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Fuel return line

in 912 / 914 Technical Questions 13 years ago
I'm looking for some clarification from Rotax here. Paragraph 14.4.1 of the installation manual makes mention of the orifice required in the fuel return line to the fuel tank. I have highlighted the details in the attached document.

I get people who interpret this info differently.
Interpretation 1: The orifice must be installed if you have a fuel return line in order for the fuel system to operate correctly; and
Interpretation 2: A fuel return line must be installed in order for the fuel system to operate correctly.

The way I read it is as per interpretation 1, that if you have a return line, the orifice must be installed, but a return line is not essential to the system.

Some clarity would be appreciated.

Dave
orifice.gif (You do not have access to download this file.)

  • Re: Fuel return line

    by » 13 years ago

    I'm not sure where you got that it's not essential from this description. I admit, it's not the easiest to read, is it? Rotax does want a return line, and one must use the orifice in that line. The "optional" part, concerns a fuel pressure gauge. If you happen to have a fuel pressure gauge, the line going to that gauge must also have an orifice in it.
      
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  • Re: Fuel return line

    by » 13 years ago

    David,

    I am pretty sure you 912 has a fuel distribution block similar to the one in the schematic I am posting here.

    Part#7: You conect there the Fuel Return Line.

    I you check Part#7 it has a Banjo Bolt Part#9.... This Banjo Bolt#9 has inside a RESTRICTOR that is furnished with a very small orifice (0,35mm = 0.014in) to allow to pass a small amount of fuel to return to the main tank and this also allow to keep fuel presure to the carburators.

    The OPTIONAL part in that schematic will go were part#7,9 goes. You remove the part#7,9 and install there the optional part. The only diiference is that this part has another nipple B to conect a Fuel Presure Gauge. But if you check, it also has a restrictor on the side A in ordert to conect the fuel return line.

    The Fuel Return Line "IS" ESSENTIAL to the system. "Thats why ROTAX recomend it".

    This Fuel Return Line helps relieve some fuel presure in the system and also prevent fuel vapor lock.

    Hope this info help you

    Isaac
    XXXXXFuelReturnLine.JPG (You do not have access to download this file.)
    xxxxDibujo.JPG (You do not have access to download this file.)
      
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  • Re: Fuel return line

    by » 13 years ago

    Thanks Isaac

    Most of are Rotax installations are in homebuilt aircraft and do not use the Rotax fuel distribution block. The system is made up using fuel hose and T-pieces. I would say that 70% do not have the return line and many have run for hundreds of hours without problems. That is why I am querying the ambiguous wording.

    Can you please explain why you need to relieve some fuel pressure? Do Rotax install a mechanical pump that over pressurises their fuel system?

    How does the orifice prevent vapour lock? If there is vapour between the pump and the carb, it will vent into the space above the fuel in the carb bowl, which is in turn vented to the atmosphere. If the vapour is before the pump (which is most likely as this fuel could be at a lower pressure due to the pump sucking the fuel) the orifice does not enter into the equation.

    Dave
      
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  • Re: Fuel return line

    by » 13 years ago

    Thanks Robert.

    Maybe I can explain more with an example of good English.

    First section being queried…

    Original text: “The connection nipple is furnished with an orifice essential for operation of the fuel system.” In this sentence, essential refers to the orifice.

    My alternate test: “The connection nipple, furnished with an orifice, is essential for operation of the fuel system.” In this sentence, the essential word refers to the connection nipple.

    There is a difference.

    Second section being queried…

    Original text: “If the pressure gauge connection is not used and a hose nipple installed, the banjo bolt assembly marked with a colour dot or marked with “FUEL” is furnished with an orifice. This is essential for the operation of the fuel system as it prevents a loss in fuel pressure.”

    There is no doubt here that it is essential to have the orifice. If you don’t install the orifice, you will have a loss in fuel pressure. But I don’t see them saying you must install the return line, just that if you do, it must have the orifice.

    Dave
      
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  • Re: Fuel return line

    by » 13 years ago

    That "optional" piece is intended to be used when you use *both* a fuel return line and a fuel pressure gauge. If you don't have a mechanical fuel gauge, then you don't need that optional part. If you have a fuel pressure gauge and the optional part is used, you would not use parts #6, 7, 9 of the standard assembly. Instead, you use the restrictor that is located in the one end of the "T" of that optional part.

    "Must" you install the return line? Legally it depends on what rules you must follow. If you aircraft is certificated in such a way, where following Rotax's instructions is mandatory, then you do need to install it. If your aircraft is not required to follow Rotax's instructions, then I suppose you don't need it. I personally think it's a good idea to have the fuel return line in the system.

    The bottom line is that if you do have the fuel return line, that line must have the restrictor in it.

    Yes, I agree that sometimes their words are hard to understand. That manual is written by people who's first language is not English.

    The fuel return line's purpose is to prevent vapour lock, by simply keeping a small amount of fuel always moving thru the system. The restrictor's job is to prevent the fuel pressure in the system from getting too low, while still allowing a small amount of fuel to pass thru the system, and be returned back to the fuel tank. Without the restrictor, fuel would flow at max rate, which might be too much for the fuel pump to handle. This would be similar to turning on your garden hose. Unrestricted, water gushes out of the hose, and the water pressure inside the hose is not very high. Now, place your thumb over the hose. Water still flows, but you have reduced the amount of water that flows, and you've raised the pressure inside the hose.
      
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