Its a Vapour return line
Hi,
I have read this post with interest, as an owner of a 912 UL in my Europa
As I understand it, on my aircraft the return line is a vapour return line. This is to prevent vapour lock in the fuel lines. After the mech fuel pump, the lines to the pair of Bing CV carbs is brought over the top of the engine, and although sleeved in heat protective shield, is clearly prone to heat soak ( especially true on a hot day, with a line up delay)
There is a "T" piece inserted at the highest piont on the fuel line, this then returns vapour to the fuel tank. As there is no vapour separator, to prevent the carbs being bypassed by the fuel, a restriction is included in this return line, that being a spare carb main jet orifice inserted into the return line.
I have never had a vapour lock problen in a 912 engined Europa... and I've flown many hundreds of hours in them, on various different airframes.
I'm sure some small quantity of fuel will come back down the return line, on my aircraft it goes back to the single fuel tank, but I do not know how much this is. Does the OP's twin tank set up have a balance line between the tanks?
I have a pressure gauge fitted, the pressure is more or less constant in flight, as the pump is capable of delivering many more times the volume of fuel than the carbs can use.
I tend to use it more to check the initial operation of the backup electric pump in my preflight checks. I then turn the electric pump off ( having primed the carbs by testing it) and check the fuel pressure returns to zero -via the orifice, I presume- and then check the mechanical fuel pump pressure on starting the engine. As part of my line up checks the electric pump goes back on, and I have now ensured, as far a practicable, continuous fuel delivery for take off.
I hope this is of some help. FlyingApe.