Primary finding
Probable cause
A partial loss of engine power for reasons that could not be determined based on the available information.
Investigator assessment
Analysis narrative
The pilot reported that, while descending in the single-engine airplane toward the destination airport, the engine lost partial power. He performed a forced landing to a dirt road, during which the airplane contacted a bush, resulting in substantial damage. Postaccident examination of the engine and an engine test run revealed no anomalies that would have precluded normal operation. Atmospheric conditions at the time of the accident were not conducive to the development of carburetor icing. The reason for the partial loss of engine power could not be determined based on the available information.
Source record
Factual narrative
On May 5, 2019, about 1756 mountain standard time, a Cessna 172F airplane, N8887U, was substantially damaged when it was involved in an accident near Red Rock, Arizona. The commercial pilot and three passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he canceled flight-following about 15 miles west of his destination. During the descent, the engine rpm dropped from 2,600 rpm to 1,300 rpm once or twice, then dropped to 1,000 rpm and remained there. The pilot reported that he checked the magnetos, verified that the fuel selector was on BOTH, and set the fuel mixture to full rich. The pilot performed a forced landing to a dirt road, during which the left wing clipped a bush, resulting in substantial damage. The pilot mentioned that he did not use carburetor heat, as the humidity was very low; calculated relative humidity for the area of the accident site was 27%. Additionally, the pilot reported no additional anomalies with the airplane that would have precluded normal operation. Review of the Carburetor Icing Probability Chart indicated that conditions were not conducive to carburetor icing. The airplane was equipped with a Continental Motors O-300-D engine, serial number 29754-D-5-0. Maintenance records revealed that about 6 days before the accident, the Nos. 2- and 3-cylinder assemblies were replaced, and the lower forward crankcase through bolt and hardware were replaced. Examination of the engine revealed no anomalies, and the engine operated normally during a subsequent test run to about 2,280 rpm.