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  • Re: Third fuel pump on 912 iS engine installations

    by » 6 years ago


    I don't know if it is applicable to your case but I have noticed a good number of aircraft (including high wing) have a header tank between the fuel tanks and the fuel pumps. The return fuel goes back to the header. In this configuration the pumps are guaranteed a good fuel supply and, so, the pressure drop and its attendant problems is reduced. It strikes me that IF you're not getting enough fuel into the supply to the pumps then adding a third pump is not going to make it any better. Of course, if it is vapour lock then this will not help at all.

    BTW, I wasn't trying to be facetious when I suggested leaving both pumps on. There was an article in a recent LAA magazine (UK EAA) which talked about pump operation on the 912iS. The conclusion, while not definite 'advice', seemed to 'suggest' leaving them both on. Not to maintain pressure over a dodgy pump or setup but in order to keep the engine going if the operating one failed. Of course, they should both work properly and provide redundancy and while both on would (maybe) solve your problem it does mask an underlying snag.

    What sort of aircraft do you have and does it have space for a header tank?

    Thank you said by: james porter

  • Re: Third fuel pump on 912 iS engine installations

    by » 6 years ago


    Glenn,
    Thank you for your input.
    My most serious occurrence with vapor lock also involved a refueling stop. "Hot" means different things to different people. My hot is temperatures are at least 100 F degrees on the ground. Living in central Texas I regularly see them during at least 4 month and sometimes more. The auto fuel in Texas can have up to 10 percent ethanol.
    My worst occurrence with vapor lock happened with fuel said to be ethanol free at an airport in Missouri after refueling on a very hot day of at least 100 F degrees.
    I didn't intend for my post to indicate that anything was wrong with my airplane. In my post I hoped to point out that I experienced vaporization of the fuel while using a suspected higher RVP auto fuel at a warmer than normal ambient temperature.
    My question in the post was would adding an additional fuel pump in a cooler (temperature) location have any negative consequences?
    Again, the negative pressure at the pump inlet and the lower air pressure at 8,500' combined with the ambient temperature probably resulted in some vaporization of the fuel being used. Could lowering the ambient temperature by locating the low pressure (at the fuel pump inlet) in the engine compartment to an area inside the passenger cabin (the inlet side of the AUX pump) result in decreasing the vaporization?
    Yes, as I mentioned in my post I switched to AVGAS. Clearly the easiest choice to prevent vaporization.

  • Re: Third fuel pump on 912 iS engine installations

    by » 6 years ago


    Hi James,
    thanks for clarifying. Seems like you need Avgas for a few months of the year or to source non ethanol fuel if possible.

    If the vapor lock is due to low pressure on the suction side than adding a third pump may not solve the problem. However it is an easy experiment put a standard inline bosch automotive pump inline in the cockpit with an on/off switch and try with the ethanol fuel and replicate the problem with the third pump on or off.

    Of course you have already done a lot to decrease temps,

    Increasing the inlet fuel line size will not be a bad thing to do, and perhaps a header tank.

    However in the end better fuel or Avgas for a few months may be the simplest solution.


    I do like the suggestion

    Thank you said by: james porter

  • Re: Third fuel pump on 912 iS engine installations

    by » 6 years ago


    Paul,
    Thank you for your reply.
    I don't have a header tank. I haven't done any research as to why a high wing airplane would need one over say a low wing aircraft.
    My previous airplane was a Cirrus and it had header tanks inboard of the fuel tanks- still in the wings though and not inside the fuselage.
    As I recall the reason was to have a usable supply of fuel to use during maneuvering so as the fuel tank outlet would not become "un-ported"
    I suspect that the same would apply for the high wing design. with low fuel levels and in designs that would "un-port" the tank outlet connection (during medium or steep bank turns or turbulence) it would be desirable to have a header tank with 10 or 15 minutes of uninterrupted fuel supply available.
    However, don't think supply quantity is the problem.
    I suspect that regardless of available supply, the fuel pumps will always have a negative pressure (suction) at the inlet. Fuel pressure (PSI) at the inlet is very low even in a high wing airplane where the gravity flow is a benefit. The pump obviously produces a high pressure (PSI) at the outlet. The result will always be a negative inlet (PSI) at the fuel pump. One way to decrease the inlet pressure at the inlet pump is to add another pump upstream in the system. The AUX pump. But the AUX pump encounters the low pressure at the inlet (suction). You are just passing the problem upstream. However, the variable of temperature can be an advantage since the temperature in the cabin is lower than the engine compartment. Thereby increasing vaporization margins. Not much but some. Also, once fuel is pressurized the vaporization margins increase, around 2.2 degrees F per PSI. Therefore a small pressurization of the system for fuel at the inlet of the main fuel pump would increase vaporization margins.
    Again, there may be too many negative impacts to justify adding an AUX pump.
    It is far simpler to lower under cowling and fuel temperatures as much as possible and switch to a lower RVP fuel.
    Yes the fuel system should be as friction free as possible so that the negative pressure at the fuel pump inlet is as low as the design allows, but again, I suspect that the pumps are designed to run at some negative (PSI) number.
    Regarding leaving both pumps on during all phases of flight, that is up to the operator. The only requirement that I know of is that in the Operators Manual-(and/or specific airframe Operators manual) requiring both be "ON" during takeoff and landing.
    I don't leave both on partly because, depending on which fuel pump fails, I am not sure how I would know if one fails until I attempt the next "run up" before takeoff. I suppose that there would be a drop of 1 to 2 psi , but my engine monitoring is not set up to alert me unless the fuel pressure drops out of the normal range. My instruments would show the pressure but I don't traditionally look at the pressure only if it is in the green.

  • Re: Third fuel pump on 912 iS engine installations

    by » 6 years ago


    Another possible solution...
    I once worked in industry where we needed good fluid pressure downstream of a pump and valves. often when a valve switched or the pump switched mode we would get a short pressure drop which caused problems.

    The solution was a surge protector or bladder accumulator. This was a closed reservoir of 200cc with an internal bladder pressurized to the required pressure or a spring on a diaphragm. What this meant was if the pump pressure fell for a short while the accumulator maintained pressure for up to the rated 200cc of fluid flow. By then the pump/valves had stabilized and the system never suffered from significantly low pressure. One of these downstream of the pumps would maintain pressure and flow when the second pump was turned off.

    Bosch makes these for fuel injection systems, they plumb inline and also dampen pressure pulses. search for "bosch fuel accumulator".

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