What happened
On March 1, 1998, a Robin DR 360, registration F-BRZM, was conducting a flight for travel purposes when the pilot experienced a progressive loss of engine power. While cruising at 2,500 feet just below the cloud base, the pilot attempted to rectify the issue by activating the carburetor heat; however, this action failed to restore power. The pilot proceeded to check the magneto operation and switched fuel tanks, but the power loss persisted. Consequently, the pilot decided to perform a precautionary landing in a field near Saint-Pavace. During the final approach, the engine power began to recover, allowing the pilot to land the aircraft without any damage to the plane or injuries to the two occupants.
The investigation
The investigation focused on the cause of the engine power fluctuation and the effectiveness of the pilot's corrective actions. Post-flight inspections and engine tests conducted on the ground revealed no mechanical anomalies or malfunctions. Meteorological data from Le Mans around the time of the incident indicated a temperature of 10°C and a dew point of 2°C, with broken cloud layers at 2,600 feet. An analysis of the icing risk indicated that the atmospheric conditions in the area presented a significant risk for carburetor icing during cruise flight.
Findings
- The primary cause of the engine power loss was carburetor icing.
- The application of carburetor heat was not effective because it was initiated only after the symptoms had already manifested, whereas the system is primarily designed for preventive use.
- Environmental conditions, characterized by high humidity and specific temperature profiles, created a high risk of ice formation in the carburetor.