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Explain why prop oil B4 startup?

in 912 / 914 Technical Questions 13 years ago
I religiously turn prop to get all oil burped up into oil tank before startup,
but admit I don't fully understand why.

I get that if I don't do that the some oil will be pooled down where it shouldn't be,
but don't really understand where or how it being there could do damage on startup.
(After all, whereever that extra cupfull of oil is sitting would _seem_ to have to be part of the normal oil circulation path.)

Anyone willing to educate me, or point me to an article that would directly address my question without taking a 3 week factory maintainance course? :S

Al

  • Re: Explain why prop oil B4 startup?

    by » 13 years ago

    It is necessary to get all of the oil out of the crankcase so you can check the level with the dipstick.
    David
      
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  • Re: Explain why prop oil B4 startup?

    by » 13 years ago

    Al,

    In Lycomings or Continentals, the oil reservoir is always well below the crankcase and the oil always drains back into the reservoir. The Rotax reservoir Is liable to be mounted incorrectly in many cases and is often located much higher than it's specifications call for. In these cases, the oil cannot drain back to the reservoir naturally, but must be pushed back by crankcase pressure. With the engine off, It is possible for all the oil in a high reservoir to slowly siphon back into the crankcase. In extreme cases, this can cause a hydraulic lock with disastrous results if not burped back into the reservoir before starting. Even a properly mounted reservoir is beside the crankcase rather than below it and unless you burp it, you never know exactly how much oil you have in the system. A cupful of oil in the crankcase wouldn't hurt anything. The way the Rotax system is designed however, never lets you know whether there is a cupful or a quart in in there.

    Bill.
      
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  • Re: Explain why prop oil B4 startup?

    by » 13 years ago

    "The way the Rotax system is designed however, never lets you know whether there is a cupful or a quart in in there."

    I'll continue to burp it.

    But not sure I understand what you wrote:
    Like many 912's ours burns essentially no oil at all.
    When it's fully burped and the oil is cold it's at full mark on the dipstick.
    So if we see it's not more than half the distance down between the high and low mark,
    befor burping, we DO know that there ain't more than about half a cup of oil unaccounted for.
    Right? Am I missing something?

    I also vaguely recall SOMEthing about the oil maybe being in a valve lifter and potentially causing a problem --- but I'm probably mixing up several different stories here. :S

    Al
    Thank you said by: bill martin
      
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  • Re: Explain why prop oil B4 startup?

    by » 13 years ago

    Al,

    You didn't miss anything and you are right. I was a little out of context there. I'm used to checking other people's planes and if the dipstick shows a low oil situation, I don't know if it is really low or its in the crankcase unless I burp it.
    Thank you said by: Al C
      
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  • Re: Explain why prop oil B4 startup?

    by » 13 years ago

    Hi Bill,

    You wrote --
    "....In extreme cases, this can cause a hydraulic lock with disastrous results if not burped back into the reservoir before starting. .."

    Yup, see that in the Rotax docs and other places.

    I believe it. Similarly, I read that if one ever runs the prop backwards one can pump oil into a potentially nasty place where it can do serious damage on startup.

    But I can't really picture/understand where that oil is and exactly, or even approximaely how it's (potentially) able to cause havoc/damage.


    Having been able to find any digrams of the oil system at all, let alone any that would shed enlightenment on that. I'm willing to dutifully follow a rule from reliable sources, but would love to understand it.

    You, or anyone, care to take a crack at educating me on that if you understand it ;) ?

    Al
      
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