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.
