Feathering props
 

Hi all.

While reading about the recent Melbourne Kingair accident, I've been seeing 'feathering' mentioned a lot, so trying to understand what it is (Non-pilot, so excuse my poor lingo here).

I've been googling, and it seems it's a way for the pilot to a) reduce drag on a failed engine and b) protect the engine from windmilling (Possibly causing further damage). And what happens is the blades are angled so that they cut cleanly into the airflow (So, leading edge of the prop into the wind?). And that helps cause wind resistance in the spinning direction, as the blade is flat (Like a cricket bat? Unless you're playing a cut shot.. :oh:)

And if all is OK, (i.e. You have time), you would then attempt to restart it.

Would you un-feather .. (de-feather... not sure what the term is) it to start it, as now it's in a resistance mode (Trying to stop it from spinning). Or is the engine powerful enough not to care about that. I'd have guessed that running a prop when it's feathered would strain the engine, and be pretty useless for providing thrust? So, would the engine be started while feathered.. and then ... de-feathered completely to run normal again?

And then auto-feather - does that mean the engine is feathered automatically in the event of a failure? If so, why would you ever disable auto-feather?

Yes you are correct about the reasons for feathering. Primarily it is a means to reduce drag, in terms of restarting mid air it depends on the aircraft type. For my type when a relight is attempted the propeller is moved out of the feather position by the unfeather pump. The windmill effect you speak of actually is used to provide initial acceleration (usually done on the ground with the starter generator). This is only ever done at altitude where the effects of the windmill are safe. In terms of auto feather this is engaged on takeoff and landing but disengaged in the cruise.

If it's a recip engine, unfeathering the prop usually creates enough torque to restart the engine, provided it is not badly damaged. Think of a rolling start with a clutch and manual transmission in a car or truck.

Single shaft turbines can also be restarted this way, because th prop torque is directly coupled to the compressor rotor. With a two- or three-shaft machine, no such luck, although the engine starter may be used.

The Pratt & Whitney PT6A-41/42 turbine installed in a King Air is a free turbine, not a fixed or single shaft turbine. The propeller and power turbine are not mechanically connected to the Compressor Turbine and Compressor.

you would have to think long and hard before restarting a turbine. Turbines are extremely reliable and for one to shut down or fail there must be a very good reason for it.

Thanks pinkpanther1 - When you mention safe conditions for the windmilling, is that because of the drag it causes - resulting in a lot of yaw towards the dead engine?
Also, why would you ever want auto-feather disengaged? Is there a time that, if the engine fails, you wouldn't want it to feather?
Thanks for explaining.

Thanks barit1 - the 'push starting a car' is a good way of explaining it for us earth-bound people. Makes sense.

Yes, the drag is so significant on all such twin engine types that the aircraft cannot maintain its altitude. All propeller powered aircraft (with 2 or more engines) have the ability to feather the props of failed engines for this reason.

Yes, there is yaw towards the dead engine.

Things get very interesting when you try to answer this question. From what I have read in this thread, it appears this operator had a policy of taking off with autofeather (AFX) disengaged - for the reason that other King Airs in their fleet were not AFX equipped and they wanted the takeoff procedure to be standard across the fleet. There are sound safety arguments behind this approach.

But, on the other hand, it can be said that to intentionally disable a major safety feature such as AFX is foolhardy! Takeoff is the phase of flight for which the AFX is primarily intended.

Yes, after takeoff, when the aircraft has climbed to a comfortable altitude (say 5000-10,000 feet). In this case, with the possibility of being able to restart the engine, it is more simple to do it with the propeller windmilling, rather than feathered.

Gets interesting with a fixed shaft engine such as a Garrett? Engine components start disembarking at a rapid pace! :}

On the King Air with the Raisbeck mods it is mandatory to have serviceable auto feather and to use it.

That is interesting as VH-ZCR appears to have a Raisbeck conversion:

http://australianaviation.com.au/wp-...g_01052016.jpg

FGD
I think that comment from the EN thread (is that the thread you were referencing?) was in relation to the AAV crash, not the EN crash, but please correct me if I'm wrong.


Megan,
Please let me just clarify your statement, you can have "Raisbeck mods" without requiring the AFX on the B200. James developed more than just props on the B200!

A more correct statement would be B200s fitted with Hartzell-Raytheon or Hartzell-Raisbeck 4 blade props require auto feather.
A B200 fitted with McCauley 4-blade props does not.

FDG135 - technically there are twin-engine aircraft where the propellers can't be feathered. An example:

https://en.wikipedia.org/wiki/Champion_Lancer


Cralis - with regards to reasons for having it disengaged, I can see two (in addition to the one posted above).

