N756CLCessna U206G · 1977

On June 7, 2024, about 1505 Alaska daylight time, a Cessna 206 airplane, N756CL, sustained substantial damage when it was involved in an accident near King Salmon, Alaska. Both pilots onboard were uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 flight.

The pilot reported that, shortly after takeoff, he observed a high engine oil temperature indication and initiated a turn back to the airport. The engine began losing power and the pilot performed a forced landing to the tundra, which resulted in substantial damage to the horizontal stabilizer and elevator.

The engine was rebuilt by Continental Motors in February 2018, and had accrued about 1,282 hours since the overhaul. A preliminary engine examination revealed that the oil filter media contained a considerable amount of metal. The engine was shipped to Continental Motors in Mobile, Alabama, for further examination, which revealed that the No. 1 piston was eroded down to the steel ring lands and scuff marks were observed on the side of the piston skirt. The upper and lower spark plugs in cylinder No. 1 had molten aluminum deposits covering the 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 found worn beyond limits in accordance with 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 it was drained. The No. 2 and No. 5 crankshaft rod journals had severe thermal discoloration. All connecting rod bearings exhibited abnormal wear. Connecting rod bearings Nos. 2 and 5 were severely damaged. All damage discovered was consistent with a pre-ignition/detonation event.

The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25B), states, in part:

Detonation is an uncontrolled, explosive ignition of the fuel/air mixture within the cylinder's combustion chamber. It causes excessive temperatures and pressures which, if not corrected, can quickly lead to a failure of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness, or loss of power.

Preignition occurs when the fuel/air mixture ignites prior to the engine's normal ignition event. Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage to the cylinder that causes a part to heat sufficiently to ignite the fuel/air charge.

The pilot was attempting a takeoff from a frozen, snow-covered pond that had just been plowed. During the takeoff roll, the airplane's left main gear hit a snow berm, and the airplane turned abruptly left. The right wing contacted terrain, and sustained substantial damage. The pilot stated there were no preaccident mechanical anomalies with the airplane.