What happened
On 10 November 2001, a Eurocopter AS332L Super Puma, registration G-BKZE, landed on the helideck of the West Navion drilling ship, located approximately 80 nautical miles west of the Shetland Islands. The aircraft was undergoing refueling with its rotors still running. While the commander remained inside the cockpit, the co-pilot disembarked to assist the ship's crew.
During this period, the vessel's Dynamic Positioning (DP) system unexpectedly reverted to manual heading control. This change occurred without the knowledge of the helicopter crew or the personnel on the ship's bridge. As a result, the West Navion began to drift slowly to the right. At the time, the wind was westerly at 32 knots, with gusts reaching 42 knots. As the ship's heading shifted, the helicopter was increasingly exposed to a lateral wind component.
Approximately seven minutes after the ship's heading began to drift, the helicopter toppled onto its right side. The impact caused the main rotor blades to strike the helideck and fragment. The co-pilot, who was outside the aircraft, sustained severe injuries from flying debris. The commander managed to evacuate the aircraft through the left door, though the process was complicated by fire-fighting foam entering the cockpit.
The investigation
The AAIB investigation examined the vessel's DP system, the helicopter's flight data recorder, and mathematical models of the forces involved. Investigators established that the aircraft's maintenance and flight crew qualifications were in order, and there were no mechanical defects found in the helicopter itself.
Analysis of the flight data revealed that the helicopter had landed with a slight roll of 2.5 degrees to the right. The investigation also noted that a similar failure of the DP system had occurred on the same vessel just one month prior to this accident. Mathematical modeling confirmed that the primary force responsible for the toppling was the aerodynamic pressure generated by the increasing lateral wind as the ship yawed.