Light Twin Feather Detent

[ROB-x38]

I believe I have a reasonably good understanding of CSUs and variable pitch props and how they relate to twins and singles but the more I think about it the bigger the hole I dig.

My question is, following a confirmed engine failure in a typical light twin (PA-44), what exactly (mechanically) does pulling that pitch lever all the way back do? Is the only difference between a very low RPM setting and the feather position that in the feather position the feathering locks are inhibited from engaging? So basically does pulling the prop to feather stop the feathering locks from engaging and allow the counterweights, nitrogen and spring pressure to drive the prop to feather?

When taught to test feather in the runup bay the procedure was 1500 RPM then pull the pitch lever into feather, ensure it can travel all the way then straight back to fine pitch. If indeed all the feather position does is to inhibit the feathering locks from engaging should the correct procedure be to check RPM can drop below 950 the return it to fine pitch?

Thanks in advance

 

[minitour]

The way I understand it is this:

You pull the prop lever back to Feather...

The speeder spring in the governor gets adjusted appropriately and the flyweights move out (prop spinning "too fast") and the oil pressure "valve" opens letting the oil (pressure) out of the prop dome. A spring (and sometimes nitrogen) help to push on a piston that rotates the prop into feather due to the lack of oil pressure pushing back against it.

Remember that oil (pressure) you just let out of the prop dome? Well, when it's in the dome, it pushes on a piston that rotates the prop into a high RPM/low pitch setting. When you let it out, it can no longer push back against that piston...so that's where the spring/nitrogen works.

Then, your prop goes into feather.

You MUST do this before 950RPM in the PA44...if the RPM drops to 950, you will NOT get the prop to feather. If you notice a drop in oil pressure and rise in temperature, I'd recommend to investigate quickly and then feather...get that drag gone before you can't.

The pins are engaged at 950 RPM and lower regardless. Your only chance is to feather PRIOR to that speed.

To get the prop out of feather, if you've got accumulators...the pressure stored in the accumulator helps to "push" that piston into a lower pitch/higher rpm setting to get the prop turning. IF you don't, you may need to kick the starter a bit here and there to get the prop turning and windmilling...then the governor says "hey wait a minute...the prop isn't turning fast enough because those flyweights are movin' on in"..that "valve" opens back up, oil pressure flows into the dome and...low pitch/high rpm.

Once that happens and the engine kicks up, you should be in jolly good shape. Just make sure you follow the checklists on the re-start procedure though...and the engine failure too.

Remember, do it quickly, but there's no "!" on a checklist.

....that's my understanding of the system anyway.

-mini

 

[gkrangers]

Theres a big difference between doing it quickly and rushing.

Rushing is bad.

It was my main problem during my multi training.

Once you realize you don't have to Mixture, Prop, Throttle, Flaps Up, Gear Up, Identify, Verify, and Decide in half a second, things flow much smoother.

 

[greyhound]

Ggkrangers is corrrect

Rushing is a bad thing in probably any in-flight emergency.

Thinking things out and not allowing the "I have to do something now" syndrome to take over will work out better.

Back to the original Q.... The prop levers do not prevent the pins from engaging. Those pins are only set to engage anytime rpm drops to a certain value since they are held out by centrifugal force.

That is why you must feather a prop before RPM drops to a certain value, otherwise the reduced rotation of the prop causes less centrifugal force to be applied on the pins and they will engage.
If you allow that to happen,you can pull the prop lever back all day long and you won't get them feathered.

 

[ROB-x38]

Thanks for the replies. Is it therefore correct to say that when a prop feathers correctly it will hit the feather position before the RPM drops below 950? ie: for at least a brief period the prop will be spinning in feather at > 950 RPM?

In our flight training we only simulate feather and as a result i haven't had the benefit of actually seeing it. Don't suppose there's any videos out there?

 

[SPilot]

http://video.google.com/videoplay?docid=-6996877986231375539&q=engine+stop (totally unrelated)

http://video.google.com/videoplay?docid=-1303721238647607181&q=prop (quite unrelated)

(sorry, google dont seem to have videos on feathering props)

http://www.aopa.org/asf/publications/sa06.html (AOPA on propellers)

If you have FS 2004, look on the last CD in the set. It contains training videos, in which the instructor shuts down an engine on a B58. You dont see very much details on the propeller though.

I think you should talk your instructor into doing at least one full engine shutdown with you. During my ME/MEI training and FAA checkrides I've performed many full shutdowns, maybe against school policy, but nevertheless its an invaluable learning experience. Make sure you're above an airport without too much traffic, be at least 5000ft, and simulate engine failure and then proceed to full feather... close the cowl flaps, and restart it as soon as you've secured it (do not let it cool down too long). If it doesnt start up again, you'll do as you're trained to do, and it should not cause any damage to the airplane. You can help the starter motor by increasing the airspeed a little during the inflight restart. If it does start, let it run on low power setting for a while to allow the temperature to build up slowly again (rapid temperature changes is stressfull to the engine parts).

You'll probably experience that handling the aircraft with the failed engine feathered is much easier than handling it when you've set the engine to "zero thrust". It is hard to tell if the engine is at actual "zero thrust" during simulated engine failure, and you still have gyroscopic forces. When it is shut down, it acts only as dead weight, and it becomes very quiet in the whole airplane because you dont have the vibrations from unsynchronized props. You'll also be able to determine the *actual* single engine performance at your airplane weight and density altitude.

And yes, you're right to assume the props will go into feather before the feather locks prevents it. You can essentially think of the controllable pitch system as a "pressurized fight" between the spring and the controllable oil pressure. When you put the prop control all the way to feather, your governor opens up so that the oil pressure drops completely, and the spring (and nitrogen) in the propeller hub squeezes the prop into feather right away. If you try to do it in the airplane you'll notice that the RPM drops very quickly once you move the control lever back, because it suddenly changes the propeller pitch and gains immense resistance (drag) to rotational movement.

As mentioned before in this thread, the feather locks are very simple in design, with calibrated springs pulling the locks in, and centrifugal forces on the propeller shaft pulling them out. During normal shutdown, the governor doesnt deliberately drop the pressure, and so the RPM will drop so low that the feathering locks will engage before the governor looses enough pressure to allow the prop to feather. That allows for easier starting.
...just my $.2 :smash:

 

[CalifDan]

Isn't a complete feather and restart in the requirements for a multiengine rating? I had to do one during training. Frankly, I don't remember if I had to do one for the test, too long ago. I did have to do one for the commercial multi.

Just curious.

 

[gkrangers]

I did one on my comm multi ride. Did plenty of full and simulated and had 1 real failure along the way during training.

 

[minitour]

Yep...gotta shut one down on the ride. I'm surprised you haven't shut one down in training...very surprised.

-mini

 

[GravityHater]

I have often wondered why some turboprops are designed to be feathered on normal shutdown, and some not.
Is there any advantage in one system over the other?