Turboprop Q

Discussion in 'Flight Following' started by Let'sgoflying!, Jul 18, 2021.

  1. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    Never addressed in training.
    Neither CP nor head mech have answer.

    What is the purpose of this cruise checklist action? (ie what use the downside of leaving it on) AD003E2B-EFC9-4718-8C13-A789874C154A.jpeg B8F2B6EB-A8A0-42CE-A2DE-EDCE17C250EE.jpeg
     
  2. luvflyin

    luvflyin Touchdown! Greaser! PoA Supporter

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    I dunno but will take a guess. It’s the cruise checklist so I’m guessing it’s about once it’s feathered, getting a restart don’t work so well.
     
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  3. bijanmaleki

    bijanmaleki Pre-Flight

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    FlightSafety teaches leave it on at all times…. That’s what we do in our 425.
     
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  4. luvflyin

    luvflyin Touchdown! Greaser! PoA Supporter

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    Based on @dmspilot ’s reply above, I’m guessing my guess was right. That once it feathers, you can’t get a restart. At least not using a windmilling prop turning the engine to do it. So what’s the logic behind FlightSafety’s teaching to do it anyway, despite the POH’s checklist instruction?
     
  5. bijanmaleki

    bijanmaleki Pre-Flight

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    It’s a pt-6 not a direct drive engine it is viscously coupled on a second spool between the prop and core. Prop motion is relatively negligible for the core to spool. I believe Cessna may have opted for shutting it off in cruise for the systems of the day. Perhaps hydraulic tq sensing failure modes, or the circuit related to it. Worst case the circuit senses a false fail in cruise or a pilot bumps a power lever individually and the engine auto feathers and you rely on the over speed governor to do its job which is also a first flight of the day check.

    A engine out on a relatively large diameter prop, is amazing in the time to react vs a piston bird. Especially on something like a 425 which has a similar moment arm and rudder size to its piston cousin the 421. In cruise in the sim one an oei situation with auto feather inop it goes from cruise to vmc in seconds….
     
  6. Craig

    Craig Line Up and Wait

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    Probably it’s that way for needed crew reaction time. Engine failure in cruise doesn’t require drag race speeds for identification of the problem and correctly reacting to it. Failure at high power and or low speeds generally requires very rapid reaction to identify and cope with the problem.
     
  7. jonnyjetprop

    jonnyjetprop Cleared for Takeoff

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    Not sure for PT-6, but some autofeather systems work by sensing differential power and other work when the windmilling prop drives the engine (negative torque). Both could happen during normal operations and could lead to an inadvertent feathering.
     
  8. Kritchlow

    Kritchlow Final Approach

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    As said earlier, it’s a PT-6. The prop is not physically connected to the rest of the engine. The windmilling prop will not drive the engine.
     
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  9. James_Dean

    James_Dean Pattern Altitude

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    When you’re in the cruise phase there is no need for anything to happen quickly, and no reason for an automated system to do any intervention. Let the pilot sort it out.

    Having said that…. I leave it on in cruise.

    It is a system that I REALLY hope works when I need it, and it is a mandatory preflight test every time before taking the runway. Vmc on the 425 without the prop feathered is north of 130kias. Those few seconds between Vr at 90 and 130 could be exciting if it doesn’t work. I’ve done it in the sim more than a few times, and can save it nearly every time…but I know it’s coming. IRL it would be spicy.
     
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  10. Z06_Mir

    Z06_Mir Pattern Altitude

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    The demonstrations in the sim are a scary and harsh reality for what would happen without autofeather between rotation and blue line.

    I'm 99% sure on the checklist we use it says under cruise "Autofeather - Off/As desired", not just "off"
     
  11. Tarheelpilot

    Tarheelpilot Final Approach

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    I never turned it off. Not sure why it would be necessary to turn it off for cruise.
     
  12. IK04

    IK04 Pattern Altitude

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    DING DING DING! Correct answer.
     
  13. James_Dean

    James_Dean Pattern Altitude

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    IIRC, the 425 system relies on a throttle position sensor and Ng indication(84%). IOW, if the throttle is up and Ng falls below 84% it will feather. If you pull the throttle back it won’t feather if it is set up correctly.

    It does not go from cruise to Vmc in seconds. First off, if you’re at altitude you’re below max torque due to ITT limitations. If you’re low enough to be torque limited you’ve got a 60+ window of IAS deceleration. I’ve shut one down at FL270 during training and it is a yawn.

    The tail and rudder between the 421 and 425 are quite a bit different.
     
  14. bijanmaleki

    bijanmaleki Pre-Flight

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    James while the tail and rudder as you mention are significantly different I’m not so certain the arm and control surface area are that far off. I believe you are a Blackhawk bird as are we. Most of the time I’m still close to 450hp as we don’t temp out till Fl 230+ in the summer and 250+ in the winter. While I haven’t simulated it in the plane I was astonished in the sim climbing out at 150kias with sim auto feather inop, the time it took to slow enough to Vmc was not very long, if autofeather is working and does it’s thing it’s easy breasy, but you have experience doing it In the plane which trumps my knowledge. I’m used to turbofan land where oei leaves you all the time in the world to troubleshoot, or piston land, only other t-prop time is in the pc or the old Merlin(which was nasty if Nts didn’t work). Awesome memory on both micro switch and ng, good call.