912 is Fuel Selector

Hi Michael

It sounds like you are making things more complex than needed.  For an aircraft with 2 tanks you must always return fuel to the one you are drawing from.  the type of fuel valve is called a duplex fuel selector.  if you select right it returns to the right tank.  If you select left it returns to the left tank.  The alternative is to use a header tank but with the iS you have a very high fuel flow so the tank size is important.  Please watch the videos from Rotax-owner and read the installation manual.  i would use at least a 5 gallon header if you can.  if you do not then heated fuel begins to circulate and this is not good for the fuel system. 

https://www.andair.co.uk/product/duplex-fuel-selector-fs2520-d2/

Cheers

Thanks for the reply.  I understand having to return fuel to the supply tank.  I can't count the times I've watched the Rotax-owners CH750 install  videos.  Our header tank is larger than the SkyTek header tank, so should be adequate. I was hesitant to buy a Duplex fuel selector valve when I don't need the selector feature.  I just need a duplex cutoff valve, but that doesn't seem to exist.  I could take a selector valve and plug one of the inputs on each circuit, but that seems wasteful.

Just re-watched the video and it looks like they use a cut-off valve on the supply line and a check valve on the return line.  I guess that would serve the purpose. 

My goal is to make sure that fuel will not flow into the engine compartment in the case of an emergency.  The return fuel line is my concern, since I had already bought a nice fuel valve.

==

Michael

Hi Michael,

   I have a Sting S4 912IS engine two wing tanks. Only a on/off valve needed/ used. KISS the wing tanks drain into the main tank, effectively a big header. The return goes back to the header, only one return needed. works very well. the fuel valve will stop fuel flowing into the Engine bay as you want.

Glenn,

Do you have a check valve on the return line at the main/header tank? 

Lets say that the return line gets damaged.  What would keep the fuel from draining via the return line?   On our header tank, the return is toward the top, so we'd always have 2.5gallons to use, but I could see it emptying the wing tanks. Looks like Andair sells a check valve for the 912iS just for this purpose.  I'm trying to "keep it simple.....stupid", KISS, but also trying to account for possible failure points.

Thanks,

Michael

Michael,

  the fuel pressure regulator as in the schematic and as is standard on almost all EFI systems regulates the fuel pressure to about 45 PSI. This is also in effect a checkvalve as it closes if pressure is less than 45 Psi.

There are many hoses in modern Automotive EFI systems the aircraft is no different. You cannot design a system that is fool proof in any scenario. I would ask in what way could a well designed and routed high pressure fuel hose ever be cut or fail in an aircraft ? This is far more likely in an off road vehicle but almost never happens. In addition Rotax Paranoia dictates that all these rubber elements are changed every 5 years, again many vehicles last decades with the same fuel lines and no issues. When was the last time you heard of a car stopping because the fuel lines failed ?

I just flew 900 km over the Southern ocean in the roaring 40's and back on such a system and never had a concern about any technical issues with my 912IS or aircraft.