What happened
On August 23, 2019, a private Cessna 170-B, registration LV-FNO, was performing a general aviation ferry flight from Tres Arroyos to Necochea. Approximately 25 minutes into the flight, while in the cruise phase, the engine began to vibrate violently and experienced a loss of oil pressure. In response to the mechanical failure, the pilot executed an emergency landing on unprepared terrain in a field near San Cayetano, Buenos Aires. The pilot escaped the aircraft without injuries, though the engine sustained significant damage.
The investigation
The investigation focused on the mechanical failure of the Continental O-300-A engine. Upon inspection, investigators found damage to the engine block near cylinder number one, which was covered in oil. A subsequent teardown revealed that the connecting rod for cylinder number one had been damaged due to the failure of a connecting rod bolt.
Laboratory analysis of the bolt identified a fracture at the junction between the rod body and the head. Microscopic examination revealed "beach marks," which are definitive evidence of a crack that progressed through the component over time due to cyclic loading. No evidence of corrosion or overheating was found. Additionally, the investigation reviewed the pilot's credentials and the aircraft's regulatory compliance, noting that the pilot's medical certificate had been expired since May 2017 and the aircraft lacked a required Emergency Locator Transmitter (ELT).
Findings
- The engine failure was caused by the fracture of a connecting rod bolt due to progressive fatigue cracking.
- The crack originated from stress concentrations likely caused by the long service life of the component.
- The failure was not detectable through standard visual inspections or the existing maintenance program, which focuses on internal cylinder surfaces and oil filter debris.
- The aircraft was not in compliance with regulatory equipment requirements as it lacked an ELT.
- The pilot was operating with an expired medical certificate.