What aircraft was involved and where did it happen?

The accident on 2020-11-23 involved a aircraft, registration OB-1838, at PE.

What was the probable cause of the accident?

The failure of the connecting rod cap bolts on the first cylinder caused an imbalance of forces that fractured the crankshaft and seized the engine, leading to the loss of power.

Engine failure leads to emergency landing in Peruvian Amazon

A Cessna C-205 experienced a catastrophic engine failure and subsequent forced landing on a riverbank near San Lorenzo, Peru, after internal component failure.

What happened

On November 23, 2020, a Cessna C-205, registration OB-1838, operated by Servicios Aéreos Ruiz E.I.R.L., was performing a scheduled passenger flight from Yurimaguas to San Lorenzo. While cruising approximately 10 nautical miles from the San Lorenzo aerodrome, the pilot in command noticed an abnormal engine noise, a significant drop in RPM, and a loss of oil pressure.

Shortly after these indications, a loud explosion occurred within the engine, resulting in a complete loss of power and the cessation of propeller rotation. The pilot executed emergency procedures and performed a forced landing on a sandy beach along the banks of the Marañón River. All five occupants—the pilot and four passengers—survived the impact without injuries.

The investigation

The investigation focused on the mechanical failure of the Teledyne Continental Motors IO-470-S engine. Metallographic and fractographic analyses of the engine components revealed that the connecting rod cap bolts for cylinder number one had failed. The analysis showed that these bolts contained significant surface defects, including non-metallic inclusions and pitting, characteristic of aged or poorly preserved material.

Following the failure of the upper bolt, the lower bolt attempted to maintain the connection, leading to the deformation of its hexagonal head before eventually being torn away. This caused the connecting rod to detach from the crankshaft, creating asymmetric forces that damaged the crankshaft and caused it to fracture transversely. The resulting internal friction and erratic movement of the powertrain components eventually brought the engine and propeller to a halt.

Findings

The primary cause of the incident was the failure of the connecting rod cap bolts on cylinder number one, which induced asymmetric cyclic loads on the crankshaft, leading to its fracture and the subsequent seizure of the engine.
The bolts, which had been recorded as newly installed, were actually aged, used, or improperly preserved, making them unable to withstand design loads.
There was evidence that the installed bolts may not have had the correct part numbers required for this specific engine model.
The engine's internal components experienced high-energy impacts within the engine cowling due to the erratic movement of the fractured powertrain.