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I note the latest fuel system recommendations from Rotax include a check valve between the pump and the outlet of the high pressure filter with a minimum opening pressure of about 11 psa (0.8 bar). My understanding is that this would allow fuel (albeit unfiltered) to bypass a clogged fuel filter and keep the engine running.

Now, like the Rotax diagram, I have a pressure gauge before the filter so  that would read the standard rail pressure (assuming the pressure regulator is working correctly) plus the pressure drop across the filter.

So what would that gauge read just before that check valve opened? The answer has to be close to 11 psi plus 46.4 psi, say 57.4 psi. If it gets that high, you definitely have a clogged filter according to rotax.

 

So what SHOULD be an upper limit for that indicated fuel pressure? Should I just assume that there is no pressure drop and set the upper limit at 46.4? That I think would see me changing a lot of filters (at $90 each!) or should I make an allowance for the pressure drop of a partially loaded filter say 5 psi, that would mean changing filters before 100 hours if the pressure went over 50.4 psi?

 

What are your thoughts? When should the filter be changed?

  • Re: Fuel Filter And Fuel Pressure.

    by » 4 years ago


    I would say you are on the right track.  For reference, my POH has the caution range starting at 50.8 psi.


  • Re: Fuel Filter And Fuel Pressure.

    by » 4 years ago


    Fuel peressure over 50psi is an indication of a problem.

    After the FIRST 100hr filter change, The schedule calls for additional changes every 200 hours and there is no reason to expect that should not be easy to make.

    If your fuel filters are clogging in less that 100 hours, something is wrong with your fuel supplier.

    Have you had a look at the Coarse Filter before the pumps?

    It is not clear if you are trying to solve a real fuel problem or just asking for where to set the gauge alarm points;  48=Yellow and 50=Red would be reasonable.

    Dont expect to see either of these alarms at any time.  Expect to be changing otherwise perfectly good filters.

     

     


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: Fuel Filter And Fuel Pressure.

    by » 4 years ago


    Thanks Bill, I have no problem with dirty fuel (yet) and the aircraft is yet to fly although I’ve done a few engine runs. I’ll set 48 and 50 for the caution and warning limits. I filter everything with “mr funnel” and I hope to get the 200 hour life after the first 100 hour change. There are places in remote parts of Australia where dirty fuel might be an issue.


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