JohnBurke

Negative torque sensing. Positive torque values occur when the engine drives the propeller. Negative torque values occur when the slipstrip drives the propeller, and thus, the engine. Any value in which the prop is flat and is being driven by the slipstream, or when negative torque occurs, involves a steep rise in drag. NTS works to increase prop pitch to reduce negative torque, but will not feather the engine. It drives toward feather, or more accurately reduces oil pressure to allow the prop toward feather to reduce drag and negative torque. The prop must still be manually feathered.

NTS, when operating, will be felt as a pulsing as though the prop keeps taking a load, and alternately driving to feather. It's not unique to the garrett/honeywell offering, but found on other systems such as the Allison turboshafts, too. One could think of it as a temporary partial feathering, automatically dumping oil pressure, but it doesn't actually feather the aircraft. It only acts long enough to decrease the negative load.

Ironically, Honeywell states that the 331 will run for 1/2 hour with no oil; it runs like a top, in fact. Just no oil pressure, and still doesn't feather, and may not get much NTS action, either. Nor does it produce torque, during that time; it may produce a little, depending on the amount of oil remaining and the nature of the problem, but not much. An engine failure can occur such that the engine does produce considerable drag, which may or may not be relieved by NTS, and under such a circumstance, depending on how the engine has been rigged (there are a lot of ways to set up a TPE-331), the drag rise can be substantial. Feathering is critical.

Consider that when feathering is required, we all have taught not to rush. Sit on the hands, count to ten, etc. On short final, selecting the correct engine and feathering it may allow less time, and may be much more critical, as one sinks below the glide path. If one cannot maintain that glide path, then eliminating that drag is essential. Consider the ramifications of not eliminating that drag, eliminating it too late, or attempting to feather the wrong engine. Or, in the case of the rapid drag rise that can occur with a shaft-drive engine, an increase of power on the other engine, exaggerating the differential problem. Consider also, the effect of increasing power on the failed engine, and the possibility of a drag increase there, too (as simple as pushing both power levers up together: under the right circumstances, pushing up the power on the failed engine my act at cross purposes to NTS and may cause a drag rise, increased yaw, decreased aircraft performance, and directional control issues).

I find that most pilots who have been checked out in TPE-331 aircraft don't have a detailed grasp of NTS or operations in that regime; mostly just the memory items and procedures and the understand that the engine is either working or it isn't, and if it isn't, then feather it. With the TPE-331, there's a lot that can take place between working and not working, and if one is at low altitude when in that region, available time and altitude may be both compressed significantly so far as finding a solution and executing it. I say that from the perspective both as a pilot who has flown various 331 aircraft, experienced engine failures in them, worked on them as a mechanic, and put them on the ground following an engine failure in a regime where nothing works as one might expect. The TPE-331 is a great engine when it's functioning as intended; relatively light for a lot of power. Small, compact. When it's not functioning as expected, the results can be rather disappointing.