Accident Overview
On June 7, 2024, at approximately 1505 Alaska daylight time, a Cessna 206 airplane, registration N756CL, sustained substantial damage during a forced landing near King Salmon, Alaska. The two pilots aboard were uninjured. The flight was conducted under Title 14 Code of Federal Regulations Part 91.
Pilot Report
The pilot reported that shortly after takeoff, a high engine oil temperature indication was observed. He initiated a turn back to the airport when the engine began losing power. A forced landing was performed on the tundra, resulting in substantial damage to the horizontal stabilizer and elevator.
Engine Examination
The engine had been rebuilt by Continental Motors in February 2018 and had accumulated approximately 1,282 hours since overhaul. A preliminary examination found considerable metal in the oil filter media. The engine was shipped to Continental Motors in Mobile, Alabama, for further analysis.
Detailed examination revealed:
- The No. 1 piston was eroded down to the steel ring lands with scuff marks on the piston skirt.
- The upper and lower spark plugs in cylinder No. 1 had molten aluminum deposits covering electrodes and insulators.
- The upper and lower spark plugs from cylinder No. 3 and the top spark plug from cylinder No. 4 displayed chipped porcelain insulators at the electrodes.
- All spark plugs were worn beyond limits per the Champion Check-A-Plug chart.
- The crankshaft rod journals exhibited signs of foreign material contamination in the oil.
- Dark oil was observed in the sump case when drained.
- The No. 2 and No. 5 crankshaft rod journals had severe thermal discoloration.
- All connecting rod bearings exhibited abnormal wear; Nos. 2 and 5 were severely damaged.
All damage was consistent with a pre-ignition/detonation event.
Relevant Definitions
According to the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B):
- Detonation is an uncontrolled, explosive ignition of the fuel/air mixture within the cylinder's combustion chamber, causing excessive temperatures and pressures that can lead to failure of piston, cylinder, or valves. In less severe cases, it causes engine overheating, roughness, or loss of power.
- Preignition occurs when the fuel/air mixture ignites prior to the normal ignition event, often caused by a residual hot spot in the combustion chamber such as a carbon deposit on a spark plug, cracked spark plug insulator, or other damage that heats sufficiently to ignite the fuel/air charge.