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Why NOT turn prop to drain ALL the dirty oil?

in 912 / 914 Technical Questions 11 years ago
SOMEwhere on this site I saw some official looking comments regarding changing the oil.

On a document on this site it says:
"See SI-912-010 “Oil change” and “Oil change for the 9 series” article for more information.
The complications happen when people do not follow the published instructions:
WARNING: The following are maintenance errors:

  • Re: Why NOT turn prop to drain ALL the dirty oil?

    by » 11 years ago

    Hi Alan,


    If you turn the prop or drain the oil lines the system will be full of air. Hydraulic lifters do not pump or work with air in the system and they will absolutely cause you very expensive damage. If you pull the prop through or drain oil lines you absolutely must do an oil purge as described in the manuals. Failure to do so will cause lots of crying and an empty wallet. Oil pressure is not an absolute indication oil is getting to all the lifters and air in the system is gone. It only means you have pressure at the pressure sender.
    If all you do is pull the oil plug and allow the oil to drain and replace the plug then change the oil filter
    (without rotating the prop) and then fill the oil tank no purge is necessary. If you do a hose change and change the oil hoses then an oil purge is necessary. More or less any time you break open the oil system.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: YEN NIEN YU
      
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  • Re: Why NOT turn prop to drain ALL the dirty oil?

    by » 11 years ago

    Much thanks for your quick and detailed explaination!

    I can see how getting air in there can cause a problem, and apparently at least some owners have, per your comments, been seriously blindsided by this practice.
    So I'll take it as a no-no for myself on my Rotax, but that I'll continue that practice on my auto engine oil change. Been doing it over 30 years on a variety of engines.

    But, with all due respect to your obvious expertise, I'm not sure it's as absolutely guarenteed a disasterous practice as you suggest. Here's why I say that:
    Turns out right after posting my question I caught the guy who does the frowned-on turn-the-prop-while-filter's-off practice out at the airstrip that evening and discussed it with him. He was surprised to hear it was a discouraged practice. (That said, he ONLY turns the prop, doresn't drain the oil cooler or the other lines.)
    Turns out he had just done exactly that (turned the prop with oil filter off to get more dirty oil out) five days prviously and since then had flown two flights totalling 3 hours and had not fallen out of the sky yet or heard any unusual sounds or symptoms, and describes his 800 hour 912 80hp 12 year old Rotax as "purring like a kitten and full power." Sounded that way to me, too, when he took off while I was there.


    Do you suppose he was just lucky and dodged a bullet, or are you seriously predicting he's already got serious damage that he doesn't know about yet?


    By the way: Do you see anything wrong with, while the filter is still on, but the oil tank has stopped draining, proping it to get a bit more oil to come out of the tank drain? Or is that as likely to suck air into the system?
    I'm beating on this because I'd still like, if safe, to get as much of the old dirty oil as practical.

    Any suggestions for _safer_ ways to get the last of the dirty oil out?
    How about, after draining the dirty oil, pouring a quart into the oil cap, prop it to circulate it, and let it drain through the system. Would waste a quart of oil expensive oil, but seems like that would flush out the dirty oil without risking getting air in there.
    What do you think of that?
      
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  • Re: Why NOT turn prop to drain ALL the dirty oil?

    by » 11 years ago

    Hi Alan,

    Yes anyone can dodge a bullet and do it more than once. Kind of like Russian roulette. You may dodge that bullet a lot of times or the first one will count. The problem here with air in the oil system is you never know when it is too much or when it will affect a lifter. If the owner does happen to have too much air in the system and he starts the engine the damage will be done before he knows what's going on and shuts it down and he did all this damage just to get 2 ounces more oil out. Even if you dodge that bullet so to speak what are you doing to the longevity of the engine when you are on the boarder line more than once and from what I have seen through the years many owners don't know when an engine is running poorly so they just keep running it until something makes it quit and the whole time what lead up to that moment was their maint. practices over the last couple of years.



