Accident Details
On June 21, 2015, at approximately 1130 central daylight time, a Mooney M20K (registration N1168Z) executed a forced landing in a wheat field located about 5 miles south of Sheboygan County Memorial Airport (KSBM), Wisconsin. The pilot, who was the sole occupant, was not injured. The airplane sustained substantial damage.
The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91. Instrument meteorological conditions were present, and an instrument flight rules flight plan had been filed and activated. The flight originated from KSBM at 1113 with a destination of Elkhart Municipal Airport (KEKM), Indiana.
Shortly after departure, the pilot reported that he was returning to KSBM due to low oil pressure. The airplane subsequently lost engine power, leading to the forced landing in the wheat field five miles south of the airport.
Examination Findings
A Federal Aviation Administration (FAA) airworthiness inspector examined the airplane and reported finding no oil in the crankcase. The inspector also noted that the rear bulkhead was wrinkled and the empennage was twisted.
On July 1, 2015, the inspector examined the engine and turbocharger. The turbocharger was dry and did not appear to be the source of the problem. On August 17, the inspector performed another examination of the engine at a teardown facility in Poplar Grove, Wisconsin. Evidence of pre-ignition in the number 5 cylinder was found, which appeared to have been caused by failed spark plugs. The top and sides of the piston head were eroded, which allowed exhaust gases to pressurize the crankcase and force engine oil out of the breather tube.