Accident Details
A pilot reported that the engine suddenly began to run rough and backfire during a pipeline patrol flight at an altitude of 300 feet above ground level. The roughness worsened, and the pilot executed a forced landing onto a dirt road. During the landing rollout, one of the wings caught vegetation bordering the road, causing the airplane to veer off the road and into a ditch.
Findings
Examination of the airplane revealed that it was supplemental type certificated for the use of automotive gasoline, and that it had been fueled with auto gas. The engine, a Lycoming O-290-D2, was examined. The #4 exhaust valve was found stuck open, with heavy carbon deposits present on the valve. Additionally, the valves in the #3 cylinder were found to be burned.
Manufacturer Information
Lycoming does not recommend the use of automotive gasoline in Lycoming aircraft engines. According to Service Letter L199, gums can form in auto fuels which can cause sticking valves. FAA Advisory Circular 91.33A contains several cautions on the use of auto fuel concerning quality control, gum formation, volatility, and carburetor icing.