DHC6 autofeather

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jg24
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DHC6 autofeather

Post by jg24 »

Question: have a pretty big debate with some colleagues:

How come, on the twin otter, the autofeather system is not armed on the approach part of flight. Would it not be usefull duing a go-around? Can’t find anything in the POH.
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FL007
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Re: DHC6 autofeather

Post by FL007 »

jg24 wrote: Fri Oct 26, 2018 12:47 pm Question: have a pretty big debate with some colleagues:

How come, on the twin otter, the autofeather system is not armed on the approach part of flight. Would it not be usefull duing a go-around? Can’t find anything in the POH.
Autofeathers in general shouldn't be on during any phase other than takeoff. Autofeathers can fail or cause unscheduled prop feathering etc. The risk of an unscheduled prop feathering on takeoff is negated by the benefit of automatically feathering the engine in case you lose one at V1.

That's why Autofeathers are turned off after takeoff and not selected in cruise, etc.

On an overshoot hopefully you're a couple of hundred feet above the ground already so you don't need that instant feathering of an engine.
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switchflicker
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Re: DHC6 autofeather

Post by switchflicker »

There was some kind of a delay relay on some and not on others. If my most senior memory serves correctly.
If you didn't have the delay relay, on the overshoot, if the power was not applied perfectly symmetrically, the lagging engine would command an autofeather. Not a factor on TO because the power application starts at the lowest power lever angle.
This system is the haziest one in my poor brain
What do the other DH6 pilots say
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av8ts
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Re: DHC6 autofeather

Post by av8ts »

All of the Dash-8’s only have autofeather on takeoff also
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NWONT
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Re: DHC6 autofeather

Post by NWONT »

For what it's worth. Everything is modified to suit the present conditions and operations. At the MNR in water scooping conditions, on floats of course, the auto-feather is selected on for the entire water-scooping operation. The aircraft is very complicated with all the water-bombing systems and hydraulics for the landing gear operation, no need to make it even worse!!!
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valleyboy
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Re: DHC6 autofeather

Post by valleyboy »

Then there was the old school that left the auto feather off all the time. It's such a poorly designed system in the first place with far more uncommanded feathering of good engines than actual failures. It was simply left off.
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Qdrivermann
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Re: DHC6 autofeather

Post by Qdrivermann »

Going back over 20 years ago - we left the system turned off most of the time... floats/wheels or skis. We NEVER had the system on during the approach - only the British were that stupid enough to do that, and it caused some horrific incidents/accidents. The poor system design is the reason we left the system turned off - esp. on floats.
Hope that helps.
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Brantford Beech Boy
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Re: DHC6 autofeather

Post by Brantford Beech Boy »

Both companies I flew for that operated DHC6 (First Air and Borek), AFX was armed for takeoff and approach/landing.
Reason for arming during the approach was, as stated previously, to allow autofeathering in the event of an engine failure in a go-around.

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PanEuropean
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Re: DHC6 autofeather

Post by PanEuropean »

jg24 wrote: Fri Oct 26, 2018 12:47 pmHow come, on the twin otter, the autofeather system is not armed on the approach part of flight. Would it not be usefull duing a go-around? Can’t find anything in the POH.
The takeoff phase of flight is characterized by low kinetic energy, low potential energy, and power levers stationary near the full-forward position. Engine torque is both high and constant.

During the approach and landing phase of flight, the aircraft has substantially more kinetic and potential energy, and the power levers are quite far aft (near the arming switches for the autofeather system) and may be moved frequently to manage speed and/or altitude. Engine torque is low, often at or below the threshold at which the autofeather system can be enabled.

For the above reasons, use of the autofeather system is mandated during the takeoff phase of flight, and prohibited during all other phases of flight.
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Re: DHC6 autofeather

Post by PanEuropean »

switchflicker wrote: Fri Oct 26, 2018 2:02 pm There was some kind of a delay relay on some and not on others.
The 2 second time delay (requiring 2 continuous seconds of low torque prior to autofeathering a propeller) was introduced by Mod 6/1329 at SN 290, around 1968 or 1969. At the time the mod was introduced, de Havilland recommended that it be retrofitted to earlier serial numbers, and made a kit available to accomplish this.
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TG
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Re: DHC6 autofeather

Post by TG »

jg24 wrote: Fri Oct 26, 2018 12:47 pm Question: have a pretty big debate with some colleagues:

How come, on the twin otter, the autofeather system is not armed on the approach part of flight. Would it not be usefull duing a go-around? Can’t find anything in the POH.
Something to keep in mind if you get an uncommanded feathering during an approach, overreact and not realize it's the system going bonkers on you.
-Your engine showing the highest torque will be the one feathered, which could lead to confusion.
-If you decide to go around regardless, setting max power on both levers will overtorque your feathered one.
Well you will on a Basler, I don't know about Twin Otter's torque limits versus prop's RPM but I suspect the same.



