Cessna 150M N9432U accident near Greenwood, Indiana

Probable cause

The flight instructor's failure to ensure that carburetor heat was used while performing maneuvers at reduced power settings in conditions conducive to the formation of carburetor ice, which resulted in a partial loss of engine power due to carburetor ice. Also causal was the flight instructor's failure to secure the engine before landing, which resulted in a sudden increase of engine power and an aerodynamic stall at a low altitude.

NTSB analysis

The flight instructor reported that the airplane sustained a partial loss of engine power after his student completed several training maneuvers at a reduced engine power setting. The subsequent application of full throttle did not increase the engine power, and the engine continued to operate with reduced power. The flight instructor then took control of the airplane and established best glide speed while he verified that the magnetos were operating properly, that the fuel selector was in the "on" position, and that the mixture and throttle were full forward. Applying carburetor heat did not noticeably affect engine operation. The instructor remembered maneuvering toward a nearby field for a forced landing; however, he did not recall the landing or impact with terrain. The student pilot stated that, shortly before flare/touchdown and when the airplane was about 25 to 30 feet above the ground, the engine power increased suddenly, which caused the airplane to pitch up and enter an aerodynamic stall. The airplane subsequently impacted terrain.

An examination of the engine did not reveal any preimpact mechanical malfunctions or abnormalities that would have precluded normal engine operation. The atmospheric conditions at the time of the accident were conducive to the accumulation of carburetor ice at cruise and descent engine power settings. Additionally, an aviation mechanic confirmed the existence of carburetor icing conditions at the departure airport within 1 hour of the accident. The instructor confirmed that carburetor heat had not been used during any of the training maneuvers completed at reduced engine power settings. The failure to use carburetor heat while performing flight maneuvers at reduced engine power settings while in conditions conducive to the formation of carburetor ice likely resulted in the partial loss of engine power. The sudden increase of engine power shortly before landing was likely due to a reduction of accumulated carburetor ice following the flight instructor's application of carburetor heat and the increase in ambient temperature as the airplane descended during the forced landing.

Text from the NTSB. Open the NTSB docket for CEN14LA161 for the full record.