Aircraft

Failure of a critical mechanical component caused the fatal crash of a Sikorsky S-76 helicopter near Tallinn, according to the draft final report presented to the Estonian government by the country's air accident investigation commission. The final report is expected to be published within about two months.

The accident happened on 10 August 2005 when the Copterline aircraft went out of control within 2min of departure from Tallinn on the short scheduled flight to Helsinki, Finland. It had just established cruise at a height of 1,500ft (460m) and airspeed of 130kt (240km/h), but the problems began when the captain - the pilot flying - announced his intention to add power and climb to 2,000ft.

Then, says the report, as the pilot flying began to raise the collective to climb, the S-76 rapidly pitched nose-up and rolled left, and the vertical acceleration increased from 1g to 3g in 1.5s. The pitch had increased to 40° nose-up, the roll to 40° left, and the heading had changed from 355° to 320°. The helicopter climbed about 200ft, then spun around 13 times during its descent to hit the sea.

An autopsy established that all 14 people on board drowned.

The report confirms that a fault in the helicopter's main rotor servo caused the crash. The US National Transportation Safety Board said in its initial factual report three months after the accident that the helicopter's forward main rotor control actuator had failed. The actuator's function is to enable the pilot to control flight by varying the main rotor blade pitch, but at the time Sikorsky denied such a fault was the cause of this accident.

The actuator, a part ordered in 2003 from Sikorsky-owned Helicopter Support, failed with its piston in the fully extended position because flakes of plasma coating, which was applied slightly too thickly, separated and blocked the two return flow ports, immobilising it and freezing the main rotor blades at maximum pitch. Copterline managing director Kari Ljungberg says the company had since learned that the servos were secondhand, adding that if they had known the parts were reconditioned they would not have bought them.

Sikorsky says: "This matter remains the subject of litigation, and our policy is not to comment on litigation.