First, it doesn't always work. If you're expecting it to feather automatically, and it doesn't, then in a situation that's already extremely busy you're going to have a plane that's not behaving how you expect (because the propeller isn't feathered) and another action (manually feathering it) to deal with. Better to just have it disabled, so that you fully expect that it'll need to be handled manually.

Second, I know some pilots are wary of automatic systems. Better, perhaps, to have a plane that's behaving exactly how you expect than to have one that's trying to make things better for you and just adding to the confusion. We saw this sort of thing in the Asiana 777 crash; the pilot assumed that the automatic systems were taking care of airspeed control, and didn't realise that they weren't until it was too late to recover.

So, what law would you be breaking if you chose not to arm the AFX prior to taking off in your 4 blader King Air?

138- Pilot to comply with requirements of aircraft's flight manual etc


The AFX requirement is stated in the Limitations section.


I would also suspect that if you ignored it, killed people etc, you'd probably be up for negligence, misconduct etc.

There's a reason why the equipment is certified with that requirement. Do some people really think they know better?

Thanks for the clarification Ramrod, but since we are talking auto feather I took the assumption folks would read "Raisbeck mods" as referring to engine/props. My error. The Raisbeck mentions -52 engines and Quiet Turbofan Props, whereas ZCR had -42, so not applicable? Is it a prop thing, engine thing, or combination of both? And for what reason to make auto feather mandatory over and above the stock standard machine?

Slatye,
Ha! I just knew somebody would come up with some ultra obscure twin that had fixed pitch props - and that it would be British!

Well, in this case this type is American. A quick look over that Wikipedia page brought a few laughs - thanks! Here are a few quotes from that page:

Still applicable, Megan - applies in all cases where the prop has four blades. That is my understanding.

No probs Megan. I took it as the props but thought I'd get a little pedantic :}
You can get Raisbeck props on -42 too :ok:

If I remember correctly, the D4N-3A designator is the Hartzell-Raisbeck, and the E4N-3G is Hartzell-Raytheon.

Nothing to do with feathering props but in line with the philosophy of training in case of a safety system ( auto feathering props ) not working . Emirates used to do all it's single engine work on the B777 in the sim with the TAC failed . The TAC is a computer that senses thrust diffencial between engines and automatically adds rudder to assist the pilot with control.
After a few real life engine failures where the TAC worked as designed and the pilots massively over controlled the aircraft . This training philosophy was revoked. ( till the next flight training manager anyway )

Thanks, Slatye - that makes a hell of a lot of sense, as to why you wouldn't/shouldn't leave it engaged all the time.

I use X-Plane, a flight sim (I know - pa! Not the real thing! But it's the best I can do for now...). And on the KingAir, I've always thought this thing (circled in red) had something to do with feathering. I'm now guessing I'm wrong, but might go a bit crazy when you feather an engine?

http://i66.tinypic.com/maat5w.jpg

I've always thought this thing (circled in red) had something to do with feathering

Check page 7-28.

Synchroscope - thanks! Absolutely nothing to do with feathering, although, I'm guessing if you feathered one while running (If that's possible), the synchroscope would start spinning towards the unfeathered engine.

Thanks John.

Cralis that bit you've circled is the prop synch. There's an automatic system to synchronise the propellers. In the B200 there's a Type I and a Type II, but I won't go into the differences now as it isn't really relevant.

The little black and white "spinny thing" is called the synchroscope. It spins when the props rpm is not synchronised. It spins clockwise if the right one is rotating faster, or counter-clockwise if the left prop is rotating faster.

Not that it really means much for this discussion, but you've put up a C90 King Air panel pic.


HOWEVER, I don't quite agree with what Slatye has posted. The AFX system does a much better job than most pilots and it is a fairly robust system in Beechcraft twin turboprops. I've never had them go wrong- but then again maybe there are tightarses out there with poor maintenance who neglect their aircraft and thus take the "it doesn't/might not work very well so I'll just turn it off" stance. That's a topic of discussion for another day!

There's a very simple procedure- when the engine fails you check that it has auto feathered. If not you manually feather it. Takes no longer than let's say turning off the system and planning to do it manually anyway.
It is a very good safety system and, if it's an option, may as well use it.


Megan,
Sorry I missed some of your post earlier re mandatory AFX. I'll have to go digging through my notes to see if I can find it, however my best recollection is that the Hartzell-Raytheon/Hartzell-Raisbeck props require AFX all because of Vmca considerations when they were certified.

