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.

 

[avbug]

With autofeather armed, the T-56 would NTS on an idle high angle descent; getting the lights on the drops wasn't uncommon. Not all T56 installations used both, I believe. I'd dig out my manuals if they weren't stuck in storage. (Thinking early series C-130, here).

Autofeather shouldn't take over for NTS during normal operations, as the prop isn't going to feather.

 

[Donsa320]

With autofeather armed, the T-56 would NTS on an idle high angle descent; getting the lights on the drops wasn't uncommon. Not all T56 installations used both, I believe. I'd dig out my manuals if they weren't stuck in storage. (Thinking early series C-130, here).

Autofeather shouldn't take over for NTS during normal operations, as the prop isn't going to feather.

Yep, the autofeather would not actuate even if armed in a descent because the throttles would be at or near flight idle. The throttles have to be up quite high to arm the autofeather.

The Allison 501D engines and Aeroproducts 606A propeller were my meat on the Allison Convair 340/440 (580) <grin>

 

[transpac]

The C-130s I flew didn't have autofeather. (C-130B &E)

 

[pilotyip]

Avbug and Donsa, I left that part out. The power lever had to be at around 66% coordinator to arm auto feather. So at Flt Idle you could NTS. Also taking off with birds in the area, you were not suppose to use auto feather, because a bird might cause a momentary power drop and activate auto feather on an engine that would normally stay on line. AAL lost one at rotate in BOS back in 1960 due to this problem. #4 auto feather and shut down, #3 NTSed, VMC air 2 eng 138KTs, rotate 115 Kts, equal upside down airplane.

 

[avbug]

This comes back to the differences between autofeather and NTS, again. Autofeather is generally an customer (airframe manufacturer) supplied function, like prop sync, while NTS is an engine function. The autofeather system is the airplane letting the engine know what it wants, whereas the NTS is the engine letting the airplane know what it's doing, and when the engine doesn't like what the airframe has told it to do...doing what it thinks is necessary to fix it.

 

[GravityHater]

OK, read and tried to digest all that, thanks.

Next a question about operating an twin turboprop with AF in the event of an engine failure. Completely hypothetical.

Would you expect most flight manuals to say when one quits, do NOT pull a power lever to idle (ie the dying engine) as this will inactivate the AF system?

 

[pilotyip]

When it autofeathers, you don'ta toucha nothin. Unless of course it is on fire then follow fire procedures

 

[Donsa320]

OK, read and tried to digest all that, thanks.

Next a question about operating an twin turboprop with AF in the event of an engine failure. Completely hypothetical.

Would you expect most flight manuals to say when one quits, do NOT pull a power lever to idle (ie the dying engine) as this will inactivate the AF system?

I can only speak for the Allison engine installation and there, once the feather button has been sucked in by the autofeather system, it would make no difference what you did. The fuel has already been shut off and the feather solenoid actuated and electric pump is already running. In my experience, turbine engines usually don't gradually die. They either run or they don't. The exception could be bird ingestion, hence the need to turn off autofeather if you see birds about to collide on take off. My failures were instantaneous. Only had one on take-off at about 90 knots. I heard the bang, perceived the left prop feathering as the nosewheels started to howl and I stopped. Enroute with the autofeather not armed of course, bang and the safety coupling let go so that the prop and engine were no longer connected. The NTS never had a chance to operate and prop sync never even was broken. Manually feathered it and landed.

 

[avbug]

As others said, with autofeather, your first indication of a problem is quite possibly that the propeller has already feathered. Your function at that point is securing items after you address flying the airplane.

The failure I experienced late this summer was a case of the engine continuing to run, but the prop feathering, without an autofeather system available. All my oil ported overboard with a turbine bearing seal failure, and the prop moved toward feather (kept turning, however), while I had engine response. I could push the power up, see the EGT climb, but had no thrust through the prop as I'd lost propeller control. I didn't have much time to evaluate it, but in the back of my mind I was thinking about a possible shaft failure. The propeller didn't fully feather, the engine didn't quit turning, and while I had appropriate power response through the power lever, I had no torque. In that case, the trained response for that engine installation was to keep pushing up the power lever, as the lever controls both the fuel controller and the propeller; it offered the best chance for minimal prop drag in the event the propeller didn't automatically feather. Lots of conflicting signals and no altitude to sort it out. Temps went through the roof, but the engine was already toast, and my only concern was making my forced landing site.

With autofeather, once the system has done it's job, you follow up with the appropriate pilot actions per normal. However, in order for it to work, the power levers must be far enough advanced to activate the microswitches that arm the system.