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Dry Nitrogen Prop Unfeathering

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Aircraft Systems

I'm getting my mutli-engine comm rating in a PA30 Twin Commanche. The POH states that it uses Dry Nitrogen to Unfeather the prop. Can you explain exactly how that works? The POH doesn''t go into details on how this system works.

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2 Answers

  1. Best Answer

    Paul on Aug 11, 2010

    In order to explain this you have to go back one step. When you feather the prop, you opened a valve that took oil out of governor and sent it back to the engine. Now if you decide to restart the engine, the blades are in a position that makes it extremely hard on the engine to turn them over. You need oil pressure in the governor to put the blades at an angle where wind can assist in cranking the engine. On some aircraft, manufacturers included a unfeathering accumulator to assist in getting that high pressure oil to the governor ASAP. In this unfeathering accumulator is where your dry nitrogen lives.

    This unfeathering accumulator has 2 sides. One side, in your case, has nitrogen and the other side has high pressure oil that flowed into the accumulator during normal flight. It builds pressure in this accumulator, compressing that nitrogen. A check valve keeps that oil in there when you feather the prop.

    When you unfeather the prop (take it out of detent) the accumulator springs into action pushing all that oil back into the governor to assist with putting the blades in a position that makes it easier to start the engine.

    Here is a good link that has some valuable images of a governor and unfeathering accumulator:


    Unfeathering Accumulator

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  2. Felix on Aug 11, 2010

    Thanks Paul! Makes sense now.

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