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Fuel pumps

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  • Fuel pumps

    Rotax motors generally run two fuel pumps. The 912 has one mechanical and one electric, the 914 has two electric. There is some discussion at the moment whether both pumps should always be on while flying or as the flight manual says, both on for takeoff and landing only.
    Both sides of the argument have merit, be interested what you fellas think about it?

  • #2
    I don't have a Rotax but a 6cyl Jab. I fly with the electric pump on (manual says okay to to that). Some aircraft have experienced fuel vaporisation in the heat. There is no additional cost for running them, they are robust and is a back up in the event the mechanical pump fails. Having said that, plenty of people don't use it and don't have an issue. What does Rotax recommend?


    • #3
      The rotax issue is that with both pumps on in the non turbo'd engines the total fuel pressure can overcome the needle valve in the carby and cause flooding. The old argument for the Subarus was that having both pumps on just tired out out both pumps at the same rate with the possibility that when you needed the spare it would fail as well (most unlikely in my opinion) Never the less unless you are spending all of your time monitoring the fuel pressure if it stops suddenly the first indication will be engine stoppage in which case part of your immediate action will be to activate the spare, check pressure and commence a restart if you still have the height.



      • #4
        Mechanical diaphragm fuel pumps have a pressure regulator spring built in them. When they reach a certain pressure they effectively stop pumping until a pressure drop demands it. Although they are plumbed in series which should increase the pressure, the mechanical pump cannot add to the pressure because of they way it works. Feeding a Rotax mechanical pump with a low pressure (approx 3.5psi) electric pump will not increase fuel pressure for this reason. If however you have too high of a pressure electric pump you will run into problems with the carbies flooding.

        I have a 3.5psi electric fuel pump that feeds the mechanical Rotax one on my 912 uls. As soon as I turn the key on comes the pump, no switch. There is no bypass and check valve as my electric fuel pump allows fuel to be drawn through it when it is off, verified on each service by pulling a fuse and running the engine for some time. I have had 500hrs of trouble free service with this setup. This is not the Rotax recommended setup but just sharing my experience.

        The 914 has two electric pumps in parallel which effectively doubles the flow rate but not pressure.

        The question is like catch 22. More fuel pumps the better but then there are more joins and hose clamps to fail. A single mechanical diaphram pump is nearly bullet proof. Each aircraft would have to be evaluated according to the fuel pressure with both pumps on. If it was too high I would only use as directed in the POH or in a emergency but if the pressures were in check I would use them both at all times.


        • #5
          for those that know I have twin SU carby's. and too high a fuel pump pressure was always a problem and still be able to give the flow at full noise. eventually [ after multiple pump purchases ] I scored some from NZ. can't believe I printed that. these pumps manufactured for the vintage car market answered all my problems.
          they are an electric pump for low pressure for low pressure carbs. rated at just .5 to 2.5 psi. [ yes point 5 psi.] @ 33lt minute each. i have 2 in parallel. very smooth , can't hear them pulse or feel them pulse. so I put a low pressure gauge in the line [ 0 to 15psi. ] and yes they run at about just 1 psi. looking at the pressure gauge at start up is part of my pre-flight. never had a problem since. strongly recommended to solve problems.


          • #6
            its not only the fact that both pumps were running at the same time, arrr I don't know, but 1 of the failures was I believe blocked filters? so if only 1 pump was being used the only filter would have got him home? maybe? maybe not? hell I done a touch over 3000hrs in 3 different machines, most of the time below 300' , and apart from the 914 machine(only running 1 pump) I never even switched on the electric pump, taking off from some pretty ordinary places too mind you, but I ALWAYS checked the filters at service time, which in most cases was every week. I never had an engine go to sleep, must have been just lucky huh.
            In the 2 seater training, I would throw the duty filter and move the stand by to duty and new in stand by. but anyway do what tickles ya fancy, its your machines not mine.


            • #7
              Yep, I hardly ever run my Electric pump on the 912 during flight, (probably count the times on one hand) If it's been sitting for awhile I'll use the electric one to pump fuel up into the bowl, I figure if they were that worried about the mechanical one being unreliable they wouldn't have it on there....


              • #8
                Bones, I couldn't agree more. I only run the electric pump because your buddy in the center of Oz. He said that having an electric pump below the fuel tank was a good idea to combat fuel vapourisation when parked with a hot engine and/or a hot day. It would push fresh cool fuel to the carbs rather than vapours and could save a engine failure on take off. I have absolutely no dramas running just a mechanical pump on a 912 or one good electric one on a 914.