What happened
On 16 July 2002, a Sikorsky S7lam S76A (registration G-BJVX) was performing a scheduled passenger transport flight in the southern North Sea. After completing several successful sectors between gas installations, the aircraft departed the 'Clipper' platform to transfer a passenger to the 'Global Santa Fe Monarch' drilling rig.
While cruising at approximately 320 feet, witnesses on the drilling rig heard a loud explosion. The aircraft was observed diving steeply into the sea, with one witness reporting seeing the main rotor head and blades separate from the fuselage before impact. The crash resulted in 10 fatalities (two crew and eight passengers) and one person remained missing. The aircraft was completely destroyed.
The investigation
Investigators recovered the aircraft's combined voice and flight data recorder from the seabed. Audio analysis revealed that the crew noted an increase in vibration roughly four and a half minutes into the final flight sector. The crew performed a rotor track and balance procedure, but the vibration intensified shortly before the recording ended abruptly with unusual, structure-borne sounds.
Examination of the wreckage revealed that the main rotor gearbox and rotor head had broken away from the fuselage. While three main rotor blades remained relatively intact, the fourth blade had fractured. Metallurgical analysis of the fractured blade identified a fatigue failure in the titanium spar. This failure originated at a point where a manufacturing anomaly—a folded edge in the titanium erosion strip—had caused the strip to press against the spar. This contact point also showed evidence of thermal damage.
Findings
- The primary cause of the accident was a fatigue fracture of the main rotor blade's titanium spar.
- A manufacturing defect, present since the blade's production 21 years prior, created a localized area of increased thickness in the erosion strip, leading to contact with the spar.
- A lightning strike sustained by the blade in 1999 likely exploited this existing manufacturing anomaly, causing thermal damage that served as the initiation point for the fatigue failure.
- The subsequent separation of the outer portion of the blade created a severe imbalance, which caused the main rotor gearbox to detach from the aircraft's fuselage.