N7379U

On August 7, 2018, about 1215 Alaska daylight time, a Cessna 207 airplane, N7379U, was substantially damaged when it was involved in an accident near Iliamna, Alaska. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 flight.

The pilot reported that, after takeoff, he started a climbing left turn toward his destination. As the airplane reached his intended altitude of 1,000 ft mean sea level, and as he began to configure the airplane for cruise flight, the engine began to run rough and lose power, so he elected to immediately return to the departure airport. He switched fuel tanks and activated the auxiliary fuel pump in an effort to restore full engine power. He noted that the engine roughness improved momentarily, followed by a severe engine vibration and complete loss of engine power. The pilot selected a forced landing area on tundra-covered terrain, during which the airplane sustained substantial damage to the fuselage and wings.

The airplane was subsequently recovered and returned to the operator’s base. A Federal Aviation Administration inspector reported that there was no obvious damage to the engine, and an engine run was attempted. Although the engine eventually started, it ran very rough and began to vibrate and the test was discontinued. The engine was sent to the manufacturer’s facility for further examination.

The examination revealed that the No. 2 piston was eroded on the bottom side, and a hole was burned through. The No. 2 piston and cylinder head were mechanically damaged by loose piston ring debris. Closer examination of the No. 2 cylinder head revealed re-solidified molten metal around the bottom Heli-Coil tang notch. All damage discovered was consistent with a pre-ignition/detonation event. The cause of the preignition and/or detonation could not be determined.

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.”