Cessna 150G N4041J accident near Fitchburg, Massachusetts

Probable cause

A partial loss of engine power due to the formation of carburetor ice and the flight instructor's improper application of carburetor heat.

NTSB analysis

On the day of the accident, the student pilot conducted a touch-and-go landing and then entered the traffic pattern to perform another one. While on the downwind leg of the traffic pattern, the engine began to run roughly, so the student applied carburetor heat and kept it on until touchdown. During climb after the second touch-and-go landing, the engine began running roughly again. Subsequently, the flight instructor took control of the airplane, lowered the angle-of-attack, and reapplied carburetor heat. According to the flight instructor, the engine subsequently ran more roughly, so he immediately placed the carburetor heat to “off.” He stated that, although the engine then ran less roughly, it still ran roughly. The flight instructor decided that there was not sufficient remaining runway to “land.” He was not sure how much power the engine was producing or if he could maintain level flight. He stated that he thought about turning back but decided against it. He said that the airplane was low and slow, so he decided to continue straight ahead. He tried to keep the airplane flying and was successful for about 15 to 20 seconds, but then he observed a warehouse ahead and banked to the right to avoid it. The airplane then settled and struck trees, substantially damaging the airplane and seriously injuring the student pilot.

Examination of the airplane and engine revealed no evidence of any preimpact mechanical failure or malfunction that would have precluded normal operation. However, a postaccident review of weather data and a carburetor icing probability chart also revealed that, at the time of the accident, the ambient temperature and dew point favored serious icing. Industry and Federal Aviation Administration guidance advised pilots to be aware of the warning signs of carburetor ice, including loss of rpm (with a fixed-pitch propeller) and rough running (both of which occurred before the accident), and advised that the pilot should respond to these warning signs by immediately applying full carburetor heat, and that the engine may initially run roughly for a short time while the ice melts. Therefore, the flight instructor should not have turned off the carburetor heat when the engine was still running roughly after he turned it on.

Text from the NTSB. Open the NTSB docket for ERA12FA143 for the full record.