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Fuel starvation after electrical failure?

in 912 / 914 Technical Questions 6 years ago

  • Re: Fuel starvation after electrical failure?

    by » 6 years ago

    Dear Rob,

    Thank you. I checked my airplane and it seems that it is correctly wired. Nevertheless I'm going to double check it with the information from the installation manual.

    Kind Regards, Renz
      
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  • Re: Fuel starvation after electrical failure?

    by » 6 years ago

    Hi
    Question to Roger.
    I´ve noted that the pumps are in a series configuration. Is this the correct config? If you have a parallel config with separated power sources could be better?
      
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  • Re: Fuel starvation after electrical failure?

    by » 6 years ago

    You know that the engine runs with either one of the pumps operating singly.
    This confirms that the first pump can push fuel through the second pump,
    and the second pump can draw fuel through the first.
    Pumps in series are preferred because with both pumps operating, the flow rate remains the same and will not overwhelm the pressure regulator.
    The pressure is capable of being doubled but the pressure regulator just dumps the excess fuel back to the fuel tanks.
    Never return fuel to an unvented header tank as any air or vapors will just be recycled.

    Turning OFF both pumps with the power switches WILL kill the engine.
    The redundancy is that each pump should be separately power sourced.
    Blowing one fuse or the failure of one switch should NOT kill the engine.
    If you lose the alternator and then subsequently lose the battery, that will end the flight. Fact of life!

    You have a 914 in a Low Wing aircraft.
    If it was a high wing aircraft the engine might still idle with both pumps OFF.
    The carburetors are at vacuum levels at idle so gravity fed fuel would still allow fuel to flow.
    Unfortunately, gravity does not work uphill as in your case.

    With the turbo operating at full throttle and ~40" of MAP pressure, the carbs are at ~+5PSI above ambient. (Boost!)
    Engine driven fuel pumps and Garden variety electric fuel pumps only have 2-5 psi of pressure and would not be able to push fuel into a turbo engine. It might idle but it would not throttle up.

    Your pumps operate at ~15psi.
    The pressure regulator makes the fuel pressure at the engine equal to the air box pressure plus ~4 psi.
    At idle it might be about 2psi. At WOT it will be closer to 9psi.

    It sounds like your plane is plumbed correctly.
    And it is expected that the engine will not operate without at least one of the pumps operating at all times.
    Just as you check that the engine will operate on each ignition singly before the flight;
    Checking that both fuel pumps are operable singly is just as important.

    If there wasn't any risk, flying wouldn't be half as exciting. ;)

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

    Thank you said by: Renz van der Toorn
      
    Report

  • Re: Fuel starvation after electrical failure?

    by » 6 years ago

    As Bill said.
    In series and individual power sources.

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

    Thank you said by: Renz van der Toorn
      
    Report

  • Re: Fuel starvation after electrical failure?

    by » 6 years ago

    Here's a link to the best discussion I have seen regarding 914 fuel pumps configuration:

    http://contrails.free.fr/engine_pierburg_en.php
    Thank you said by: Renz van der Toorn
      
    Report
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