What happened
On 15 February 2005, a Cessna 150 K, registration EI-CIN, was conducting aerial work near Derrinturn, Co. Offaly. The flight involved a qualified instructor and a student performing stall recovery exercises. The weather conditions were favorable, characterized by good visibility, light winds, and minimal cloud cover.
During the training session, the student had successfully completed several stalls in various configurations. However, while recovering from a stall performed in a left-hand turn with 20 degrees of flaps selected, the engine lost power and ceased operation. The instructor assumed control of the aircraft, established a glide configuration, and identified a suitable landing site. Despite three attempts to restart the engine during the descent from above 3,000 feet, the engine remained non-functional. The pilot issued a MAYDAY call and executed a landing in a field. There were no injuries to the two occupants, and no damage was sustained by the aircraft or any property.
The investigation
The investigation focused on the cause of the engine failure. The pilot noted that during the low-power stall exercises, the carburetor heat had been engaged, and the engine temperature was approximately -3°C at 3,500 feet. The pilot suspected that the engine was not warming sufficiently to prevent ice formation in the induction system. Notably, the engine functioned normally once the aircraft was on the ground.
Meteorological analysis provided by Met Éireann confirmed that at an altitude of 3,000 feet, the temperature and dew point were approximately -1°C and -5°C, respectively. This data was used to evaluate the susceptibility of the engine to induction system icing.
Findings
- The engine failure was caused by carburetor icing.
- The aircraft was operating at a low power setting during stall maneuvers, which increases the risk of icing due to the temperature drop at the venturi and the restricted airflow through the butterfly valve.
- Atmospheric conditions, specifically the temperature and dew point at 3,000 feet, were highly conducive to the formation of ice within the induction system.
- The use of carburetor heat during periods of low power may have been insufficient to prevent ice buildup or may have even contributed to the issue by melting ice particles that then refroze in the induction system.