TALLINN - After nearly six months of investigation, experts believe they have discovered what caused the Copterline Sikorsky 76C helicopter crash in August last year resulting in the death of 14 people. Laboratory examinations of the Finnish-owned helicopter's front servo, which was sent to the National Transportation Safety Board in the United States for investigation, showed that parts of the plasma-sprayed coating that is made from a mix of copper and aluminium had broken off from the piston crown.
Simply put, the servo, which controls the helicopter's throttle, fell apart.
Experts explained that, after breaking off, the two large pieces of the servo layer blocked one of the ports by which the device is operated. This, in turn, caused the servo to break down completely, which triggered an unexpected malfunction of the rotor blades and the subsequent crash.
The ministry pointed out, however, that the conditions which led to the servo's dismantling had to be proved in further tests and surveys. Since a servo is a piece of high-tech equipment, all laboratory examination requires long-term planning and careful execution, a ministry spokesperson added.
But the investigating commission is convinced that the accident was caused by the uncontrolled self-extending of a servo located in one of the helicopter's main rotors.
Commission representatives announced the news after a meeting that took place in Tallinn on Feb. 15.
Meanwhile, the helicopter's Finnish manufacturing company, Sikorsky, has been testing its equipment with a similar servo, using extreme loads on a specially built device. The series of tests began on Jan. 30, and is due for completion in March. An Estonian representative has overseen the experimentation.
An in-depth metallographic survey of matters related to the plasma coating of servo pistons is also being conducted, the outcome of which should provide clues for improving
An in-depth metallographic survey of matters related to the plasma coating of servo pistons is also being conducted, the outcome of which should provide clues for improving the overall reliability of hydraulic devices used in aircraft.
In mid-December, the investigating commission announced that, immediately before the accident took place, the attack angle of a rotor blade changed suddenly due to a malfunction with the main rotor servo. As a result, the helicopter veered out of control and crashed.
The commission must present its conclusions by Aug. 10, exactly one year after the Copterline Sikorsky crashed into the Tallinn Bay. The aircraft went down shortly after takeoff while on a scheduled commercial flight from Tallinn to Helsinki, killing all 12 passengers and two crew on board.