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Mechanical pump back-flow valve?

in 912 / 914 Technical Questions 7 years ago
Can anyone verify that the mechanical fuel pump on the 912ULS has a built in back-flow check valve?

The fuel system design of a particular aircraft tees the electric boost pump output directly into the line that feeds the carbs - which is the pressurized side of the mechanical pump. So, it is important to understand if the mechanical pump is capable of allowing back-flow if it should fail.

I know this config is a little different than Rotax factory recommendations, that's not at issue. I'm asking from a purely technical perspective.

Thanks!

  • Re: Mechanical pump back-flow valve?

    by » 7 years ago

    "The fuel system design of a particular aircraft tees the electric boost pump output directly into the line that feeds the carbs "
    I haven't seen one like this. It's always plumbed into the mechanical pump inlet side hose upstream somewhere. How about can you move its tee in point?

    It should not backflow, but I'm not sure it won't leak bacik? I guess you could call it a floating check valve that moves to allow fuel in then shuts off when the fuel pumps out so it can build pressure and the fuel doesn't just move back and forth.

    What do you other guys think??????

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

      
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  • Re: Mechanical pump back-flow valve?

    by » 7 years ago

    I believe, although speculative, the design goal was to create a redundant parallel fuel supply path. For instance, if the mechanical pump had a perforated diaphragm, the aux pump would not pump additional fuel overboard. I don't think that is necessarily a bad idea or config...but that is all predicated on the factory pump not allowing back-flow....hence the question.

    I'd really like to know for sure if it does or not. Anyone know for sure?
      
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  • Re: Mechanical pump back-flow valve?

    by » 7 years ago

    The mechanical pump is designed as two, one-way valves with a variable volume between them.
    As the diaphragm moves, the volume changes.
    One of the valves allows fuel to only enter from one side and the other valve only allows fuel to exit out the other.

    So, assuming the Mechanical pump is operating correctly, the flow through the pump should only be one way.
    And any backflow will be very minimal.

    There are two failure modes for the mechanical pump.
    Either the diaphragm becomes punctured and leaks, or the or the one-way valves fail.
    The good news is that both valves would need to fail at the same time to allow backflow.
    The bad news is that if the diaphragm leaks, the mechanical pump keeps pumping the same amount of fuel overboard despite the aux pump operating in bypass mode.
    It will keep the engine running but you are still going to run out of fuel just as soon.

    The Aux pump is normally installed in Series with the Mechanical pump.
    The Aux pump can pump through the Mech pump's valves without issue.
    The ruptured diaphragm failure produces the same result either way.

    The advantage is that an Aux pump installed in series and low on the firewall can rapidly purge the system of any air or vapor lock conditions in a few short seconds.

    When a parallel installed Fump as you have is energized, will require the engine to deal with twice the fuel volume it is expecting causing the fuel pressure to increase.

    Series pumps just share the pumping load, deliver the same amount of fuel at the same pressure with the Aux pump On or OFF.
    The mechanical pump essentially goes to sleep when the Aux pump goes to work.

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

    Thank you said by: James Redmon
      
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  • Re: Mechanical pump back-flow valve?

    by » 7 years ago

    That certainly jives with the drawing in the heavy maintenance manual. Good explanation, Thank you.

    Yeah, a broken diaphragm would leak out regardless, but at least the aux pump isn't adding to the flow rate in this design. OK, unless someone has some other explanation, I think I'm going to leave this setup as-is and not bother with an additional check-valve.

    Thanks again!
      
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  • Re: Mechanical pump back-flow valve?

    by » 7 years ago

    In a pump with a leaking diaphragm, the pump will continue to supply fuel to the engine at nominal pressure.
    Additional fuel will be lost to the leak.

    With an Aux pump in series and before the mechanical pump, the Aux Pump essentially delivers pre-pressurized fuel to the Mechanical pump.
    The system pressure remains unchanged so the Diaphragm leaks at the same rate.
    Then again, the Mechanical pump's diaphragm will not flex as much to maintain the pressure.
    There is a 50/50 chance that the diaphragm leak rate will remain the same or it may decrease with the Aux pump engaged. Do you feel lucky?

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

      
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