Autofeather System

[Shadowfromthesky]

Hi all,

My question is in reference to King air with PT6, although relates to any aircraft fitted with the system. Why would you turn autofeather off? Wouldn't it be safer to leave the system armed in all stages of flight?

Appreciate your thoughts..

Shadow.

[ConfigFull]

Gday,

The reason for arming autofeather on takeoff is that if the engine fails, the autofeather system will immediately feather the engine - useful on takeoff/go around because you will lose the drag of a windmilling engine in a potentially time-critical/terrain-critical situation - you seem to already know this.

Imagine now if the engine failed during cruise for whatever reason. It would be a much preferred situation if the problem could be fixed via non-normal checklists etc rather than immediately feathering the engine.

I guess it all comes down to how much time you have - fix or feather in the end.

[triadic]

The good old shed, the Shorts 330/360 had an autofeather system that was armed from the moment you pushed the power levers fwd for takeoff. Had one failure on t/o prior to V1 - it was caged before we had the power back and stopped. Another in Cruise saw the engine caged within seconds of the fuel pump failing. A good system which always was there. Cant help with the KingAir, but guess it is a mater for the manufacturer.

[LeadSled]

Folks,
For real excitement, you neeed to trrrry the auto-feather on a Bristol "Frightener" --- all in the days before modern electronics --- where even a half way decent VHF radio was something to look forward to --- not an old STR-9X.
For me, almost 50 years ago, and I can remember like it was yesterday ---- some things you never forget !! Shirt, or something similar, does come to mind.
Tootle pip!!

[Fr8dog44]

If the King Air Auto-Feather system is the same as the BE-99, which it may well be, with the auto-feather armed and you lose one engine, that engine will auto feather. If the 2nd engine then fails, the 2nd engine will auto feather and the 1st engine will come out of feather. This is what occurs when you "Test" the auto-feather system during your run-up. So to answer your question, you wouldn't want the auto feather system armed when in cruise should a dual engine flame-out occur.

Remember, the PT-6 is a free turbine engine. The prop is not directly connected to the compressor turbines. So the only drag that occurs is from a flat, wind-milling prop. Auto feather is, I believe, a redundant system to assure the prop will feather upon engine failure. The prop on a PT-6 will normally go into feather position on shut down do to loss of oil pressure. It's oil pressure that keeps the prop in the tractor position. It's been ages since I flew one, but as I recall, there's another reason why the system does what it does, just can't think of it and I no longer have my manuals to look it up!
Sorry! Hope that sheds some light for you.

[triadic]

As I recall the system in the Shorts on the PT6-45setc was that if one engine autofeather operated, it would in the process un-arm the system on the other engine. Both power leaves had to be at least in the cruise position of it to be armed.

[morno]

The autofeather system in the King Air will not work as you describe for the BE99 Fr8dog44.

If a propellor were to feather by means of the autofeather, it immediately protects the propellor of the operative engine, which would prevent it from feathering should the situation you describe, occurr.

Willing to be proven wrong though, but from my time on B200's that's what I understand of the system.

morno

[FGD135]

I'm with you morno.

If the autofeather worked as described by Fr8dog44 that would be a really silly design.

Surely, the second most important design consideration of an autofeather system would be that it didn't feather both engines at the same time.

(The first most important would be that it didn't feather a prop unnecessarily.)

My understanding of the King Air system is that:

1. To feather a prop, the system for that engine must be armed, and
2. to be "armed", the power lever for that engine must be sufficiently advanced, AND, the torque of the opposite engine must be greater than 400 ft-lbs.

So, with the King Air system, the scenario outlined by Fr8dog44 could not occur as the autofeather of the second engine would have been disarmed as a consequence of the first engine failing.

[The Green Goblin]

Autofeather is for girls.

You should get a fully awesome NTS system like a Garrett

[j3pipercub]

Love ya work GG

[Howard Hughes]

You should get a fully awesome NTS system like a Garrett

Perhaps you could explain how the 'propellor can drive the engine' on a PT-6!

PS: I wish I still had Garretts, much more noise for your buck!

[prospector]

And for a real buzz, lose the centre engine on the Tri-Lander wth Auto Feather operative, not that it was very often.

[OpsNormal]

Back to original poster's question, should an autofeather system inadvertantly activate at a high power setting it is goodbye engine as it will cook-off in mere milliseconds. From memory (getting bad now...) there was an incident a few years ago on a Dash 8 (?) where the autofeather inadvertantly activated on an engine producing either climb or cruise power.

[m.r.a.z.23]

In the King Air the Autofeather system operates on the difference between air pressure in the bleeds of the engines or something along those lines, and activates when there is a difference in pressures greater than a set parameter. It is therefore also necessary to dearm the system before practicing engine failures, as a 1-INOP G/A with one throttle at idle and the other at max allowable torque could also cause the system to feather the eng producing lower pressure in the bleeds, hence creating a real problem.

