DHC6 autofeather

[shimmydampner]

Yes, thank you. I was not mixing them up. A question was asked, and I threw out a possible answer.

[jg24]

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

thanks for that

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

Michael, thanks for that very clear explanation. Really helps!

[co-joe]

One small reason to leave the auto fx off, is that if you will be attempting an air start, it saves one or two steps, streamlining the process. I’ve never heard of an auto fx feathering an engine at high power, that would be ugly, I have heard of it feathering an engine on landing when both power levers were brought to idle, but not on an otter.