What happened
On 24 November 1998, a Cessna 150 aircraft, registration EI-CDV, was conducting an introductory training flight when the pilot experienced a partial loss of engine power. After climbing to an altitude between 1,200 and 1,300 feet, the engine began to malfunction, prompting the pilot to execute a precautionary landing. The aircraft approached a field in the Oldtown area of Summerhill, Co. Meath, from a westerly heading and touched down approximately 60 metres past a hedge, completing a ground roll of about 240 metres. There were no injuries to the pilot or the passenger, and the aircraft sustained no apparent damage.
The investigation
The AAIU investigation focused on the cause of the engine's rough running. Following the incident, a maintenance engineer performed a ground run of the engine, which confirmed the irregular performance. Upon inspection, the engine's carburettor was replaced with a serviceable unit, after which the aircraft successfully returned to Weston Airfield.
Analysis of the removed carburettor revealed the presence of sediment within the bowl and around the mixture control metering valve. The investigation examined the history of both the engine and the airframe, noting that both components had undergone significant maintenance and periods of storage prior to the flight. The airframe had recently undergone extensive rework, including paint stripping and the renewal of all fuel and engine hoses. The engine had been previously overhauled and re-installed in EI-CDV in September 1998.
Findings
Technical examination of the debris found in the carburettor bowl identified several contributing factors:
- Corrosion: X-ray analysis of the carburettor drain plug revealed that the steel bolt and its lining had suffered significant ferrous corrosion. This was attributed to a failure or damage in the cadmium coating on the bolt threads, which allowed water to react with the steel in the presence of fuel chemicals.
- Contamination: The investigation identified aluminium-based particles and organic matter, including the remains of a dead insect, within the carburettor. The source of the aluminium debris could not be definitively determined, though it was noted the aircraft had recently undergone paint stripping.
- Maintenance Gaps: The investigation concluded that the accumulation of rust, aluminium particles, and organic matter caused a blockage in the mixture control metering valve, leading to the engine power loss.
Safety action
Based on the findings, the following safety recommendations were issued:
- Aircraft manufacturers should issue service bulletins requiring periodic inspection of the carburettor drain plug for aircraft manufactured between 1963 and 1968.
- Such inspections should include checking the drain plug for wear or damage to the cadmium plating, with replacement required if necessary.
- Engine manufacturers should update their inspection schedules to include the periodic removal and inspection of the carburettor drain plug.