Primary finding
Probable cause
The pilot’s delayed use of carburetor heat which resulted in the loss of engine power due to carburetor icing.
Investigator assessment
Analysis narrative
The pilot reported that he had just been cleared to fly the GPS approach in night instrument meteorological conditions when the engine rpm decreased. The pilot turned on the carburetor heat, enrichened the fuel mixture, and advanced the throttle forward in order to troubleshoot the loss of engine power; however, there was no change in the engine operations. The pilot did not recall the accident sequence. Security video footage showed that the airplane touched down on the top of a one-story building, then impacted a retaining wall and the ground. The airplane sustained substantial damage to the engine mounts and forward fuselage. A postaccident examination of the engine revealed no mechanical anomalies that would have precluded normal operation. The investigators determined that there should have been adequate fuel to complete the flight. The temperature was about 37° F and the dew point was about 35° F. According to the carburetor icing probability graph conditions were conducive for serious icing at cruise power. The loss of engine power was likely due to the accumulation of carburetor icing during the flight. While the pilot did activate the carburetor heat, it is likely that the ice had already accumulated to a point that the carburetor heat was insufficient to melt the ice and restore full engine power.
Source record
Factual narrative
On April 29, 2024, about 0027 central daylight time, a Cessna 172M airplane, N12805, was substantially damaged when it was involved in an accident near Clearbrook, Minnesota. The pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot departed Stanley Municipal Airport (08D), Stanley, North Dakota, about 2132 on an instrument flight rules flight to return to Bemidji Regional Airport (BJI), Bemidji, Minnesota. The pilot was cleared by air traffic control for the GPS 13 approach to BJI. Shortly after the clearance for the approach, the engine rpm decreased “as if the key was shut off.” The pilot turned on the carburetor heat; however, there was no change in the engine operations. The pilot enrichened the fuel mixture and advanced the throttle forward; however, there was no change in the engine operations. The pilot was in night instrument meteorological conditions and chose to focus on flying the airplane. During the descent, he turned the airplane toward the town of Clearbrook but did not recall anything after that. Security video footage showed that the airplane touched down on the top of a one-story building and then impacted a retaining wall and the ground. The airplane sustained substantial damage to the engine mounts and forward fuselage. A postaccident examination of the engine revealed no mechanical anomalies that would have precluded normal operation. A representative of the aircraft owner believed the airplane initially departed BJI with full fuel tanks and that the pilot added 16 gallons of fuel at 08D before the accident flight. The temperature was about 37° F and the dew point was about 35° F. According to the carburetor icing probability graph contained in Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35, conditions were conducive for serious icing at cruise power.