adjusting k factor on GEM III

[TheTurtle]

After a few months of owning the plane Im starting to be more detailed in logging fuel burn.  I used to just fill up whenever it looked like it needed it and I dont do much XC so that was fine.  After resetting the initial fuel to 64gallons last fill up I checked today before topping off.  It said I had 20.9 when I shut down.  They put in 47.6.  By my math thats 68.5 gallons.  So My GEM thought I had 4 gallons that I didnt have.

However last fill up the math said I had 65ish gallons, which I thought was close enough for government work. 4.5 gallons off is a bit to much for my tastes.

This assumes its 64gallons when filled up all the way to the top enough that it overflows when you stick the cap back on. maybe that amount of fullness is an extra 2gallons per side? 

I was reading the GEMIII manual about adjusting the K factor and it looks like its just magic and guess work.  Theres no if you are showing X amount to much reduce K factor by Y. 

anybody have an tips for adjusting it?  Seems like this will take a few fillups to dial in using this point and shoot method.

[carusoam]

Turtle,

Skip the magic and the guess work. The system has an accuracy that you can exploit if you set it up properly.

It would really help if you have the procedure and know some details about your tanks.

The K factor is the adjustment that is used...

Most often people fill the tank to the top.  This requires filling it the same way on the same level surface a few times...

Do you have something near the top of the tank that you can fill to each time?

the basic procedure is to...

1) fill the tank,

2) fly the plane,

3) fill the tank again

4) compare the fuel bill to what the G3 said you used.

5) repeat...

After a while you get pretty good at filling the tank the same way each time.

6) follow the procedure to adjust the K factor.  

7) then test it again...

When you get it right, your fuel used according to the instrument will be within a few tenths of a gallon of the fuel billed.

The fun part is this doesn't take filling all the way up and burning it all the way down... that is more of how fuel level is determined.

Do you have 52 gallon useable tanks? Have you built a stick to stick the tanks with?

How is that?

PP advice only. Not a mechanic.

Best regards,

-a-

Edited March 7, 2017 by carusoam

[kortopates]

Turtle, You are making it more complicated than what it is. First, its important to understand that the K factor has nothing to do with your tank capacity - just treat it as an unknown (since it is, till you measure). But what's important is how much fuel has been added to the tanks versus what your GEM says it burned. But the only way to get good data for how much fuel you have burned versus how much fuel you added is to pick the most repeatable filling method you can. This is easiest if you just use the legal POH definition of "full" which if the bottom of the filler neck that you can see easily. Above that point is room for expansion and although you can easily add more fuel you can't be assured you are always filling it to the same point since air gets trapped inside and it can be difficult to burp it without taking some time. Once you know how to fill it so you can be confident each fill is identical in quantity then you'll be able to do this with minimum error. Your repeatable full point doesn't even need to be official full but it does have to be 100% repeatable by you. 

Next is put a spreadsheet together. Its silly and just adds error if you try to adjust your K factor on 10-20 gallons of fuel. Instead use a spreadsheet to track every time you added fuel and what your GEM recorded as burned via remaining fuel. This way you can re-zero the fuel burned to zero every time you fill it. After you have gone through about 100 gallons you are ready to re-calculate the K factor. Waiting till you have gone through 100+ gallons ensures a 1 gal discrepancy between fills will be only a 1% error. You should be able to fill it back to your identified full mark within 1 gal to get precise data. This is only critical wrt to being identically filled on the first fillup and the last fillup. All the other in between fills can be filled to anywhere you want by anyone. But the last one need to be done identically to the first one, so preferably by you at your home base self serve fuel island where you have the best chance of repeating the same fill point.

FWIW, the JPI procedure has you only changing the K factor by 50% of the difference you re-calculate it each time. They have you do this because pilots tend to introduce a lot of error. If you do it as outlined with a 100 gal or more you should be very accurate as long as you can fill it accurately.

As a separate data collecting exercise, I suggest at some point drain your tank completely, level it, add the amount of un-usable back in and then see how the actual capacity compares to the POH as you fill it and make yourself a calibrated dip stick. Fill it to the the bottom of the filler neck and see how close it comes to your POH capacity. You can then see how much additional fuel you can overfill it by or what it takes to make up any missing capacity.