhaust. My engine was producing normal EGT temperature, but without any torque. Additionally, no oil was available for propeller operation, and when I pushed the power up, the temps came up because the gas sction was still burning...the engine was producing power, but I couldn't use it because I had no control over the propeller.
My first clue that something wasn't right occured on the descent. I was doing a formation 3,500' descent in a canyon with another aircraft, and I couldn't slow down to stay behind him. At flight idle, I didn't get the drag out of the engine that I expected, and the NTS system didn't pulse. When I reached the bottom of the descent and pushed up the power, temps came up, but no torque, because there was no oil left to increase the blade angle...none for the NTS system to work it's magic on the way down, nothing to hold the propeller in place for the governor to do it's work on the descent, and nothing for the governor to use to increase propeller blade angle as I increased power with the power lever as I reached the bottom of the canyon.
It was a very unusual situation. Had I realized what was occuring (I was pretty focused on the airplane ahead and trying to stay out of the canyon walls on the descent...the windscreen was covered in ash and I was flying through smoke into the sun), I would have probably tried something differently and opened up more options...but the lack of the NTS operation was probably the biggest clue, and I missed it.
In this case, the propeller didn't really feather. It tried to move in that general direction, but I couldn't have feathered it because I had no oil pressure to dump, and it was trying to move in response to aerodynamic twisting force and it's own RPM...it was still being driven by the engine shaft, but there was no way to get torque out of it because I couldn't increase or decrease the propeller blade angle...neither could the governor...it was on it's own. It sought the position of least resistance, or itself. Not for the engine, nor for the airplane, but for itself, which was a compromise that neither myself, nor the engine, nor the airframe really wanted.
The actual propeller speed was a function of engine speed...the faster the engine tried to turn, the faster the propeller tried to turn, because it's attached through gearing to the engine driveshaft. At the same time, because it was free to act on it's own according to the various forces in flight, it wanted to align itself toward the feather or coarse position to alleviate the drag of the slipstream. It was producing more drag than I wanted, certainly more than if it were feathered, but no useful torque...sort of a perpetual state of NTS.
As airspeed reduced, it tried to align itself more and more with the plane of rotation, which meant moving toward flat or fine pitch (high RPM position), and I had pushed the power ever all the way up as I had been instructed to do in the event the propeller didn't feather in an emergency...so the engine was trying to go great gangbusters and was only being controlled in it's desire to overspeed by the governor removing fuel. It was a very rare, unusual situation. After I came to rest on the hillside, the propeller was turning, as the engine was still turning (it ran quite some time without oil), but with the power lever retarded to the idle position the blades moved toward feather on their own...no oil pressure to keep them anywhere else, and no engine RPM or aerodynamic twisting force to move them to the high RPM position any more. When I shut it down, the propeller stopped in the feather position. I didn't pay a lot of attention to it right then, as I was busy tossing gear out of the cockpit and egressing, but when I came around the front of the airplane it was solidly feathered.
The feathering action wasn't the result of NTS, which required oil pressure...it was the natural position of the blade...when fixed shaft engines are shut down, unless they're locked in a particular pitch position ("pitch locked" is a term unique to these engines, but with multiple appliciations depending on the engine) they'll go to feather with a loss of oil pressure...or they should. Without oil pressure, they should move to feather. With oil pressure, the NTS system forces the propeller toward feather/coarse/low rpm position, but for entirely different reasons than a feathering system. The autofeather or feathering system is designed to reduce drag on the propeller disc as a whole, whereas the NTS system is designed to increase drag on each blade to create resistance against the driveshaft (increase torque), while also reducing the drag on the prop disc by moving the propeller and engine out of a condition where it's being driven by the slipstream.
Feathering and NTS sound similiar, and appear similiar, but are actually very different. I hope that explaination didn't confuse you more...the relationship between the engine and propeller in a turboshaft engine such as the Garrett or Allison is perhaps the most complex of any aircraft engine, and it can be confusing. The pilot tells the engine in broad terms what he wants, and the fuel control talks to other parts of the engine such as the governor, to make it happen. However, the pilot isn't really controlling any one particular thing in the engine...in the turboshaft engine, the pilot is only controlling a small air valve (and mechanical linkages rough fuel control functions)...it's the air valve that meters air to the fuel control unit which tells the fuel control unit what to do, and that in turn talks to the propeller governor and other system components.
From the pilot end of things, what you need to worry about mostly is pushing up the power makes things go faster and burn more fuel. In an emergency, the engine should feather, but you can't take that for granted, and must still follow through with identification, verification, and shutdown, just like you're familiar with in piston equipment (no matter w(h)eather autofeather or other similiar systems are functional, or not). In my case, I didn't do that this last time, as the failure occured at 150', and I reached a luxurious 300' before starting back down again...I just didn't have time to do much other than make a turn and put it on the ground. Once it's quit, then NTS isn't really your concern any more...if there's oil pressure for it to do it's job it will keep trying until you dump the propeller oil by feathering (or the autofeather does it for you)...but NTS won't feather the propeller for you because it's job is to increase torque to keep things running. NTS is there to put you back in the game and keep you going, whereas feather functions such as autofeather are there to pack it in for the day and end the madness. Which applies to you at any given time depends entirely on what's going on, and in general, what you tell the engine to do.
