Incident Details
The flight instructor (CFI) reported that the engine began missing when power was applied after practicing stalls. A decision was made to return to the airport. Carburetor heat was applied, and the throttle was retarded to descend, while the mixture remained in full rich. After a couple of minutes, the engine started running rough and losing power. Adjustments to carburetor heat and throttle were unsuccessful in restoring power. The pilot then decided to land on an unopened highway. During the final approach, the aircraft struck a power line and subsequently impacted the ground nose-first.
Engine Findings
Approximately 10 gallons of fuel remained in the tanks. Examination revealed that all but one spark plug were found oily and sooty. The engine was started and ran smoothly to 1200 RPM. The engine had 50 hours of operation since overhaul.
Operational Context
The pilot handbook stated that cruise should be conducted at 65 to 75% power for the first 50 hours or until oil consumption has stopped. A Champion spark plug publication indicated that wet, oily plugs may be caused by an engine still in a break-in period.