We had a interesting event while on a test flight on a 69' Champion that just completed a metalized wing conversion. After sitting idle for about a year, waiting for the new wings to arrive. We were all excited to see how the old bird would fly. Instead we were startled to hear loud back fires approximately 800 ft after take off. Fortunately, the aircraft was able to successfully return to the field for a safe landing. Our first thoughts were clogged fuel lines, inefficient venting or fouled plugs. After reading another forum post on a similar event in 2015, we pin pointed our focus on the flex fuel line between the gas-collator and the carburetor where the culprit revealed itself. I thought I would share this experience in the hope it could help someone experiencing something like this or prevent future incidents with a simple inspection. Attached are photos of what was found and checks you can perform for high wing gravity fed systems.
So as you can see from the attached photo of the flex fuel line and a foreign piece of rubber that we were able to blow out. The question is where did it come from since this airplane was regularly flown before and never had a this problem in the past. The answer is it probably happened when a former owner replaced the line with a new 303 tubing. The tool used to swage this hose has a metal bulb that protrudes into the hose and prevents the sharp edge from cutting the rubber tubing. But this flexible fuel line has a 90 degree bend in the metal attachment tube which prevents this bulb's use. So when swaging this piece a little extra lubricant can prevent the metal attachment end from cutting into the 303 tubing. In addition to visually inspecting the tubing prior to installation for any severed rubber. As mentioned earlier, it worked great each flight before until it didn't. That's why we wanted to share this and maybe others might want to check for this condition on the next 100 inspection or Annual inspection.
We took it a step further and did a free flow drain test from the gas tank to the gas-collator as well.
FAR 23.957 for gravity fed high wing aircraft requires the fuel flow rate to exceed engine demand by 150% for take off fuel consumption. So just to be sure we had a good fuel flow rate we tested it. We measured approximate fuel flow from the tank to the gas-collator of 1 gallon in 1 minute or 60 gallons in 1 hour. The Citabria flight manual I have does not give fuel burn for take off but does give a cruise performance burn for the 7BCAA (O-320) of 11.8 GPH at 2700 RPM for comparison. The Lycoming Operators Manual (Throttle Fuel Consumption) for the O-320 shows the fuel consumption at 2700 RPM of 12 GPH to 13.8 GPH that I can determine after converting Lb per hour to Gal per hour. The formula below was used for the flow rate in percent.
60 GPH / T/O GPH (100) = Flow in percent
60/ 12.5 x 100= 480%
480% is well over the 150% minimum and further test flights seemed to resolve the back firing problem.
As a matter of procedure, check the math as well as the formula. Hope this helps, thanks for reading
