Cessna 172S N5311H accident near Tonopah, Nevada

Probable cause

The pilot's failure to enrich the mixture during the descent, as required by the Pilot's Operating Handbook (POH), and his subsequent failure to follow the POH emergency procedures, which resulted in an excessively lean fuel/air mixture and subsequent loss of rpm. Contributing to the accident was the pilot's improper decision to land on unsuitable terrain despite the availability of two nearby airports.

NTSB analysis

The private pilot departed for a long cross-country flight with the mixture leaned to the high field elevation. He then further leaned the mixture for lean-of-peak exhaust gas temperature as he climbed the airplane to 10,500 ft mean sea level (msl) for improved fuel efficiency. Thirty minutes into the flight, he descended the airplane to about 8,000 ft msl. The pilot did not enrich the mixture during the approximate 2,500-ft descent. Shortly after, the engine sputtered, followed by a loss of about 700 rpm. He verified the position of the throttle and enriched the fuel/air mixture with one full rotation of the control knob, but he never advanced the knob to the "full rich" position. The pilot then decided to perform a precautionary landing because he was concerned the airplane would not have the power to cross an approaching mountain range. He chose a dry river bed despite the availability of two nearby airports with dirt runways. During the landing, the airplane nosed over and came to rest inverted, which resulted in substantial damage to the rudder.

Postaccident examination of the airframe and engine did not reveal any evidence of preimpact mechanical anomalies that would have precluded normal operation, and the engine achieved full and continuous power during a test run.

The Pilot's Operating Handbook (POH) required a mixture adjustment during the descent for smooth operation, which the pilot did not do during the 2,500-ft descent. An engine manufacturer representative reported that the engine may suffer leaning misfires if the fuel/air mixture is not enriched during a descent. Therefore, it is likely that the engine sputtered and lost partial power due to the pilot's failure to properly enrich the fuel/air mixture during the descent in accordance with the POH. The pilot could have remediated this condition if he had advanced the mixture to the "full rich" position in accordance with the POH emergency procedures; however, instead the pilot made a minor mixture adjustment, which did not restore engine power.

Text from the NTSB. Open the NTSB docket for WPR18LA002 for the full record.