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Fuel Used per G3X Touch on RV-12iS is 10% Low

jcl777

Active Member
I've kept meticulous track of fuel added vs fuel used per the display. Over the course of ~370 gal and 2+ years, I finally noticed that the fuel used per the display to date is almost exactly 10% less than actual.

Has anyone else noticed such a thing? Did you bother correcting it? How?
 
The fix is simple since you have records. It involves adjusting the K-factor in your avionics settings.

Multiply the number of gallons used per the G3X with your current K-factor, then divide that product by the number of gallons you actually burned, per your records. The resulting number is the K-factor you should update the avionics settings to reflect.

This method works for any fuel totalizer and isn’t specific to the G3X.
 
And

The g3x touch allows you to correct it. On the fuel page press menu button. Calibrate fuel button appears. Change the fuel used to actual fuel boarded. Press back button. K factor is updated.

Now for the hook. It will only calibrate within 10%. If you are already at 10%, you will need to go into the configuration mode, calculate a new k factor as described above, and enter it. From this point forward, use the fuel calibrate menu to fine tune it…
 
The caveat here (if I’m not mistaken) is that we have recirculating fuel systems. So, not all measured fuel is actually being burned by the engine, some is returned back to the tank. (This has big advantages too.) The numbers we do see should always be considered approximate; just another data point in the big picture. Some will see more accuracy than others, but I know at least one 12 driver that removed the fuel flow transducer completely due to the inherent inaccuracy of the setup.
 
It is possible to add a second fuel flow transducer, adding it to the return line, and a G3X Touch can deduct one from other to get an accurate figure.
Never heard of anyone doing it.
 
The two-transducer solution for engines with return lines is very common on Continental fuel injected engined aircraft. You could add one to your return line and get more precise calculations. Perhaps the Rotax return flow is so little Vans decided it wasn't worth adding a second transducer. There is also a chance the newer Rotax models have flow built into their ECUs which would feed the G3X. Jus guessing on that one.
 
Location of transducer

Is there a reason that the fuel flow transducer cannot be located after the return line, so that it measures just the fuel flow that is directly to the engine? That is the way that it has been done with the 912ULS installation in my Foxbat and it is accurate.
 
There is also a chance the newer Rotax models have flow built into their ECUs which would feed the G3X.

They do, and the G3X Touch system can easily be configured to read this data instead of using a fuel flow transducer. I don't personally know how accurate the fuel flow data supplied by the Rotax FADEC is, but if it was my airplane I might try it. No wiring changes required and no harm done if you wanted to change it back afterwards.
 
Rotax Fuel Flow

Displaying Fuel Flow on the G3X Touch as transmitted by the Rotax FADEC can be accomplished by setting the Fuel Flow Input Configuration to "Rotax FADEC".

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Please let us know if you have any other questions.

Thanks,

Justin
 
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