Fuel flow vs fuel consumption

Eric Page wrote:

For current fuel flow (instantaneous LPH or GPH), that's correct, at least for Dynon systems.  However, the calculated fuel used (and therefore fuel remaining) can be corrected to very near actual.  The Dynon installation manual says the following (page 7-37):

"If you have not installed physical fuel flow sensors (Dynon Avionics P/N 100403-003; Electronics International FT-60 “Red Cube”), the fuel flow information from 912 iS ECU used by the SkyView Fuel Computer can be adjusted if necessary: SETUP MENU > HARDWARE CALIBRATION > EMS CALIBRATION > FUEL FLOW CALIBRATION > PUL/GAL (C37 P14) > (value). The default value is 68000. Higher numbers will reduce reported fuel flow; lower numbers will increase reported fuel flow. For example, if the reported fuel flow is 10% low (determined by careful records of fuel fills and fuel used), set the value of C37 P14 to 61818 (68000 – 10%). Changing this value will not alter the displayed fuel flow; it will only alter the fuel used calculated by the SkyView Fuel Computer."

I'm pretty sure Garmin and others offer similar functionality.

Nope, Garmin does not allow us to adjust fuel used / fuel remaining on Rotax iS engines. The only adjustment is K-factor, which is only if a fuel flow transducer is installed. 

All...

For the fuel injected engines we need to understand that the fuel flow and fuel burn are indeed not the same thing.  The ECU uses a calculation based on how long the injector is open and it is accurate within some reasonable limits.  Typically engines that are run past the ECO limit (approximately 97% throttle position) obviously will use a lot more fuel.  Indeed even something as simple as the fuel type can alter the fuel used.  The only accurate way is to have a fuel gauge in the system like most aircraft for fuel remaining.  Fuel pressure variables account for some inaccuracy also.  Let us note that the fuel pressure regulator is mechanical in nature and not electronic.  (there will always be over and undershoot pressures)  To measure fuel flow correctly, as has been noted by some comments, must be done by using sensors that match the instrument you are using.   In your setup phase work with your instrument supplier there may be adjustments to get accurate readings.  You can't use a calculated fuel burn value to determine an accurate reading of fuel remaining.   

We posed this question to some very smart people, Micheal Stock of Stock Flight Systems who was the developer of the original instrument used on the type 912 iS engines.  He said that on his aircraft the readings were very accurate.  In looking at what was different it turns out that the rates are accurate only if all the details are exactly the same as noted.  Fuel type, fuel system setup (line size, filters, pump pressures, etc) and a dozen other parts that go into each aircraft.  The calculated burn is very much dependent on prop matching to the engine also.  The primary work was done on all aircraft using constant speed propellers for example.  

I think that the takeaway for all of us is this is a calculated burn and not measured.  If you measure your flows you need to calibrate that part to account for the variables. Just my view on it all. 

Cheers

Of course aircraft should have fuel quantity indicators - that's beside the point. The point is the Rotax ECU is providing erroneous information. 

Everyone with experience flying aircraft with iS engines is reporting 5-10% MORE burn than what the ECU is reporting. I haven't heard of anyone reporting less. It is at best marketing puffery; claiming lower fuel burn than what it is in reality. At worse it's an outright lie, reminiscent of VW intentionally setting up their vehicles to deceive emission tests. This could be a major liability for Rotax.

This is should be an easy fix for Rotax: push out a firmware update which adjusts the numbers to closer match the reality we're all experiencing, or provide a method for us to do the adjustment.

I will seek help from Garmin on this as well, pushing them to provide an adjustment method as Dynon already does. But really, this should be Rotax's problem to fix. I have been a huge public cheerleader for Rotax aircraft engines, but they need to do the right thing on this one.