What happened
On 16 July 2002, a Sikorsky S-76A+ helicopter, registration G-BJVX, crashed in the North Sea near the Leman 49/26 Foxtrot platform. The aircraft was operating in the Leman Offshore Gas Field when a catastrophic failure occurred in one of its main rotor blades. The failure of the blade led to the loss of the aircraft.
The investigation
Investigators examined the wreckage and the specific main rotor blade that had failed, which featured a yellow colour-coded identification. Metallurgical analysis of the titanium spar revealed that a fatigue crack had initiated and propagated through the component.
Researchers conducted experiments using a representative blade to study how crack growth affects blade droop. By creating various simulated cuts in a blade spar, they determined that a significant change in blade droop—a measurable physical indicator of structural compromise—only became apparent when the crack reached a certain length. This specific length corresponded to a timeframe shortly before the accident, suggesting that the crack might have been difficult to detect during standard visual inspections conducted the previous evening or during the pre-flight check.
Analysis of the fracture surface through striation counting allowed investigators to estimate the crack's progression. The data suggested that the crack growth period was relatively short, with an estimated time of approximately 7.3 hours for the total crack growth, though the final period of rapid propagation leading to failure was estimated at roughly 2.4 hours.
Findings
- The accident was caused by the catastrophic failure of the main rotor blade spar due to a fatigue crack.
- The fatigue crack initiated in the titanium spar and propagated through the component.
- The crack growth rate accelerated significantly in the final hours of operation.
- The physical deformation (droop) of the blade caused by the crack was not detectable via visual inspection until the crack had reached a critical size shortly before the flight.