What happened
On August 6, 2017, a Neiva EMB-720D, registration PT-RPU, was conducting a local flight departing from Aeródromo Novo Campo in Boca do Acre, Brazil. Approximately fifteen minutes into the flight, the pilot experienced abnormal engine vibrations that intensified until an audible bang occurred. This was immediately followed by a significant oil leak that coated the aircraft's windshield. Realizing that level flight was no longer sustainable and unable to reach the departure runway, the pilot executed an emergency landing on the banks of the Inauini River. The pilot survived the impact without injury, but the aircraft sustained substantial damage to the landing gear, wings, engine, and propeller.
The investigation
CENIPA investigators discovered that the aircraft had been disassembled and moved from the crash site to Rio Branco via barge without the required authorization from the investigation authority. Despite this, technical analysis was conducted on the Lycoming IO-540-K1G5 engine at a certified workshop.
Laboratory examinations of the engine components revealed that the cylinder #3 studs had fractured. Metallurgical analysis by the Institute of Aeronautics and Space (IAE) identified macroscopic characteristics of fatigue fracture in the studs. While corrosion and physical damage prevented the identification of specific fatigue striae under scanning electron microscopy, the evidence pointed to a fatigue-driven failure. The investigation also noted that the pilot was operating with an expired medical certificate and an expired single-engine land rating. Furthermore, the engine had undergone a period of inactivity between 2012 and 2014, which may have contributed to the degradation of the metal structure.
Findings
- The primary cause of the engine failure was the rupture of the cylinder #3 mounting studs due to fatigue.
- The failure of the studs caused the cylinder to detach from the engine block, leading to piston misalignment and subsequent breakage of the connecting rod bolts.
- This mechanical failure caused a drop in oil pressure, resulting in overheating and the melting of the rod bearings.
- The pilot was not legally qualified to operate the aircraft due to expired medical and rating credentials.
- The aircraft was moved from the accident site without proper authorization from the investigating authority.