What happened
An instructor pilot was conducting commercial flight instruction for a dual student, who held a private pilot certificate. The aircraft was observed flying at a low altitude in the vicinity of the Fertile Airport. One witness reported seeing the aircraft depart the airport and turn south, noting that the pilot initiated a steep, left-banked turn just above the tree line. After looking away momentarily, the witness could no longer locate the plane.
A second witness, positioned one mile east of the accident site, observed the aircraft performing a steep left bank before descending toward the ground. The witness described the aircraft's altitude during the turn as being similar to that of a crop duster completing a pass over a field.
The investigation
Mechanical examination of the wreckage identified a 3-inch long crack in the exhaust pipe of the number 5 engine cylinder. This crack circumscribed the pipe at a location approximately 1/4 inch to 1 inch below the flange where the pipe connects to the cylinder. No other mechanical anomalies were discovered during the inspection of the aircraft.
Toxicology testing was performed on the dual student pilot by the State Medical Examiner's Laboratory, which revealed blood carbon monoxide levels of 1.3 percent. However, FAA toxicology testing of the student pilot's specimens returned negative results for all conducted tests, as the FAA does not classify carbon monoxide levels as positive unless they reach 10 percent. Carboxyhemoglobin testing was not performed on the instructor pilot due to issues with blood contamination.