Where I fly (DC3T) We keep it Off on approaches for the reasons already mentioned in this topic.
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jg24
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Re: DHC6 autofeather

Post by jg24 »

Thanks everyone for the responses
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Zaibatsu
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Re: DHC6 autofeather

Post by Zaibatsu »

I fail to see the difference between an AFX failure on takeoff vs on approach. Both will give excessively high torque readings with operating engines and could result in engine damage. That’s why we train for these things.

A go around is much like a takeoff, rather than having high energy and low power settings like an approach, you have low energy and high power settings like takeoff. This is why, on my aircraft, its automatically deactivated at low power settings. This is also why it’s armed the approach checklist and remains so until landing.
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FL007
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Re: DHC6 autofeather

Post by FL007 »

Zaibatsu wrote: Wed Oct 31, 2018 3:24 pm I fail to see the difference between an AFX failure on takeoff vs on approach. Both will give excessively high torque readings with operating engines and could result in engine damage. That’s why we train for these things.

A go around is much like a takeoff, rather than having high energy and low power settings like an approach, you have low energy and high power settings like takeoff. This is why, on my aircraft, its automatically deactivated at low power settings. This is also why it’s armed the approach checklist and remains so until landing.
Well the difference is you're 200ft higher and well above V2 on approach.

Dehavilland doesn't like their autofeathers on in any phase other than takeoff. In fact in dh8 its been known to cause issues.

Which plane do you fly that requires autofeather on approach?
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shimmydampner
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Re: DHC6 autofeather

Post by shimmydampner »

FL007 wrote: Wed Oct 31, 2018 3:51 pm Well the difference is you're 200ft higher and well above V2 on approach.
Not in a DHC6 doing MPS you're not.
FL007 wrote: Wed Oct 31, 2018 3:51 pm Which plane do you fly that requires autofeather on approach?
I seem to recall the mighty Beech 99 being one such machine, although it's been a while so I could be wrong.
valleyboy wrote: Sat Oct 27, 2018 2:53 am Then there was the old school that left the auto feather off all the time. It's such a poorly designed system in the first place with far more uncommanded feathering of good engines than actual failures. It was simply left off.
It's a requirement for MPS. I've heard of guys leaving it off because in a simulator they "proved" they could manually feather just as fast. Seems easy enough when you know it's coming; quite another thing I would imagine if you didn't and you've just launched off a 500' esker and you're still below Vmc when it happens.
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joshiscool26
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Re: DHC6 autofeather

Post by joshiscool26 »

My thoughts on this would be,
According to the Viking AFM for the 300's a single engine go-around must not be conducted once more than flap 10 has been selected. A normal landing is between considered between flap 20-37.5 . 3 things need to happen to activate the AFX system a) its turned on b) torque is above a specified limit c) Ng exceeds 86-88% on both engines. In my limited experience so far and on our aircraft where I work the AFX doesn't arm on take-off until about 25-30 lbs of torque. Normally on approach on slope with 10 flap you only have about 10-15 pounds and a lower Ng so chances are the AFX would be selected but not armed and the same goes for 20 flap at about 80 kias. If you did have an engine failure in this state BOTH Ng's would have to be increased to above 86-88% to arm the system and then feather the engine which would be impossible due to one engine being failed. If you only had 10 flap out you would most likely still be at a relatively safe altitude to start a missed approach with an unfeathered engine. If its after 20 flap you are committed to the landing anyways so you would still have to feather it manually because the AFX isn't actually armed only selected. If the engine failed exactly when you applied full power and pulled the nose-up the system would be armed at that point and still have the 2 second delay or I suspect that if your engine failed at this point it would fail to reach 86-88%. I'm guessing at this point if you had more than 10 flap selected your chances of getting out of that situation regardless would be slim due to the fact that you are not to conduct a SE go-around weather the AFX was armed or not. Most of the guys I fly with land with 20 flap and unless you are over the fence at exactly ref or in the flare you would also most likely be above Vmc as well (68 with AFX 70 without AFX). It also says in a note, in the approach checklist of the AFM to not select AFX for approach and landing.
Unless you were in the flare I feel like it would just react like an engine failure while in the climb which is completely controllable. Let me know what you guys think.
J
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PanEuropean
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Re: DHC6 autofeather

Post by PanEuropean »

Hello Josh:

Your interpretation of what the Viking AFM for the Series 300 says is correct. I'm qualified to tell you are correct because I am the person who wrote the Viking AFM for the Series 300 and 400 aircraft.

