What happened
The pilot was assigned a specific aircraft to perform banner towing operations. Prior to the flight, he conducted a preflight inspection and started the engine three times for run-up checks. Each time, the engine exhibited rough running and failed to maintain proper RPM during magneto checks. After each occurrence, company maintenance personnel cleaned the spark plugs and released the aircraft for flight.
The pilot test flew the aircraft and reported that it performed normally. He then lowered the banner hook, dove down to engage the banner, and began to climb. During the climb, the engine began to run rough and lost power. The pilot attempted to jettison the banner twice but was unable to do so. He then attempted to land on an intersecting runway but crashed.
The investigation
An examination of the engine revealed that the spark plugs were oil fouled and wet. The cylinders of the engine had just been installed, and there was no record of an engine run-in procedure being performed.
Findings
The lack of a proper run-in procedure for the newly installed cylinders likely allowed oil to bypass the piston rings, leading to spark plug fouling. This mechanical issue caused the engine to lose power during a critical phase of flight.