916 Fuel Flow

Good timely topic!

Full disclosure don't really follow the technical jargon so well - so make your response in simple "man in the street language" - Thanks.

I am using a Dynon Skyview, reading two cubes. One cube on main fuel supply to carbs and second on fuel return line.

My limited understanding of the Dynon adjustment, is that the second cube (return line) will somehow automatically subtract its reading from the total, thereby giving an inflight/ real time fuel consumption reading L/Hr.

I know how to adjust the Dynon (number of pulses).

My last aircraft had just one fuel flow sensor on the main fuel line. It was quite easy to get an accurate in flight fuel flow and total fuel used fuel remaining reading.

Dynon recomend a start number (pulses) for Rotax 912 ULS, which I used for both cubes (from aged memory 70000?)

Over a series of flights, I adjusted the main fuel cube (pulses) to reflect the total fuel consumed (measured by how much fuel to refill).

Total fuel consumed and fuel remaining are now quite accurate, however real time fuel flow makes no sense to me. It's consistently about 7 L/hr too high.

Should I be adjust the return line cube pulses?

If so, will this put the main line fuel adjustment out ie need further adjustment?😈

I also have a plane with a Rotax 916 Fadec engine. The G3X value reported for Fuel used is off by 12.7% over the last three months (353.6 gallons reported as used, 398.4 gallons actual fuel pumped). I rely on my analog fuel gauges which are pretty accurate, but there should be a way to adjust the value reported to be more accurate, just like I have had on previous planes. My “FOD” reported by the G3X is always off which can cause confusion when flying close to the max range. 

My numbers show the fuel flow/fuel used reported by the G3X is low by 12.7% (353.6 reported used, 398.4 actual fuel pumped). From reading this and other threads, this is a known issue. 

I don’t understand why Garmin doesn’t provide an adjustment factor that the owner can input as a configuration input. Yes, the adjustment factor could be slightly off, but I know I can get it closer than being off now by 12.7% low! I would think that reporting values that are low by 12.7% is a risk factor to pilots. 

Doesn't the G3X have a key factor adjustment in its menu? I thought it did. It says it does  online.

Hi Roger,

The G3X does have a K-factor adjustment for instruments where there is a flow sensor wired directly to the Engine Monitor Module, but not for values reported from the FADEC.

K-factor is a calibration constant that defines the relationship between a flow meter’s output pulses and the actual flow rate of a fluid. Since the 916 fuel uses is a calculation by the ECU to determine fuel flow (and not a flow meter) K-Factor does not apply.  What would work is if Garmin had a provision to tune the reading by increasing or decreasing the flow rated provided by the FADEC by a selectable percentage. It might not be totally accurate, but at least you could then set it to a conservative number rather than an optimistic one - which it seems to be.