Cessna 206 N5152U accident near Prospect, Oregon

Probable cause

The failure of the counterweight retainer pin plate which resulted in a total loss of engine power.

NTSB analysis

About 2.8 hours into the fire reconnaissance mission flight, the pilot heard a loud bang followed by violent shaking of the airplane. The pilot believed the airplane was at an altitude of about 1,500-2,000 ft. above the ground. The pilot retarded the propeller control to extend his glide distance while he looked for a place to land. The airplane touched down in a swamp and during the landing, the left wing and fuselage were substantially damaged.

A postaccident examination of the engine revealed extensive damage to the no. 1 cylinder, piston, and connecting rod. The no. 1 cylinder valve guide rods broke through the crankcase, and the rear counterweight and counterweight lobe were broken free from the crankshaft. Numerous metal pieces and shavings were noted in the oil pan including: a broken counterweight, crankshaft counterweight attachment, counterweight retainer pin, damaged retainer pin plates, damaged retainer plate spring clips, and engine casing pieces. One of the damaged retainer pin plates displayed a deep groove, the approximate diameter of a counterweight pin.

It is likely that the counterweight retaining pin rotated over time on the counterweight retainer pin plate, causing it to wear down and eventually fail. When the plate failed, the counterweight retainer pin moved from its position and impacted the no. 1 piston rod, causing the counterweight lobe to fracture and the engine to lose power.

Text from the NTSB. Open the NTSB docket for WPR19LA217 for the full record.