CEN12LA135 · Cessna 150M

The pilot was on a local flight and the airplane had climbed to an altitude of about 10,700 feet. The pilot reduced the power and established a glide. At an altitude of about 8,000 feet, he advanced the throttle, but the engine only reached about 1,500 rpm. He checked the throttle and mixture controls and applied carburetor heat; however, the engine rpm kept getting lower. During the forced landing in a field, the airplane impacted an embankment and nosed over, coming to rest inverted. An examination of the engine failed to identify any anomalies that would account for a loss of power. A review of the carburetor icing probability chart revealed that the weather conditions were just outside favorable conditions for the formation of carburetor icing. Additionally, the air temperature and moisture content at 8,000- to 12,000-feet altitude were not conducive to carburetor icing. A review of Transport Canada’s publication, TP10737, “The use of automobile gasoline (Mogas) in Aviation,” dated March 31, 1993, noted that, “Mogas is generally higher in volatility than Avgas [aviation gas]. Mogas will thus absorb more heat from the mixing air when vaporizing, resulting in ice accumulation at higher ambient temperatures. The likelihood of carburetor icing while flying on Mogas is higher.” Despite the low power setting during the glide and the use of Mogas, the weather conditions near the accident site did not appear to be favorable for the formation of carburetor icing. The reason for the loss of engine power could not be determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset