What happened
On October 31, 2017, a private Cessna 152 was performing a local flight at the Namur-Suarlée aerodrome (EBNM). The flight, which included a pilot and one passenger, proceeded normally until the pilot began a series of touch-and-go landings. During the downwind leg of the approach, the pilot engaged the carburettor heat and configured the flaps. After the first successful landing, the pilot applied full power for the subsequent circuit, retracted the flaps, and turned the carburettor heater off.
At an altitude of approximately 500 feet AGL, the engine suddenly lost power. Despite the pilot attempting to reset the carburettor heater and checking various engine controls, the power loss persisted. After a brief, temporary recovery when advancing the throttle, the engine failed again. At 300 feet AGL, the pilot executed an emergency landing in an open field. The aircraft slid approximately 150 meters into a beet field, but there was no damage to the aircraft and no injuries to the two occupants.
The investigation
The AAIU(Be) examined the engine's fuel system, specifically the Marvel Schebler MA-3PA carburettor and the air induction heater. The investigation focused on the path of contaminants through the induction system and the design of the float chamber vent. Investigators also reviewed the Lycoming engine specifications and the Cessna 15 and the flight manual's instructions regarding the use of carburettor heat.
Findings
- The engine malfunction was directly caused by a small insect (a fly) entering the carburettor float chamber through the vent line, which subsequently blocked the main fuel nozzle.
- The carburettor design features a vent line for the float chamber that lacks a protective filter, leaving it vulnerable to external contaminants.
- The aircraft's air induction heater system does not have adequate filtration to prevent the ingestion of debris when active.
- The use of the carburettor heater while flying at low altitudes over a grass runway increased the risk of ingesting insects or organic matter.
Safety action
- BE-2019-0001: Recommended that Lycoming Engines install filters in the vent lines of similar carburettor models to prevent float chamber contamination.
- BE-2019-0002: Recommended that Lycoming Engines issue an information document to notify aircraft manufacturers of the risk of engine stoppage due to fuel chamber contamination.
- BE-2019-0003: Recommended that Cessna - Textron Aviation install screen filters in the induction heater system of Cessna 152 aircraft.
- BE-2019-0004: Recommended that the Cessna 152 Flight Manual be updated to allow pilots more flexibility in turning off carburettor heat before touchdown and to include a warning regarding the risks of using the heater near the ground.