I think that's how it goes anyway, been over 6 months since i did my course. The other facts said re: failures in the critical stages of flight compared to cruise is also right.

[sms777]

Guys... This is one of the most interesting and educational threads i have followed for a long time on Pprune. Altough i have not flown a B200 yet but fairly experienced in Garrets, my choice of aircraft ( if i was to own another one ) will be a KingAir because i just love the Beech reliability and simplicity....
Keep it coming please.... i am not old enough to stop learning yet

( Where are you Wally MKII?)

[Grogmonster]

This is the real story

The autofeather system is armed when, 1. The autofeather switch is in the armed position, 2. The N1 is above 88% and the torque is above 17% on both engines for the B350 and 90% N1 and 400ft lbs for the B200. Once the power lever on either engine is retarded below the above figures the system is disarmed so you can in fact simulate an engine failure without the possibility of a prop feathering by retarding the power lever.

It is possible to leave the system on during cruise and at normal cruise power the prop would feather were an engine to fail however as stated above you have way more time to assess and react in the cruise phase. there is also a school of thought that some of the components in the system get very hot if left on for an excessive period. The POH says turn it of in cruise so that is what we do.

I hope this helps.

Groggy

[PPRuNeUser0161]

Back to the original question.
Shadow, the AFX is turned off in the cruise to allow the pilot to have more time to think about a possible airstart or other course of action. The system is probably disarmed anyway by way of the power lever position switch that deactivates it when either lever is below the ~88% N1 position. This switch activates at approximately 88%, it is located on the power lever itself and the exact % of N1 will vary. By turning off the AFX the pilot knows it's off. If you don't it may be on or off depending on the position of the power levers.
SN

[FGD135]

Once the power lever on either engine is retarded below the above figures the system is disarmed ...

Not true - it is possible for the autofeather to be armed for only one engine. That quote was from the post titled "This is the real story".

Now for the real, real story:

This is how it is for the King Air 200. I don't know the 350 but am sure it would be fundamentally the same.

The autofeather "system" is not really ONE system for two engines, but should be thought of as two separate systems. This is why it is possible the autofeather can be armed for one engine but disarmed for the other.

A number of posts have referred to "the system" and stated that BOTH power levers must be advanced ("for the system to be armed") but that is not quite true.

Regarding the statement I quoted above: The truth is that, with both sides armed, if you retard one power lever you will disarm the autofeather for that side. But, provided you don't retard it to below 400 ft-lbs torque, the autofeather for the other engine will remain armed.

There is a green annunciator for each side which is lit when that side's autofeather is armed.

So, here again, in a little more detail this time, is how it works. It is much easier to think of as two separate systems:

A prop will autofeather if the autofeather for that engine is "armed" and the torque of that engine drops below 200 ft-lbs. That part is straightforward.

Slightly more complicated is the conditions that dictate when the system is "armed". For the autofeather to be armed on a particular engine:

1. The "autofeather" switch must be set to "ARM", and
2. The power lever for that engine must be advanced beyond the microswitch in the quadrant that denotes 90% N1, and
3. The torque of the other engine must be above 400 ft-lbs.

Note that there is only one ARM switch. It does double duty and acts for both engines. I believe that the very early model King Airs actually had two of these switches (one for each engine).

Perhaps because there is one switch, so many people think the autofeather is one system.

That switch has three positions: OFF, TEST and ARM.

You can test the autofeather system without having to run the engine at greater than 90% power by holding the switch in the TEST position. That position is spring loaded so you must hold it there.

When set to TEST, the microswitches in the quadrant (that denote 90% N1) are removed from consideration - so the only condition then required for an engine's autofeather to be armed is:

The torque of the other engine is above 400 ft-lbs.

For those that think the autofeather is disarmed in the cruise because the power levers are retarded: not true. The power levers will be set for around 96% so the autofeather would be armed - if a pilot failed to follow the prescribed practice of switching it to OFF once reaching a certain minimum altitude (1,500' AGL for some operators), that is.

[Fr8dog44]

I stand corrected!! I remembered it bass ackwards! It's been 14 years since I flew the 99. I do recall the 400 & the 800 Torque settings, so I guess that much is the same with the BE200. The BE200 came later than the 99. Many similarities as well as differences, especially the electrical.

[Grogmonster]

FGD135 is misinformed

Sorry mate but you are wrong. Once the torque on the failing engine falls below 400 ft lbs the autofeather armed light on the operating engine will extinguish indicating that the operating side autofeather is disarmed. Then once the failed engine torque falls below 200 ft lbs it will feather. That is what you are looking for in the autofeather (AFX) test sequence discussed above.

Groggy