Accident Overview
On March 3, 2021, at about 1220 Pacific standard time, a Cessna 150J airplane, registration N60290, was substantially damaged when it was involved in an accident near Lodi, California. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The instructor and pilot receiving instruction were not injured.
According to the pilot receiving instruction, the flight departed uneventfully and climbed to a cruise altitude of 3,000 ft mean sea level. The pilot donned an instrument training tool at 200 ft above ground level. At cruise, the pilot reduced power to 2,450 rpm, applied carburetor heat, and leaned the mixture for best rpm. Approximately five minutes later, a sound resembling a gunshot was heard, and the propeller stopped rotating. The instructor took control and announced "my controls" while the pilot searched for a suitable landing site and attempted an unsuccessful engine restart. The airplane glided to the end of an almond tree field, contacting trees before coming to rest.
Engine Examination
Postaccident examination of the engine revealed a fracture in the top of the engine case with a hole about 7 inches in diameter. The lower case was fractured around the circumference of the No. 4 cylinder. The No. 3 cylinder connecting rod was fractured at the shank and mechanically damaged. The No. 4 cylinder barrel rim was bent toward the crankcase. Borescope inspection of the No. 3 cylinder showed the connecting rod bushing end attached to the piston pin, but the piston ring seal was fractured with part of a piston ring exposed. No evidence of oil starvation or thermal damage was observed.
Metallurgical analysis of the No. 3 cylinder connecting rod revealed that the fracture face exhibited crack arrest marks consistent with fatigue cracking. The fatigue crack originated from the outer surface at one corner of the arm. Some features were damaged, but the origin showed no gouge or mechanical damage. The No. 3 cylinder piston had heavy combustion deposits at the outer crown, and similar deposits were present near the spark plug ports and valves.
Fuel Analysis
Postaccident examination of the carburetor input fuel line detected a smell resembling automotive gasoline. The owner stated he had only used 100 low lead aviation gasoline (100LL) in the 60 flight hours since purchasing the airplane. Fuel receipts showed purchase of 12.3 gallons of 100LL the day before and 17 gallons on the day of the accident. The previous owner, who had flown the airplane after the most recent engine overhaul, was deceased and unavailable. Fuel from the accident site was not tested.
Pilot Statements and Reference Material
The pilot stated he regularly used carburetor heat due to a "serious issue with carb ice" and adjusted it until a certain temperature, then leaned the mixture for best rpm before enrichening. The Federal Aviation Administration Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25B) defines detonation as an uncontrolled explosive ignition causing excessive temperatures and pressures, which can quickly lead to failure. It lists causes including lower-grade fuel and lean mixtures at high power. Preignition is defined as premature ignition from a hot spot, also causing power loss and high temperatures. The handbook notes that detonation and preignition can occur simultaneously or one may cause the other.