I know several airfraft owners with Rotax iS engines (912 and 915) and G3X Touch panels who are reporting that the fuel calculator is underestimating fuel burn, and therefore overestimating fuel on board. My understanding is the fuel flow is being reported directly by the Rotax to the G3X. Is this correct? Is there any adjustment to compensate for inaccuracy? 

I see this has been posted in the past over in the 912 section, but no answers. 


Anybody here have a remedy?

  • Re: Fuel Calculator inaccuracy

    by » 3 years ago

    The fuel burn that is reported in the CanBus information for the monitor is accurate.  Perhaps it is something in the G3X software that has an issue.  I have seen the information on the Stock Box system and it is exactly correct.  Perhaps it is an error in conversion of the data to the display.  

    Fuel burn/flow is based on the time that the injectors are open.  If you are running excess pressure that perhaps might account for discrepancies is all I can think of.  (the regulator is mechanical which allows for some over/undershoot of pressure in normal running)  What the computer is doing is looking at the duration of opening and it knows even if one stops working.  (it will then double pulse the remaining one in that cylinder to make up the volume required) If this was a systemic problem then it would be reported by all users however that is not occuring.  

    i  would chat with Garmin and see if they have had other complaints.


  • Re: Fuel Calculator inaccuracy

    by » 3 years ago

    Based on just my sphere of contacts as well as multiple posts I've seen on the subject, it's quite a widespread problem. The OP in the post I linked to wrote: 

    "The Rotax engine does not have an actual fuel flow transducer. It sends "theoretical" data based on the fuel injectors pulses and pulse width. Based on the data from Rotax, Garmin simply displays what Rotax sends it."

    This lead me to believe that the problem is on Rotax's side. The fact that "all users" aren't reporting it may be that many are simply unaware of the discrepancy. We started asking many owners of Rotax-powered airplanes if they had looked at the Fuel Used displayed on their G3X Touch fuel calculators and compared that the the quantity indicated on the pump when they refueled. Surprisingly, many were not making this basic comparison. But once they started doing so, they all noticed the same discrepancy.

    I'll shoot an email to Garmin, but at this point it's not clear to me whether the issue is due to Garmin or Rotax.

  • Re: Fuel Calculator inaccuracy

    by » 3 years ago

    Is there not a "K-factor" adjustment within the Garmin system to allow this data to be accurately calibrated? Both EI and JPI offer this capability, as do GRT and Dynon in the experimental world...

  • Re: Fuel Calculator inaccuracy

    by » 3 years ago

    There seems to be some misunderstanding of EFI systems in these discussions.

    Engine developers use injectors that are tested for fule flow accurately (less than 1% error). They do this by checking flows at different opening times and durations referenced to temp, and fuel pressure. So a variable fuel pressure is compensated for by the ECU adjusting the opening times.

    This is done because fuel flow needs to be very accurate in an ICE engine to get efficient burn and power.

    Therefore the ECU counting up the fuel injected is a very accurate way of measuring fuel used, much more accurate than almost all fuel flow transducers.

    Remember you need at least two fuel flow transducers to measure used fuel in an EFI system.

    Most people I have seen making fuel gauges for their aircraft, eg making sticks and marking them are not very acurate. We do not take into account temperature and pressure that can make additional errors.

    For instance the difference of say 20 degrees C can change the volume by 24ml/Litre 2.4%.

    Then when we fly fuel vapour pressure and vent tubes means we loose some, particularily when high and fast.

    Being a boring old scientist I spent a full day with calibrated measuring cylinders a plane on a flat surface checked by a spirit level and as close to a 15 degree C day as I could get

    I am planning a few "long flights" where I will burn about 200L. I have practiced these and have found the G3X to agree with my dip stick within 3-4%.

    I suspect the error is MINE not the G3X.

    Therefore to go against the flow , pardon the pun. I think the first step is for anyone having significant errors to accurately recheck your own fuel quantity measuring system, your vents etc.

    Sorry to disagree !







  • Re: Fuel Calculator inaccuracy

    by » 3 years ago

    James N Parker wrote:

    Is there not a "K-factor" adjustment within the Garmin system to allow this data to be accurately calibrated? Both EI and JPI offer this capability, as do GRT and Dynon in the experimental world...

    There is such an adjustment within the Garmin system, when using a tranducer. On the Rotax iS engines, where the engine itself sends the FF data, I don't believe it can be adjusted.

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