What happened
On 14 February 2017, a Yakovlev YAK-52 was conducting a local flight departing from the airfield of Cerfontaine. Approximately ten minutes into the flight, the engine began to sputter and suffered a significant loss of power. Unable to maintain altitude or return to the airfield, the pilot attempted a forced landing. During the descent, the aircraft struck the treetops, causing the plane to pitch forward and strike the ground with high impact. The pilot sustained serious injuries.
The investigation
Investigators examined the aircraft's fuel system and maintenance history following the accident. They discovered significant water contamination within the fuel fine filter and the compensation tank. Analysis of the carburetor revealed severe internal corrosion of the magnesium alloy casing, which had partially obstructed the pressure regulator valve.
Regarding maintenance, the investigation found that a clerical error in the June 2016 maintenance schedule had listed the cleaning of the "Oil fine filter" instead of the "Fuel fine filter." Furthermore, the investigation noted that the aircraft had been grounded for 11 months near the North Sea coast, a period during which moisture could have accumulated in the compensation tank. The investigation also highlighted that the aircraft had been released to service the day before the accident after troubleshooting efforts had temporarily masked the engine symptoms without identifying the underlying cause.
Findings
- The primary cause of the accident was the collision with a tree during a forced landing necessitated by engine power loss.
- The loss of power was driven by internal corrosion of the carburetor, which blocked fuel flow at the pressure regulator valve.
- This corrosion was likely triggered by water contamination within the fuel system.
- The pilot had limited time to react due to the low altitude at the time of the failure and the aircraft's specific lift-to-drag characteristics.
- Maintenance omissions, including the lack of a requirement to drain the compensation tank and the failure to clean the fine fuel filter, contributed to the accumulation of contaminants.
Safety action
- A safety recommendation was issued to the UK CAA to review all approved YAK-52 maintenance schedules to ensure that both the cleaning of the fuel fine filter and the draining of the fuel compensation tank are explicitly required.
- A safety message was issued regarding the risks of releasing an aircraft to service when engine malfunctions have disappeared without a confirmed, identified cause.