Accident Overview
On 16 July 2002 at 1944 UTC, Sikorsky S76A (Modified), registration G-BJVX, crashed into the sea approximately 28 miles north-east of Cromer, Norfolk. The aircraft was on a public transport flight between gas installations in the North Sea. All 11 persons on board—2 crew and 9 passengers—sustained fatal injuries (8 passengers fatal, 1 missing; crew both fatal). The helicopter was destroyed.
The flight had been routine until the penultimate sector from the Clipper platform to the Global Santa Fe Monarch drilling rig. During approach at about 320 feet, workers on the rig heard a loud bang. The aircraft then dove steeply into the sea. Witnesses saw the main rotor head with blades separating and falling into the water after impact.
Search and Wreckage Recovery
The alarm was raised by the Monarch's radio operator. The standby vessel Putford Achilles launched rescue craft, recovering four bodies and debris. Over subsequent days, additional bodies and wreckage were recovered. The underwater debris field was located using sonar, radar, and the flight recorder beacon. More than 97% of the helicopter structure was recovered by divers from a depth of about 40 metres.
Flight Recorder Analysis
The combined voice and flight data recorder (CVFDR) was retrieved and its tape successfully processed. Audio recordings showed that about 4.5 minutes into the final sector, the crew discussed an increase in vibration. They performed a rotor track and balance procedure, which did not cause immediate concern. The recording ended abruptly with three unusual sounds. Flight data indicated level flight at 320 feet, 100 knots, heading 150° magnetic, but became corrupted two seconds before the end.
Wreckage Examination
Two significant findings emerged: three main rotor blades had superficial damage, but the fourth blade was fractured about 76.75 inches from the root, with the outer section missing. Additionally, the main rotor gearbox casing was fractured, and the gearbox together with the rotor head had separated from the fuselage mountings in flight. The blade fracture was identified as the initiating event; the separation of the blade section caused severe imbalance, leading to gearbox detachment.
Metallurgical Findings
Microscopic analysis of the fractured blade revealed clear fatigue on about half the circumference of the titanium spar. The fatigue initiation point was on the upper surface near the inboard edge of the scarf joint between the two-piece titanium leading edge erosion strip. This area showed intense thermal damage resembling an electrical spot weld. During manufacture, a manufacturing anomaly had occurred: the tip of the tang on the inboard end of the outboard erosion strip was bent and folded under, doubling the thickness and causing near contact between the erosion strip and the spar. The fatigue initiation point coincided with the rear point of the tang's fold line.
Rotor Blade History
The blade was manufactured in March 1981. In 1999, while fitted to another S76A (G-BHBF), it was damaged by a lightning strike. The blade was repaired by the manufacturer and returned to service. Neither the manufacturing anomaly nor the thermal damage to the spar were detected during that inspection. At the time of the accident, the blade had accumulated 9,661 hours; its life limit was 28,000 hours. The AAIB and manufacturer concluded that the lightning strike exploited the existing manufacturing anomaly and damaged the spar.
Safety Action
On 24 July 2002, the manufacturer issued Alert Service Bulletin (ASB) 76-65-55 to remove from service any main rotor blade identified as having been damaged by a lightning strike. A modified version (76-65-55A) was issued the next day, subsequently mandated by Emergency Airworthiness Directive 2002-15-51 on 26 July. Additionally, Safety Recommendation 2002-25 was issued concerning the continued airworthiness of other S-76 blades that might have similar hidden anomalies.