What happened
On 10 August 2005, a Sikorsky S-76C+ operated by Copterline crashed into the waters of Tallinn Bay, Estonia. The aircraft was performing a scheduled flight when it experienced a sudden and catastrophic loss of control. The impact with the water occurred at a high vertical descent rate, and the helicopter sank within approximately ten seconds. All occupants on board perished due to drowning.
The investigation
The investigation examined the mechanical integrity of the flight control system, the maintenance history of the aircraft, and the oversight provided by the Civil Aviation Administration (CAA) of Finland. Investigators performed detailed examinations of the wreckage, including the main rotor blades, the tail rotor system, and the hydraulic actuators. Advanced techniques, such as computed tomography and acoustic analysis of the cockpit voice recorder, were utilized to reconstruct the sequence of events. The investigation also reviewed the operator's maintenance practices and the effectiveness of the regulatory oversight regarding Copterline's flight operations and maintenance organization.
Findings
- The primary cause of the accident was the uncommanded extension of the main rotor forward actuator, which led to the subsequent loss of control of the helicopter.
- A contributing factor to the actuator failure was the separation of the plasma coating on one of the two actuator pistons.
- The operator failed to detect internal leakage within the main rotor forward actuator.
- The investigation identified significant deficiencies in the operator's safety culture, noting that maintenance practices often deviated from approved procedures and that corrective actions were frequently delayed or incomplete.
- Regulatory oversight by CAA-Finland was found to be insufficient to identify the breadth of safety deficiencies within the company's maintenance operations.
- Meteorological conditions, specifically the presence of IMC (Instrument Meteorological Conditions), were noted as a possible contributing factor following the initial upset recovery attempt.