Cessna U206 N5229U accident near Van Nuys, California

Probable cause

A partial loss of engine power during cruise flight due to fatigue fractures of two crankshaft gear teeth from corrosion pitting, which resulted in an off-airport forced landing and collision with obstacles on the ground.

NTSB analysis

The pilot reported that the airplane was in cruise flight when the engine started backfiring and he smelled burnt oil; the engine subsequently lost partial power. During the forced landing on a soccer field, the airplane sustained substantial damage when it collided with a fence and trees before coming to rest on its left side. Postaccident examination of the engine revealed that the engine crankshaft would not rotate. A borescope inspection revealed that a valve in the No. 2 cylinder was in contact with a piston face. All of the piston heads exhibited semicircular indentations near the location of the valves. Upon disassembly, six gear teeth on the crankshaft gear were found fractured, and all of the teeth on the camshaft gear were found ground off. A metallurgical examination of the separated crankshaft gear teeth revealed that two of the fracture surfaces exhibited fatigue with areas of corrosion pitting and that the other fracture surfaces exhibited evidence of overload.

Text from the NTSB. Open the NTSB docket for WPR13LA052 for the full record.