What happened
On July 4, 2023, a private VFR flight was underway from a fjord near Kragerø, Norway, toward Copenhagen Sea Airport. The aircraft, a Cessna U206G with registration LN-TEP, was flying in formation with another seaplane at approximately 2,000 feet. As the formation approached the city of Helsingør, the pilot of LN-TEP experienced sudden vibrations and an uncommanded reduction in engine power.
Despite the pilot attempting standard troubleshooting measures—including switching fuel tanks, activating the electric fuel pump, and enriching the mixture—the engine performance did not recover. After evaluating the landscape for a safe area, the pilot decided to execute a precautionary landing on Lake Esrum. The landing was completed without further incident, and no injuries were reported among the four occupants.
The investigation
The Danish Accident Investigation Board conducted a technical examination of the Continental IO-520-F engine. On-site inspections revealed cracks in the cooling ribs of cylinder number 2, with visible exhaust deposits on the adjacent cylinder. Upon further removal of the engine components, the fractured cylinder head was found to have separated.
Metallographic analysis performed by an external laboratory determined that the fracture was caused by intercrystalline corrosion within the aluminum alloy of the cylinder head. This corrosion created small notches along the grain boundaries, which reduced the material's strength and led to a fracture progression driven by both corrosion and fatigue. The investigation also noted that the engine's time since overhaul (TSO) was 1,006 hours, but it had been in service for 17 years, significantly exceeding the manufacturer's recommended 12-year calendar limit for overhaul.
Findings
- The primary cause of the engine power loss and vibration was a fracture in the cylinder head of cylinder number 2.
- The fracture was initiated by intercrystalline corrosion, which weakened the structural integrity of the cylinder head.
- The engine had exceeded the manufacturer's recommended time-between-overhaul (TBO) calendar interval by five years.
- Operating the engine beyond the recommended calendar limit introduced a latent safety risk, as corrosion in coastal, salty, and humid environments can accelerate engine degradation undetected.