Agreed, I think the idea that Slatye has posted earlier is asking for trouble. Autofeather isn't just there to improve climb after engine failure, it is mandated on most B200's (and all a/c fitted with Raisbeck Quiet Turbofan mod) subsequent to the mid-90's to reduce Vmca from around 108 Kts to 91 kts (or so depending on the variants). If using the standard Raisbeck performance figures of 94kts for V1/Vr and 103kts for Vtoss, you are rotating 14kts prior to Vmca if you've disengaged Autofeather. You won't get that 14 knots back, if the failure occurs at rotation. Sure you could say screw it lets turn it off, but what figures are you basing your takeoff performance on as all figures are based on operative AFX and the low Vmca. It would be more like Vr of 114kts and a Vtoss closer to 120kts, but how's that affected your takeoff distances, could you even still use Runway 17 at EN using those numbers at 12,500lb? When Raisbeck first started modding the -200 in the early 80's they saw the takeoff performance issues they were having with the 4 bladed props and their associated high Vmca. They got approval through the FAA to mandate autofeather as the solution to bring Vmca down and improve their takeoff performance numbers which obviously got them sales. As the autofeather didn't require any pilot intervention the FAA were happy to certify this way and Beechcraft followed suit in the mid-90's.

Harry what a great post and excellent answer. Tippy top.

I flew B200s in the late 80's and 90's and was lucky enough to amass 5000 hours on them. I never flew one with the Raisbeck mod. We always used or activated the autofeather function I am not sure even if the MEL allowed the aircraft to be dispatched with it U/S. I am pretty sure it was never a issue, because as you said the system is pretty robust.
Even one of the requirements to take advantage of the Take-off minima of ceiling zero feet; and 550m or 800m vis is dependent on operative autofeather, for that safety factor.
The operator on the type I fly now include a check in the emergency recall to confirm autofeather. It should happen....just checking....you never know.....you know!!!! Nothing is full proof in other words.
Simulators are great tools for training on the effects of engine failures etc. I would be nice to see a cost effective generic simulator established that can demonstrate the issues that you described.
Craven.

Thanks Craven. Yes you are correct, systems fail and usually right at the point when you need them most. The Phase 1 checks for the current(ish) model B200 include "Verify Autofeather Operation" for just that scenario during an engine failure on takeoff.

Simulators are an amazing device for such training as long as they are relevant to the equipment you operate. Please correct me if I am wrong but the only B200 sim in Oz at the moment is the one at the Ansett Training Centre in Melbourne and while a great device it only simulates a 3 bladed -41 engined machine, not really relevant I would think to 90% of the operators in this country, with flight dynamics that nowhere near mirror a machine with draggy 4 bladed props. This perhaps leads to a false sense of security regarding the initial climb performance of the B200 when guys do get given such failures and are able to climb away with relative ease, with the focus being just fly the flight director and you'll get the performance you need. Once the new Level D sims are up and running this year I am sure that the training will become a lot more relevant.

On the Q400 there isn't even a thought that covers it failing at the same time as the engine, it is tested each day. Without it I doubt the aircraft would be controllable on a V1 cut, it would probably still climb but the un-commanded roll would need full control deflection to correct.

Non-feathering multis
 

The Caldwell controllable non-feathering prop received the Collier Trophy for technical achievement in 1933. Boeing, Lockheed, Douglas etc. multis certificated before this all used this counter-weight actuated (i.e. non-feathering) prop. They were soon retrofitted with Hydromatics (Hamilton Standard design) within a year or two; it made a significant boost in OEI performance.

Engine failures in a light twin result in a loss of the airplane far too many times throughout the world. Airplanes like the B200 that should be easily controllable are thrown into the ground by even professional pilots. I can accept that a PA31 with 10 people on board has no chance of being flown away if the engine should fail below, say, 300 feet, but at least the impact should be under control at the lowest possible speed for maximum survivability.

I was taught as a sprat to always confirm the engine failure before feathering. In some airplanes this is easy to do because (like in a B200) the torque and EGT will show the loss of power but for a piston twin the instruments are not much use.

The procedure for the piston airplanes was to decide the engine failed side by the dead foot method, with full throttle both engines then confirm by closing the throttle (does anyone have a better way?). If the yaw remained the same then the failure was confirmed, if not, try the other one. A partial failure would be detected this way as well.

But should the feather be quicker than that? Is there time to do this and is it necessary? After deciding which engine has failed by the dead leg, just go ahead and feather it without confirmation? Most POH procedures do not call for confirmation, but jump straight to feather.

If the rpm of a piston engine drops below 1000 the prop will not feather, and closing the throttle could cause the rpm to drop if, say, the engine had failed due to a mechanical issue rather than a fuel stoppage and then the prop would not feather. As well as the extra time taken.

In the PT6 and Garrett airplanes there are problems with decoupled props and failed FCUs that have the engine still running but out of control or the prop dragging despite good power from the engine. Closing the throttle would cause the prop to go into a high drag state so a quick feather is preferable to trying to fly inverted. The Garrett remains in full throttle all the time to protect the NTS/Beta state but I remember closing the throttle in the PT6. and RR Dart but could it actually hurt?

Is it time (at least for me) to move out of the past? Should I confirm by using the throttle at least in a light twin piston airplane or not?