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Excessive Fuel Flow Alarm

WingedFrog

Well Known Member
Yesterday on a 1.5 hour cross country flight, I experienced several High Fuel Flow alarms in the 10 Gal/h range. I wanted to share this through the Savyanalysis diagram below.
Note that I am observing a 2 Gal/h fuel flow with engine off but electric fuel pump on. This correspond to the fuel return I guess.
Note also (on the diagram) the high fuel pressure I mentioned in another post. It is due to the new Rotax pump plus the electric pump bringing me to the top range of Rotax acceptable fuel pressure
Last, you will note a correlation between density altitude and Fuel Flow which I cannot explain.
I have not adjusted the k factor for computed fuel usage and I obviously get way too high computed fuel consumption with already 2 Gal/h on top of whatever the engine burns.
Overall on this flight the fuel burn averaged 5 gal/h while the computed fuel burn would average about 60% more. I hope that increasing the k factor by 60% will improve the accuracy of my computed fuel burn. Should I change the fuel flow ranges (yellow-green-red) accordingly to avoid these pesky fuel flow alarms or will the k adjustement solve this problem?


Savyanalysis diagram (zoom on the diagram to get more details):
https://docs.google.com/document/d/1rl7MmQiIkkwIF4OCKY_nLdWx_Flk4r_Pou71YwUtENs/edit?usp=sharing
 
Last edited:
same issue here...

Hi Wingedfrog,
I just experienced the same. Did the K-factor change something?
How did you resolved the 10 gal/h warnings?

Thanks in advance
Godo
 
Hi Wingedfrog,
I just experienced the same. Did the K-factor change something?
How did you resolved the 10 gal/h warnings?

Thanks in advance
Godo

I have never been able to get the computed fuel flow right (I have the Skyview avionics). I believe my fuel flow transducer is not working as any change in the K value has no effect. I live with it by watching my fuel level (I am one of the few believers in the fuel tank windows: 0.5 Gal/window and my fuel consumption is a rock solid 4.5 Gal/h - who needs a computer?). As for the warning it was one of a kind situation, it never reoccurred. The only hint might be that it occurred after I filled my tank with Avgas which I practically never do as I am using only ethanol free gas.
 
Here is my procedure for setting RETRN AND K factors, that I have posted many times in the past, here and on the Dynon forum. It works! My post copied below. Your numbers may vary, but if you follow this procedure you will get very close to the right numbers. Further refinement can be had over time by comparing your actual fuel burn to the computed number.

Dynon Support - Thanks for the reply and the explanation. Armed with that information, I went to the airport today and did the following:

1. Set the K factor back to 68000, the recommnended value for the FT-60.
2. Went into the RETRN field and while running the electric fuel pump, with engine off, noted the fuel flow indication.
3. Began incrementing the high end of the RETRN field. As I did this the indicated fuel flow went down. When the RETRN value reached 130000 (one hundred thirty thousand) the fuel flow indication was down to zero.
4. Ran the engine. I calculated that at 4000 RPM I should see a fuel flow of about 3.5 GPH. While holding 4000 RPM, I adjusted the K factor to achieve an indication of 3.5 GPH. This value turned out to be 79000.
5. I flew the airplane. Indicated fuel fows were very close to what I believe they should be at various power settings.
 
Here is my procedure for setting RETRN AND K factors, that I have posted many times in the past, here and on the Dynon forum. It works! My post copied below. Your numbers may vary, but if you follow this procedure you will get very close to the right numbers. Further refinement can be had over time by comparing your actual fuel burn to the computed number.

Dynon Support - Thanks for the reply and the explanation. Armed with that information, I went to the airport today and did the following:

1. Set the K factor back to 68000, the recommnended value for the FT-60.
2. Went into the RETRN field and while running the electric fuel pump, with engine off, noted the fuel flow indication.
3. Began incrementing the high end of the RETRN field. As I did this the indicated fuel flow went down. When the RETRN value reached 130000 (one hundred thirty thousand) the fuel flow indication was down to zero.
4. Ran the engine. I calculated that at 4000 RPM I should see a fuel flow of about 3.5 GPH. While holding 4000 RPM, I adjusted the K factor to achieve an indication of 3.5 GPH. This value turned out to be 79000.
5. I flew the airplane. Indicated fuel fows were very close to what I believe they should be at various power settings.

John, I checked your kitlog, it looks like you have the Dynon 100 avionics. A few months ago I tried the procedure above and could not find the RETRN field in the Skyview setup, at which point I just gave-up on getting the fuel flow right.
 
Jean-Pierre,

I use 5100 RPM for cruise. After a number of flights I determined my average fuel burn for a flight was 4.5 GPH including taxi, climb, cruise and descent, so I set up cruise power in flight and tweaked the K-factor to get 4.5 GPH. In a typical 3 hour flight my totalizer is within 1/2 Gallon of actual. I didn't worry about the small return flow rate.

Rich
 
John, I checked your kitlog, it looks like you have the Dynon 100 avionics. A few months ago I tried the procedure above and could not find the RETRN field in the Skyview setup, at which point I just gave-up on getting the fuel flow right.

I just checked the Dynon SV manuals and it appears that you are correct. If they eliminated this method of approximating the return flow, then I guess there is no way to get an accurate flow indication with the current RV-12 fuel system configuration, where the sensor is in the main pipe from the tank. Seems odd. I would check with Dynon support.
 
Jean-Pierre,

I use 5100 RPM for cruise. After a number of flights I determined my average fuel burn for a flight was 4.5 GPH including taxi, climb, cruise and descent, so I set up cruise power in flight and tweaked the K-factor to get 4.5 GPH. In a typical 3 hour flight my totalizer is within 1/2 Gallon of actual. I didn't worry about the small return flow rate.

Rich

Rich, could you give me the value of your K-factor?
 
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