Rotor-blade defect blamed in S-76 crash

 - January 30, 2007, 7:23 AM

British investigation authorities implicated a manufacturing defect in the July 16, 2002, crash of a Sikorsky S-76 into the North Sea. Operated by Shell, the helicopter, G-BJVX, was completing a 13-minute leg between two rigs when one rotor blade failed, leading to separation of the entire rotor system. The two crewmembers and nine passengers died in the crash.

According to the accident report issued by the Air Accidents Investigation Branch of the UK Department for Transport, witnesses saw the crew complete a full walkaround before start-up and departure from Norwich that appeared normal to all passengers and helideck crews who were interviewed.

Seven minutes before the end of the CVFDR (combined voice and flight data recorder) recording, the copilot, who had 420 hours in type and who was flying the sector, remarked, “Got quite a vibration there.” The commander, who had more than 2,300 hours in type, agreed and said that he would request a rotor track-and-balance check. Six seconds later the copilot said, “Out of track.” The commander agreed, but there was no evidence from the recording that the increased vibration was causing either pilot any concern.

Three minutes later the crew began the approach checks for arrival at the rig, and there were some routine radio exchanges between the helicopter and the rig. The last second of the recording contained two loud cracks. When the recording stopped, the aircraft was flying with a one-degree nose-up pitch and banked 20 degrees to the right. The IAS was 94 knots and the helicopter was 323 feet above the sea.

No one on the rig was watching, but witnesses heard a muffled bang. Turning to look, they saw the aircraft tumbling with the fuselage intact. Rig personnel found no survivors. Five bodies were recovered from the sea and five were recovered from the wreckage later. One body was not recovered.

Rotor Blade Construction
The wreckage was spread over an area approximately 970 feet by 100 feet, and 97 percent of it was recovered. The first item of significance was the main rotor gearbox with the rotor head and most of the rotor blades. Three of the rotor blades were only superficially damaged, but the fourth blade was substantially damaged, with the outboard 14 feet missing. This missing section has not been found.

The rotor blade construction consists of a welded titanium spar that provides the principal load-carrying capability, and the airfoil section is produced by use of a Nomex honeycomb trailing-edge section with a filler leading edge enclosed in a titanium leading-edge erosion cover. There is a skin of cross-plied woven fiberglass with graphite trailing-edge reinforcing strips and integrated aluminum mesh to provide lightning protection.

Lightning Strike Damage
The failed blade was made in March 1981 and had flown 9,665 hours since manufacture. On Nov. 17, 1999, while fitted to a different S-76, the rotor blade received a lightning strike that caused two other blades from the same rotor system to be scrapped. In June 2001 the repaired blade was fitted to G-BJVX, and at the time of failure it had flown just over 1,404 hours since the strike.

Investigation established that a scarf joint tang had been accidentally distorted during manufacture. This did not affect the blade integrity until, during the lightning strike, the tang allowed an electrical discharge to pass between it and the spar. This caused enough heat to change the metallurgical properties of the spar, albeit in a region less than two millimeters wide.

Investigators believe a fatigue crack started during the final 100 hours of flight and may have grown from an initial through-crack to affect 50 percent of the spar in as little as 24 flight hours. There would have been no external indication of cracking. All S-76 blades that have been struck by lightning and returned to service after repair have been withdrawn.