How to figure out k factor safely

To finely tune your k factor for fuel flow, what is the safest way to do it? I'm not too thrilled about running a tank dry in flight. Is there another way?

Tell your totalizer that the tank is full then fly for a while (more is better for accuracy). Then fill the tanks back up and compare how much you out in to how much the totalizer said it should take. Then you can adjust from there.

Airport fuel pumps are calibrated by law, so there is no need to drain a tank to empty, just fill to a known point (fuel tab) or to full, go fly, come back and refuel.

You should have an fairly accurate consumption rate for that flight.

You can then compare this against what the fuel computer is telling you and then calculate the K factor.

Do this over enough flights and you can develop an accurate value.

To calculate K factor;

(computed fuel used / actual fuel used) * your current K factor.

Use this new value and do it again until you reach a resolution you are happy with, in other words your computed fuel consumption matches what you have put in at the pump.

Cheers

The Dynon Skyview Installation manual has a good section on how to fine tune this. I just went through it last night and totalled up all of my fuel receipts and notes on what the computer thought vs. what I actually put in the tanks. I ended up bumping my K to 70500 from 68000. I will revisit this in 6 months and do it again. Right now it is pretty close. Usually less than a gallon off. I'd like to see how close I can get it to perfect, but with a fuel purge set up, I can circulate fuel and have it falsely counted by the computer. Although I doubt if purging for 30 seconds would have a huge effect.

carrollcw said:

To finely tune your k factor for fuel flow, what is the safest way to do it? I'm not too thrilled about running a tank dry in flight. Is there another way?

Click to expand...

Not sure about the FII pumps but AFP (standard RV FI pump) specifically recommends against running a tank dry as the pump may have trouble repriming itself. I'm not sure I would trust the FII or the facet pumps to do any better.

carrollcw said:

To finely tune your k factor for fuel flow, what is the safest way to do it? I'm not too thrilled about running a tank dry in flight. Is there another way?

Click to expand...

So the way I did it was to start with a full tank of fuel, fly (cross countries are good or several local flights will work) until fuel level is low, fill up and note the fuel added and fuel used (according to your fuel flow totalizer). Here is the formula:

New K Factor = (Fuel Used X Old K Factor)/Fuel Added

I started with the default 68,000 and ended up with a calibrated K factor of around 71,000. I kept a log for a while and did several re-calculations of the K factor until I found the average was around 71,000 for my airplane.

Is this for the Red Cube? I never had to fine tune mine - it was great out of the box. Always within a litre or so. If it is for a different sensor then disregard.

Not a red cube, it is for the EFII. With EFII, you buy a special box that converts the fuel injector pulses into a K factor that you tell the garmin is a red box or flow scan. From there, you modify the K factor to get the consumption right. I think I am getting close based on the help in this thread. I am topping off one tank, flying for 5-10 gallons out of that tank, then topping off again and comparing the readings.

The more you use the better your numbers, a good 3 hour cross country where you use say 25-30 gal will give you a better calibration than a 10 gal re-fill.

What I said in my first reply and what Walt said will help. Burn more between flip-ups for a more accurate comparison. It should work exactly the same with the EFII system as with red cube.

As Walt says, the more fuel you use in calculating flow, the better, and multiple fills is way better than basing your calculation on a single fill, no matter how empty your tanks are. There will always be some variance in how much gas you pump in topping your tanks, but it averages out over a series of fills. When I calibrated my k-factor I probably did 10 fills before sitting down to calculate the variation between actual burn and my fuel flow gauge. Based on that calculation I made a single adjustment in k-factor and it's been very accurate since.