Helsinki, Finland-based operator Copterline in late December filed a lawsuit in the U.S. against Sikorsky, accusing the Stratford, Conn.-based helicopter manufacturer of negligence. Copterline alleges Sikorsky knew that the design of the main rotor servo actuator of the S-76 was unsafe but did not take proper action. The lawsuit further alleges “breaches of warranties, negligence, gross negligence and failure to warn.

”In August 2005, Copterline lost an S-76C+ in a crash that killed all 14 occupants. Shortly after takeoff, as it was flying at an altitude of 1,500 feet, the helicopter plunged into the Baltic Sea during a scheduled flight between Tallinn, Estonia, and Helsinki, Finland. Copterline is seeking damages of at least $60 million.

Sikorsky now appears to disagree with the NTSB, the FAA and the Estonian investigation commission on the results of the tests that were conducted last year relating to the suspect servo. At stake in the servo is possible leakage and flaking of the plasma coating on the piston’s head. The main rotor servo actuator controls the direction of flight by changing the pitch angle of the rotor blades.

According to Copterline, FAR 29 airworthiness standards require the main rotor servo actuator to be fail-safe. Last summer and then in November, the NTSB, Sikor-sky and HR Textron–manufacturer of the servo–and the Estonian investigation commission conducted tests of the part, investigating whether a hydraulic return port blockage and destruction of internal seals could affect main rotor blade pitch control.

According to Tonu Ader, vice chairman of the Estonian investigation commission, the results showed plasma coating flakes can block the servo’s return port partly or entirely. Tests also showed that, with a partially blocked return port, the servo can extend even though it received a retraction command, he added. Ader also said this condition can occur only when the servo is leaking significantly.

According to the NTSB, investigators determined that the actuator had sufficient capability to withstand a single failure (either blockage of the port or destruction of the seals). However, a combined failure at high leakage rates resulted in high airspeed loads potentially overpowering the actuator, the NTSB said.

Sikorsky Challenges Claims
In an official comment, Sikorsky challenged this claim. When a servo was deliberately subjected to stresses and fluid loss consistent with the alleged conditions of the accident, it performed perfectly, Sikorsky asserted to Finnish daily newspaper Helsingin Sanomat.

According to Ader, servo actuators with a flaking plasma coating have been found on other S-76s, although the flakes were not as big as those found in the accident rotorcraft. An industry source familiar with S-76 operations told AIN that servo actuators are regularly recycled during overhauls. This involves stripping the piston and adding new plasma coating. As a result, the thickness of the coating increases. Therefore, the more the actuator has been recycled, the bigger will be any flakes that form.

Copterline had bought the servo of the accident helicopter from Sikorsky subsidiary Helicopter Support (a co-defendant in the lawsuit) in August 2003.

One year ago, in response to an NTSB recommendation, the FAA issued a notice of proposed rulemaking (NPRM) relating to the S-76’s main rotor servo. Should the NPRM be adopted, the subsequent airworthiness directive would require the inspection of all main rotor servo actuators on all models of the S-76. The AD would require reducing the servo’s TBO and target a high rate of leakage and contaminated hydraulic fluid, which the FAA said could lead to degraded ability to maneuver the cyclic and collective controls and result in a loss of control.

Sikorsky declined to elaborate on the lawsuit. A spokesman told AIN that the company “will respond to the lawsuit through the court within the required time frame.

”Nevertheless, it seems negotiations between the two parties are ongoing. Copterline’s Web site states, “Copterline is currently negotiating with the manufacturer of S-76 helicopters.” Kari Ljungberg, managing director of the Finnish-based operator, declined to comment.

In its first official comment to the NPRM, Sikorsky stated that “the AD NPRM should be withdrawn.” However, on December 21, the manufacturer issued an alert service bulletin affecting all S-76s and focusing on main rotor servos. The service bulletin requests that operators perform additional inspections. Most checks have to be carried out within 50 flight hours. “Main rotor assemblies that exceed the maximum leakage rate as specified in the maintenance manual are to be removed from service prior to next flight,” the alert reads.

In September, Copterline sold its last S-76. It now operates seven EC 135s.