A

ao.frog

Well Known Member
AFS 3500: which K-factor to use for FF when display is set for litres?(Tread update)

Hi.

I have a 3500 EFIS in my RV-7 and I've noticed that the fuel flow probably is showing alittle too high.

Reason is that when I refuel and fill the tanks completely, the amount of fuel going into the tanks are always LESS that the "fuel used" number on the EFIS.
For example:
"Fuel used" showing 120 litres and the tanks take on 110 litres.
The difference is usually beetween 5 and 15 litres on a 100 litres refuel.
By the way: the FF-reading itself are very smooth and stable, so this is not due to the location of the FF-sensor. (the red cube)

I checked the K-factor and it's set at 180 (it was delivered to me this way I think?)
Can I adjust the K-factor alittle so to get the "fuel used" numbers more accurate?
If so; how much? What does the number "180" mean really?

The fuelgauges are usually within 1-5 litres accuracy when refueling so the error is in the FF I think.

I think the K-factor is factory-set at 680? At least when using USGLS for the FF?

Can those of you using litres for the FF-display check your K-factors so I can compare?

It'd be nice to have the FF-readings to be as accurate as the fuelgauges, because then the "fuel rem at dest", "hrs rem" etc etc would be more accurate too.

Lastly I like to add that I'm VERY happy that I choose AFS! What a great product and company!
And I've already decided to install a 4500 in my second RV-7... :)

Thanks
 
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Olaf, it explains how to adjust the K-factor in the manual. In v6.6 its on p36, but that might be an old version. Just go to engine monitor, look for fuel flow and then for calibration. Wont take long.
 
yes but....

I've seen that part of the manual, but the manual is talking about calibration for USGLS and a K-factor at 680.

Since mine is 180, I was thinking that was a huge difference so maybe the litres setup has something to do with it?

I was hoping that someone who's using litres could check what their K-factor is and chime in?

Thanks.
 
Olaf, I am 99.9% sure that gallons/ litres etc are nothing to do with the K-factor. I am in litres and am sure I id not change it. The K factor is the realtionship between the flow and the internal working of the unit. The gallons/ litres etc are just a conversion on the output to the screen. (I am again 99.9% sure.) I would just do what the manual says though I will check my number for oyu.
 
The K-factor is indeed different. If you were to switch the units to US Gallons, it should go back to 680. 180 is correct for liters. Please see our manual, available on our website, for the calibration procedure. Many installations work just fine with the 680 K-factor, but some do need to be slightly modified. We've provided a mathematical equation to make this adjustment.

Trevor(at)advanced-flight-systems.com
 
Ahhhh....

Thanks alot for chiming in Trevor! And that even on a Sunday! I'm impressed....!
Now it all makes sense to me: the K-factor is 180 for litres and 680 for US GLS.

1 US GLS is 3,7854 litres so if I multiply 180 with that, I get 681,372 .... Voila!
So I guess the 3500 by itself changes the K-factor automaticly when I switch beetween litres and US GLS. What a smart little machine!

Now, why dind't I think of that?? yeah; I know... I'm not smart enough.. :D

Anyway; now, with my mind at ease, I can adjust the K-factor and get the FF-readings to be as accurate as the tank-readings.
You gotta love these smart EFIS'es...;) And this forum too!
 
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A follow-up...

Just wanted to post a follow-up on the tread above:

A while after this tread was written, I adjusted the K-factor from 180 (the std setting for litres) to 202.
That was the K-factor I figured out after filling a few tanks to full and calculating the new K-factor as described in the AFS-manual:

New K = Old K x indicated fuel used / actual fuel used

With this new K-factor, the fuel-calculations on the AFS-3500 became more accurate.

Since then, I've recorded the amount of fuel needed when I've refueled to full tanks, a thing I rarely do (fly with full tanks I mean)

Last week, I re-calculated the K-factor, and my new one is 196.
This schould make the fuel calculations on the 3500 even more accurate.

For those of you using something other than litres, you can adjust your K-factor the same way, even though the K-factor value is different (It's 680 for US GAL for example)

So... this is just an update if any of you need more accurate fuel calculations than you get with the std K-factor.

NB: you might not need to adjust it. This depends on your installation.
 
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