A malfunction of the flight-control system played a role in the 2005 fatal crash of a Sikorsky S-76 in Estonia, according to an interim report issued by Estonian investigators. The final report is scheduled to be released next spring.
In August 2005, an S-76C+ operated by Helsinki, Finland-based Copterline crashed into the Baltic Sea from an altitude of 1,400 feet shortly after takeoff on a scheduled flight between Tallinn, Estonia, and Helsinki, Finland, killing all 14 aboard. Weather was not a factor, according to investigators. Rather, a malfunction in the flight control system occurred in the third minute of the flight, at a speed of 130 knots.
According to the report, an uncommanded extension of the forward servo actuator caused a rapid and unusual change of the helicopter’s attitude. The cyclic control stick started to move and the collective control to raise. The pilot was unable to counter these strong movements using normal piloting techniques and physical effort. As a result, the helicopter became uncontrollable and the main rotor eventually stalled. The crew issued one mayday call.
The investigators determined the uncommanded extension was caused by the failure of the actuator’s second stage. Extensive inner leakage in both stages of the actuator contributed to the extension. Simultaneously, the actuator’s return ports jammed. Coating flakes had separated from the actuator’s piston head, where a copper-aluminum plasma coating had been applied a little more thickly than the maintenance manual suggests, investigator-in-charge Tonu Ader told AIN.
In this condition, a retraction command can result in an extension. This failure did not activate any cockpit warning. According to the interim report, “Sufficient diagnostic mechanism was not set up.” Flaking during the operation of the servos was not expected.
The FAA has yet to convert its 2006 notice of proposed rulemaking into an airworthiness directive (AD), as recommended by the NTSB. An FAA spokesman told AIN that the agency will probably issue a “supplemental” NPRM late this fall and, depending on the number of comments received, issue the final AD early next year.
“After releasing the NPRM, Sikorsky introduced an improved piston head to replace the current one,” the spokesman said. Such a replacement would be needed if the piston head was determined to be leaking or during the 3,000-hour overhaul. The FAA also received comments that led it to reconsider the leakage check interval. Moreover, the FAA is mulling the requirement for a hydraulic fluid contamination check. “Because we are now proposing to replace a part–increased burden/cost to the operator– we are required to issue a supplemental NPRM,” the spokesman explained to AIN. The SNPRM now proposes the replacement of the piston head and a leakage check at 1,500 hours and 2,250 hours time in service.
All models of the S-76 (A, B, C) would be affected. The AD will be the FAA’s response to a NTSB recommendation. The FAA stated that leakage and contamination could lead to degraded ability to maneuver the cyclic and collective controls.
The interim report also confirmed that the helicopter’s emergency floats did not operate. The emergency flotation system switch on the pilots’ overhead panel was found in the “off” position. “In the emergency flight situation, it is possible that the copilot had made an effort to activate the emergency flotation devices but did not succeed, possibly due to the high acceleration forces from the rotation and the changes in attitude,” Ader explained.
According to Finnish newspaper Helsingin Sanomat, Sikorsky paid damages to the next of kin of the 12 passengers in an out-of-court settlement. The OEM declined to comment. Copterline is suing Sikorsky.