Fuel Tanks

Today I worked on calibrating the fuel tank senders. This involves booting the G3X system into Calibration mode, and then running the fuel calibration steps while filling the fuel tanks a few gallons at a time.

I built a stand to place under the tail wheel to keep the airplane in level flight attitude. Because it’s a tail wheel airplane, the fuel level reads differently between level flight and when operating on the ground with the tail down. The stand worked well for getting the plane into level flight attitude.

The first step is to perform the fuel calibration in level flight, then the process is repeated in the ground attitude. The system simply reads a set of datapoints and then computes a curve to interpolate all the in-between values. It’s up to the operator to decide what values to use, and how many points.

I coordinated with the fuel truck driver, and he helped by pumping in specific quantities of fuel while I entered the data points in the cockpit. We started with the right tank, filling in 3 gallon increments, and one final 1 gallon increment to the full 25 gallons. Everything went well, and the process was painless.

When we switched to the left tank, the sender value being read by the G3X was stuck on the same value, even as fuel was added. We stopped the process and I started troubleshooting. I quickly found the sender wasn’t grounding correctly. I was able to fix this by slightly torquing one of the sender screws.

I called the gas truck driver again, and we attempted to complete the process. Unfortunately the sender reading didn’t change when we added fuel, so we stopped again.

At that point I switched focus back to the right tank. I took the tail stand out, and set the tail down on the ground. I was startled to hear a splashing sound, and quickly realized the excess fuel was pouring out the right tank vent. I quickly placed a gas can under the vent and caught some of the overflow. Note: the tail low ground position means a practical limit of fuel capacity of about 24.75 gallons.

Rather than draining the fuel, I realized that I could calibrate the fuel level as I remove fuel, not just adding fuel. I decided to siphon gas out into canisters, as I could transfer the fuel much faster than via the vent opening. It also allowed me to control how much I was removing, which made the calibration process for the second time (the ground configuration) super easy. The gas cans were all 5 gallons, so I set points at 5 gallon increments. I left 5 gallons in the tank because I ran out of gas canisters, and the zero point reading will be the same as the in-flight value.

With that done I had run out of time, so will have to come back to troubleshoot the left tank sender.

I pulled off the right wing fuel tank, installed the fuel sender and other fittings, and then leak tested the tank. I wasn’t able to find any leaks using the soap and water technique, so I reinstalled the tank back on the wing.

Today I leak-tested the left wing fuel tank, and with a passing result, I then reinstalled the tank onto the wing.

I seem to have misplaced my two fuel strainer fittings, so I have ordered two new from Vans. Unfortunately they are on back-order, so I have no idea when they will arrive. Without those fittings I had to resort to filling those holes with something with something else. At first I tried duct tape, but then switched to a latex plug that I have from my powder-coating equipment. It slowly leaked, but that was ok since I was using soap and water to test for leaks, and maintaining pressure wasn’t needed. I just gauged the air pressure by watching the bubbles form around the latex plug. It worked out well, and I didn’t find any leaks anywhere on the tank.

With that done, I wasn’t sure how hard it would be to reinstall the tank onto the wing. I rigged up a strap to help give some support, and I was surprised at how easily it went back into position. I was able to get bolts through the spar and into the outermost and innermost z-brackets with relative ease, then I torqued them up and the tank was in position. From there I located and installed the remaining bolts, final torqued them all to 25 inch pounds (plus 10 for thread friction), and applied thread seal. I left off the lower bolts from each position to allow me to install adel clamps once the wiring runs are complete.

Today I installed the fuel sender in the left wing.

The install was fine, definitely easier with the tank removed, and easier the second time around. I sanded away the primer from the rib, around the area where the sealant would go. Then I test fit the sender and made some minor tweaks to the arm position to stop it from hitting the bottom of the tank, and the top skin too. Then I mixed up the sealant and got it done. I should have added some hole location info before I covered the holes with sealant. I had to fiddle around to locate the screws into their holes, and in the process I messed up what should have been the perfect install. Anyway, it looks ok and I think it’s a good tight seal.

Tonight I dropped the left fuel tank, and then found all the parts I needed to install the fuel senders and run the tank test. It took a while to find everything, despite my fairly organized boxes of parts.

I actually started during the day, Jen gave me a hand to lift the right wing out of the way so I could unscrew all the screws on the top of the left wing. Then later in the evening I was able to remove the bolts holding the baffles to the spar and carefully remove the tank.

With the tank out of way, I was able to get a look at some of the countersinking on the spar. Not that great to be honest. Most of the holes have countersinks down to a knife edge, and the edges are not symmetrical. There are signs of chatter during the countersinking process on some of the smaller holes for the access plates, and some of the holes are misshapen.

Next up I located the fuel senders, the floats, the tank drain plugs, the fuel return plug, and screws for the sender.

Then I took a few minutes to test the fuel senders. These should have a range of 30 to 250 ohms when the arm is moved. Both the left and right senders measured correctly, the right was exactly 30 to 250, and the left was 27 to 275, which is close enough.

Now I need to find some product to remove the thin layer of old proseal on the senders, from when I installed them on the last wing. Once they are cleaned up I’ll install them and can start the tank test. I’m planning to do one tank at a time, just to save working space.