MGL Avionics User Forum

Hello all,
I'm in the latter stages of a re-build and I'm programming my EMS (xtreme).
I have a Rotax 912 with two redcubes, one on the flow and one on the return, I'm sure there's a way to get it to calculate the difference to show a 'true' fuel usage, but I didn't see anything in the menu system which would be obvious, I did read the manual but I was no wiser.
Now to my actual issue, while trying to calibrate my two fuel tanks, one front and one rear the number that I get doesn't change, it was something like 3680, 11 litres later still 3680 and repeat.
So I tried flipping the dip switch, same.
The senders are WEMA S5 european 0-190 ohms high resolutions.
The sender in the rear with no fuel in shows 12.3v at the RDAC.
The sender in the front with 22 litres reads 10.6v at the RDAC.

Does anyone out there have an idea what I should do to get a good reading at the EMS? Again I have read the manual but it kinda wanders past the issue, which it will as there is such a massive scope of senders available...

Any help will be gratefully received,
Thanks,
Tim.

Well this is a tricky one and I have never seen anyone getting this right. I am assuming a standard 912 carburetor installation.
For this you are supposed to use a 0.4mm orifice at the highest point of your fuel system in the fuel return line. In other words - it's not a return line in the traditional sense where the return flow is very high and just a little bit less than the total flow.
This arrangements only purpose is to prevent vapor lock in your fuel line. The actual amount of fuel being shunted into the return line is very little compared to the fuel the engine actually uses.
So what we do here is we just calibrate the flow using the k-factor to cancel out the negligible return flow.
This means only a single flow sender is used.

Your main challenge is the fuel pump - if its a newer engine with the Italian pump it does not have any form of damper so the fuel pressure after the pump varies dramatically with every stroke of the pump. Since you install the flow sender somewhere after the pump and before the splitter to the two carbs you have a problem - the flow sender will have great difficulty measuring the flow as it is pulsing. Worse - if you have flexible fuel lines after the sender - the lines inflate with fuel pressure at every stroke and push the fuel not taken by the carbs back through the sender - there will be some return leakage through the pump. The only way to avoid this is to install a flexible reservoir BEFORE the sender and after the pump to take away the large pressure fluctuations. A very good one, but not exactly "aircrafty" is a simple rubber primer bulb as used with outboard engine fuel tanks (installed in the correct direction of course). The bulb holds a fair amount of fuel and the rubber provides the damping function very nicely.

The original Pierburg fuel pump was very good, had a built in damper and did not have these issues. But sadly Rotax may no longer install these.