Autofeather vs NTS

[GravityHater]

What is the functional difference between an autofeather system and a negative torque system?
I believe I was told they both serve to automatically feather a prop when power drops.

yes I should fire my past training supervisors or at least send them a rotten egg in the mail, lets get that out of the way with and learn about this.

Thanks for any info.

 

[avpro91]

You are correct, they both serve to feather the prop in the event of an engine failure. The autofeather system is most commonly found on Pratts I believe, and the NTS on Garret (Allied Signal) engines.

If my memory serves correct, the NTS uses a strain gage type of system to activate a mechanical link that opens a valve in the supply tube to dump oil pressure to the prop hub when a negative force is imposed on the prop (ie: when the prop is driven by the air). This system has a feature built in to disarm the NTS system when the power lever is placed to idle, to avoid unwanted feathering. (NTS lockout valve)

The autofeather system on the other hand I believe is actuated by power output indidcations and throttle position sensors. Both power levers must be moved past a certain point (70% N1 or something like that) and then when one engine's output is sensed to drop below a certain level, with the other engine still producing a power output signal in the acceptable range; the system operates a solenoid that dumps the oil pressure to the prop.

That what I can remember from my Jetstream and King Air days. The systems manuals for both give a good description of each system and how it is operated. Hope this helps.

Avpro

 

[xjcaptain]

Typically you have an autofeather system in a free turbine engine. The NTS systems are found in direct drive turbines (negative torque in the direct drive basically means that the windmilling prop is now driving the turbine rather than the other way around). In the free turbine there is no way for the windmilling prop to drive the compressor, so it looks at a variety of other parameters to determine that the engine has failed.

 

[PreRVSM]

Another key point is that NTS will not take it all the way to full feather. It just gives you, the pilot, a fighting chance... unless of course your flying an old Metro with original -3 engines, than it just something they teach you in ground school to give you a warm fuzzy feeling.

 

[bertengineer]

The NTS system activates at a specific negative torque to increase the blade angle to get a bigger bite of air because the low pitch is allowing the prop to drive an engine (lets say in a descent). If left uncorrected the NTS system may drive the prop towards feather...but as stated earlier not all of the way. When this ocurs...a plunger on the gearbox is driven forward by a gear ring and it will put pressure on the mechanical linkage of the prop assembly, mechanically positioning a valve to port fluid to the pitch changing mechanism to increase the blade angle until it is no longer in an NTS situation. You may feel the aircraft BUMPING as this occurs. The key to it is...when an NTS situation occurs...add a bit of throttle.

 

[avbug]

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.

 

[charter dog]

Nice, Avbug.

Even better than the explanation provided during the power management training module of the TPE-331 line maintenance course in PHX. That was the only truly challenging mx course I ever attended. All of the other courses by other providers paled in comparison.

Best,

 

[GravityHater]

Dang you guys are good; saving for tonights reading.

 

[FlyingMoose]

Avbug did a great job there; but you're all leaving out the Rolls-Royce direct drive turbo-props with the combination of an electrical Autofeather and mechanical Auto Coarsening system(similar to NTS) that functions together to perform these same tasks. Don't even get started on the prop locks and controls...

 

[pilotyip]

The Allison T-56 had both NTS and autofeather, if the autofeather was armed you woukld never see NTS. The feathering circuit would take over and feather before NTS started.