What happened
On May 7, 2021, a Cessna 177RG (registration OB-1332), operated by Red Wing Aviation S.R.L., was performing a scheduled passenger flight from Contamana to Pucallpa, Peru. After climbing to 3,000 feet, the pilot experienced a loss of engine power, a drop in oil pressure, and a subsequent engine shutdown.
The pilot declared an emergency and attempted to restart the engine twice without success. Due to the dense jungle terrain below, the pilot decided to perform an emergency ditching in San José Lake, located approximately 17 km southeast of Contamana. Following the Pilot's Operating Handbook (POH) procedures for ditching, the pilot retracted the landing gear to prevent the aircraft from flipping upon impact and approached the water at approximately 60 to 65 knots.
The aircraft landed successfully in the water, though the impact caused the pilot-side door handle to break, necessitating evacuation through the co-pilot side. The aircraft gradually sank and eventually submerged completely. The pilot and three passengers were rescued by nearby boats after remaining in the water for approximately 10 to 15 minutes. All occupants sustained only minor injuries.
The investigation
The investigation by the CIAA focused on the mechanical failure of the Lycoming IO-360-A1B6D engine. Metallographic and fractographic analysis conducted by the SEMAN-FAP laboratory revealed that the engine failure was caused by the fracture of the crankshaft idler gear assembly (GEAR ASSY Crankshaft Idler).
Investigators found that the fracture originated from material fatigue caused by a subsurface forging defect (a blowhole) that was too deep to be detected by standard non-destructive testing (NDT) methods, such as magnetic particle or fluorescent penetrant inspections, during the engine's previous overhaul in 2020. This same engine had previously suffered a crankshaft fracture in July 2019.
Findings
- The primary cause of the accident was an uncommanded engine shutdown resulting from the fracture of the crankshaft idler gear assembly.
- A contributing factor was the presence of undetected internal forging defects within the gear assembly that were not detectable via traditional NDT methods used during the 2020 overhaul.
- The pilot executed the emergency ditching procedures correctly, and the aircraft's maintenance and airworthiness records were up to date.