    Rotate the prop before any oil drain or the system is ever opened. Do not rotate it after the oil is out of the tank or the filter is off. If you have anything in the system open i.e. the tank empty or filter off or open oil lines rotating the prop will help inject air.

    There is no need to worry about the color of oil since the majority of us change oil before it is truly needed anyway. It isn't a direct reflection on its lubrication properties. Hydrocarbons that burn produce carbon which is black and then there are other by products of combustion, too. The oil is doing its job by removing these contaminates, but it doesn't mean it isn't up to the job of lubricating any more. The only way to really tell about a used oils properties is to send it to the lab. The small amount in the engine is not an issue when you are replacing the lion's share with 3 qts.

    I used to teach the Incident Command system and Homeland Security classes to fire service, emergency services, police, ect.. and even though there may be more than one way to do something history and mistakes has shown us there is what we call "Best Practices" for emergency work. We have all read and seen screw ups with emergency services. So they learned from the hard way not to repeat those practices again. In our case it is "Best Practices" as it relates to our maint. that has been learned over many years of doing some things right and doing many things wrong. A wise man will study the wrong things done and use Best Practices so bad history doesn't repeat itself.

    Let the other guys that don't want to learn from history make the mistakes and you can work from the Best Practices side.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Al C, YEN NIEN YU
      
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  • Re: Why NOT turn prop to drain ALL the dirty oil?

    by » 11 years ago

    OK, I get your point. Thanks. You made that very clear.

    Just a thought/suggestion though:
    Sometimes I find it better not to speak in absolutes (like "you WILL die if you do this <g>" if for no other reason than to preserve my credibiity when my listener finds the two guys who did that and didn't die. ;-)


    Your point reminds me of what my dad used to say:

    "A fool never learns.
    A clever man learns from experience.
    A wise man learns from _others_ experiences." ;-)



    Speaking of others' experiences:
    I think the guy I was talking about may take to heart the "that isn't a good practice" message I passed on to him. But do you think I'd be doing him a favor suggesting he needs to do some radical checking for damage like a tear down, or just mend his ways and keep an eye of his next compresion tests or what?
    Don't know if he'd take me seriously but don't want to freak him out unnecessarily if he does.

    Oh, by the way: You mentioned 3 quarts per oil change.
    I've notices in MY oil changes that the manual says 2.6 quarts capacity (or maybe it was 2.62... ) but every time I do that 2.6 quarts doesn't get us to the fill line (maybe about 1/4th of the way above the bottom mark of the dipstick. Seems like after everything settles out I end up using about 3 or maybe 3.25 quarts for an oil change to bring it to the full mark on the dipstick.
    This never really worried me, but it seemed odd to have this discrepency as Rotax is, or should be, known for being pretty precise about their specification.
    Only theory I ever came up with was "maybe the oil resovair tanks or dipsticks are not exactly identical."

    Have you or others noticed this slight discrepency between the manual (ours IS a very old manual) and what it really takes for an oil change?
      
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  • Re: Why NOT turn prop to drain ALL the dirty oil?

    by » 11 years ago

    Hi Alan,
    You're right.
    Absolutes aren't usually a good idea. I should have tempered that comment a little.

    I wouldn't freak your friend out just advise him to change his maint. practice and keep an eye on things for the future. When he does the 100 hr and or annual inspection would be a good time to pay close attention, but I wouldn't tear anything apart unless there were some signs and symtoms.
    If you break the oil system open or drain it for some reason it is okay so long as you do an oil purge before the start up.
    3 qts. will put the oil tank level at the top of the mark and is what is usually taught in the schools. One thing you can tell him; that since you can't pre-fill the new filter to rotate the prop 20 times to help fill the filter, after oil is back in the tank. The filter is going to take up some of that. Getting a true oil level reading is like one of those absolutes. It isn't always the absolute level. ;)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Al C
      
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