Incident Overview
A banner towing aircraft was being repositioned to another airport when, shortly after takeoff, the engine lost power. The pilot was forced to execute a landing in unsuitable terrain, during which the airplane struck a chain link fence.
Examination Findings
Post-accident examination revealed low compression in the engine, with air bypass present on all exhaust valves and evidence of detonation. The airplane was equipped with three fuel tanks, each holding 18 gallons. Two of the tanks contained uncontaminated fuel, while the third contained fuel to which Marvel Mystery Oil had been added. This fuel was described as pink in color and had an oily feel.
Fuel Additive Investigation
An empty can of Marvel Mystery Oil was found in the operator's trash by the FAA. The manufacturer's recommended mixing ratio for the additive is 1 part additive to 100 parts fuel. The operator, however, had no specific instructions on how or when to add the substance, nor what ratio to use. The pilot, chief pilot, and manager all denied adding the Marvel Mystery Oil to the fuel. Additionally, the manager reported that after interviewing maintenance personnel, they also denied adding the additive.
Conclusion
The investigation identified the engine power loss and subsequent forced landing, but the report does not state a probable cause.