Negative Torque Sensing and autofeather are at odds, rather than being the same. While true that both increase the blade angle, they do it for different reasons. Autofeather serves to ensure a timely feathering action independant of pilot input at a critical time when power is set high. Other posters have describedit in substance, though it functions on power lever position, rather than N1, Ng, or Np. In a typical autofeather installation, the system must be activated by pilot input, usually by setting a switch prior to takeoff. It is designed only to be operated at certain times in some aircraft, and at all times in others. It is designed as an interface system between two or more powerplants, rather than an independent system. Activation of autofeather on one engine deactivates the feature on the second engine. It is a comparator between engines, power lever settings, and actual power output to use predetermined criteria to determine when an engine failure has likely occured, and feather the engine. It does not affect any other engine operating parameter but the propeller.
The NTS system, on the other hand, is designed to load the propeller and prevent overspeed. The autofeather system slows the propeller down as a by product of reducing drag by feathering action toward the end of shutting down the engine, whereas NTS serves to load the propeller to ensure proper continued engine operation. One's trying to shut it down, the other to keep it running.
NTS prevents overspeed and returns propeller control to the engine, whereas autofeather takes propeller control away. As propeller RPM increases when the propeller blade angle defaults to the low pitch setting at low power settings, potential exists in a coupled engine to drive the propeller excessively and thereby create an enormous amount of drag, absorbed through the gearbox and engine. This can create a dangerous condition, as drag increases exponentially in proportion to the square of the airspeed...a combination of both forward aircraft airspeed and propeller RPM. In very short order, an overspeed condition on a windmilling engine can lead to a complete loss of aircraft control (ask a P3 or C130 crewmember about a decouple and overspeed on a T56 in flight).
The NTS system senses periods of zeor or negative torque (depending on where it is set), and increases to a positive torque setting by increasing blade angle. As RPM decreases, blade angle is decreased again, and this cycle continues in various degrees until the NTS system is allevaited. In a steep descent on some installations, the aircraft may distinctly be felt to pulse in and out of NTS. In garrett installations this is normal and not harmful. In other installations such as the T56, the engine operationally should not be placed into a NTS situation...Putting the engine there and experiencing a NTS failure can lead to a decoupling under a negative torque loading when the engine no longer can control the propeller, an overspeed condition exists, and a potential propeller overspeed failure at that point may lead to an unrecoverable condition. In such a condition, the ultimate overspeed governor is a fuel topping governor designed to limit engine output, but will have no effect...as the engine is no longer controlling the propeller.
With a loss of oil pressure, the propeller of the free turbine or the propeller of the driven engine will feather. Or should. NTS isn't a feathering device, however. While it moves the blade in the same general direction, it does so for an entirely different reason, and NTS does not feather the propeller. With oil pressure present and no feathering action taken manually the propeller will continue to move in and out of an NTS configuration. The free turbine engine without a feathering command at idle will only move to adjust pitch to control RPM, whereas the shaft driven engine moves not only to control RPM, but to control loading on the drive shaft and gearbox.
Autofeather functions as a certification backup system, whereas NTS is an operational system, which is unique to the specific engine, and has no function in conjunction with other engines on the same aircraft. NTS functions individually and it's operation affects only the engine which is NTS'ing. An autofeather function may be included on aircraft which feature NTS, as they provide entirely different functions.
