Background
A Cessna 172RG departed on a fire patrol flight with a pilot and passenger. The pilot demonstrated basic flight techniques to the passenger during the flight.
Flight
The aircraft proceeded into mountainous terrain. While flying southeast up a canyon, the engine performance began to deteriorate. The pilot noted that approximately 10–15 seconds elapsed between the initial decrease in performance and when the engine began to "cough." At that time, the aircraft was about 1,000 feet above the ground and 200 feet horizontally from the rising terrain.
Accident
The aircraft impacted trees and terrain along the canyon's axis at an elevation of approximately 2,500 feet above sea level. The pilot reported that he did not have time to exercise the carburetor heat when he began to troubleshoot and that he had not encountered carburetor icing previously. An examination of the aircraft at the accident site revealed the carburetor heat control in the "OFF" position.
Post-Crash Examination
Moderate carburetor icing conditions existed based on temperature/dew point extrapolation from a nearby weather reporting source. The pilot reported a ragged overcast with bases at 4,000–5,000 feet and rain showers along the route of flight. A post-crash test run of the aircraft's engine was successful; however, the left magneto was found to be inoperative. It could not be determined whether the magneto became inoperative during the flight or as a consequence of the impact.
Terrain Analysis
The average gradient from the crash site along the aircraft's direction of flight was more than +11 degrees over the 12,000-foot distance beyond the site. The aircraft had sufficient lateral distance to execute a 180-degree turn under normal power.