The helicopter accident investigation commission got together today

On 15. of February there was in the Ministry of Economic Affairs and Communications informal meeting of the investigation commission into occurred on Tallinn bay Copterline helicopter S-76C+ accident on  10. of August 2005.

On the meeting took part representatives from Estonia and Finnish side.

There was no possibility to participate in the meeting for representatives from US NTSB, but they sent to the investigation commission update about the investigations ongoing in USA.

Main items on the meetings were the precise connecting of the process of encountering of the uncontrolled attitude with the technical malfunction of the main rotor servo, discovered during the investigation and special-interest investigation of the causes of this malfunction.

The investigation commission is continually on position, that emergency situation occurred due to the uncommanded extention of one of the main rotor hydraulic actuators, particularly due to the forward servo uncommanded extention.

During the extra investigation in labs of the sent to the USA encouterd accident forward servo NTSB experts discovered probable cause of the uncommanded extention.

Namely was discovered partial flaking of the copper-aluminium plasma coating of the pistonhead in servos working side nr 2. Two major parts of flaked coating jammed one of channels of the servos control valve. Occurred jamming certainly created a situation, where the perfomance data of the servo was rapidly worsen. Nevertheless it is necessary to prove during the further investigations presence of the conditions necessary for servo uncommanded extension.

As servo is high tech device, all tests needs long term planning and carefull performing.

The NTSB and the helicopter manufacturer Sikorsky arranged similar servo performance tests for modelled extreem loads on specially created test stand. Tests series started on 30. of January and are planned to complete in March. During test starting also representative from Estonia was present.

In parallel there is ongoing special-interest metallographical investigation into circumstances of the plasma coating.

The results of the investigation should give fulcrums for improving general reliability of the aircrafts hydraulic devices.

By now on the basis of the investigation results FAA issued respective flight safety directive, where operators of the S-76 type helicopters are urged to pay high attention to the hydraulic devices, especially to servos.

The investigation commission expects to signe the investigation report before the year is passed from the accident.