Background
A Cessna 172, powered by a Continental O-300D engine, was on a cross-country flight when the engine began to run rough. The pilot applied carburetor heat, which temporarily improved engine performance. After a second recurrence of roughness, the pilot performed a precautionary landing to inspect the aircraft. The inspection revealed no anomalies.
Event Sequence
The pilot decided to continue the flight using partial carburetor heat. According to the surviving passenger, the pilot stated he would continue with the 'carburetor heat on one-quarter way.' Shortly after resuming the flight, the engine again began to run rough. The pilot was unable to correct the condition, and the engine eventually experienced a total loss of power.
Under dark night conditions, the pilot initiated a forced landing. During the final phase of descent, the airplane struck a transmission line and then impacted terrain.
Examination Findings
Subsequent examination of the airplane and a successful test run of the engine revealed no mechanical anomalies other than those associated with the impact with the wire and the terrain.
Advisory Guidance
FAA Advisory Circular AC 20-113, dated October 22, 1981, under the title 'In-flight Winter Weather Conditions' and subsection 'Prevention Procedures,' states that when no carburetor air or mixture temperature instrumentation is available, the general practice with smaller engines should be to use full heat whenever carburetor heat is applied.