You have made one rather small misinterpretation (an extropolation) that doesn't affect the overall correctness of what you have written, but needs clarification in case it confuses others. As you wrote, in order for the autofeather system to arm, it has to be turned on, torque on both engines has to be over a specified threshold, and Ng on both engines has to be above 86-88%. The latter requirement (Ng being above 86-88%) is not determined to be met by measuring Ng percentage, rather, it is met by the two power levers being sufficiently far forward that they activate two microswitches above the throttle quadrant. The maintenance manual tells the maintenance technicians to rig these microswitches so that they activate between 86 and 88%. In day-to-day operations, the actual Ng percentage reached when these microswitches close may vary slightly from the 86 to 88% nominal target due to variances in air temperature and air pressure.

The purpose of having the two microswitches closed (activated) form part of the three requirements for the autofeather system to arm has nothing to do with whatever Ng the engine is producing, rather, it is to ensure that the autofeather system does not arm until both power levers are shoved far enough forward to confirm, beyond any doubt, that the pilot is intent on making a take-off, or in the event of an engine failure after V1, CONTINUING a take-off. These two microswitches also ensure that in the event of a rejected take-off (when the pilot yanks the power levers back to reject the take-off), no propellers feather... if the takeoff was rejected due to a power loss on one engine, and the pilot rejects the take-off (pulls the power levers back) within 2 seconds of the power loss, the propeller of the failed engine will still be available to the pilot for discing (deceleration), it will not feather.

The strongest argument to explain why autofeather is used during the take-off phase of flight (it is obligatory to use it, not optional), and why it is explicitly prohibited for use during the landing phase of flight, is what I originally write in post #9 above: Takeoff is characterized by low potential energy, low kinetic energy, and steady power lever position at the high end of the power range. Approach and landing is characterized by higher levels of potential energy and kinetic energy, and continually variable power lever position towards the low end of the power range. There are some technical reasons why it is inadvisable to use autofeather during any phase of flight other that takeoff - those technical reasons relate primarily to power levers not being in a steady, full-power position during other phases of flight - but the main point is that it is just not necessary to have autofeather capability during any phase of flight other than takeoff.

Pilots who believe that turning the autofeather system on during (for example) approach and landing gives them any kind of protection are fooling themselves. It is unlikely that the autofeather system would ever arm at the power settings used for a normal, everyday two engine approach and landing (you can confirm this for yourself during your next flight - simply do your normal pre-takeoff checklist, and before you release the brakes, set both power levers to whatever torque you typically use during approach and landing... chances are, autofeather won't arm because torque levels are not high enough). For sure, if an engine fails when the autofeather system is selected on but NOT armed, it will never arm, because as you have pointed out, one of the requirements for arming the system is that torque on both engines has to be above a certain level - and if one engine has lost power, that requirement will not be met.

Michael
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Lightchop
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Re: DHC6 autofeather

Post by Lightchop »

jg24 wrote: Fri Oct 26, 2018 12:47 pm Question: have a pretty big debate with some colleagues:

How come, on the twin otter, the autofeather system is not armed on the approach part of flight. Would it not be usefull duing a go-around? Can’t find anything in the POH.
It's not armed on the Q400 either...
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joshiscool26
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Re: DHC6 autofeather

Post by joshiscool26 »

Awesome thanks for the answer Michael!
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goingnowherefast
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Re: DHC6 autofeather

Post by goingnowherefast »

shimmydampner wrote: Wed Oct 31, 2018 4:42 pm
FL007 wrote: Wed Oct 31, 2018 3:51 pm Which plane do you fly that requires autofeather on approach?
I seem to recall the mighty Beech 99 being one such machine, although it's been a while so I could be wrong.
Every Beech turbine twin that I've flown (half the lineup) requires afx armed for approach. However that's also irrelevant to a discussion on Twin Otters. Different manufacturers, different systems, different procedures. Don't mix them up.
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