What happened
On 12 December 1997, a Eurocopter AS 332L, registration G-BWZX, was performing a public transport flight near the Sovereign Explorer oil rig. After landing on the rig to refuel, the aircraft departed at 1245 hrs, climbing to 3,000 feet on a south-easterly track. The flight was occurring amidst heavy showers, though the crew had noted no prior indications of lightning activity, such as radio interference or ADF needle fluctuations.
At 1253 hrs, the aircraft was suddenly struck by lightning, accompanied by a bright flash and a loud bang. The strike immediately caused a severe one-per-revolution vibration and noise. The crew quickly identified that the main rotor blades had been damaged. The handling pilot initiated a 180-degree turn back toward the Sovereign Explorer, while the crew prepared the nine passengers for an emergency landing. Despite the intense vibration, the aircraft remained controllable, and an incident-free landing was completed at 1301 hrs.
The investigation
The AAIB investigation focused on the extent of the damage to the composite rotor blades and the electrical path of the strike. Examination revealed that all four main rotor blades suffered extensive damage. Specifically, two diametrically opposite blade tips lost approximately 40% of their surface area. The brass conductive strips, designed to protect the blades by providing an electrical path to the rotor head, had completely vaporised on the two most severely affected blades. This caused electrical arcing between the blades and the rotor head.
Investigators also analyzed flight recorder data. While the crew's statements provided the primary sequence of events, the aircraft's Combined Voice and Flight Data Recorder (CVFDR) failed to record the return leg of the flight. This was due to a G-switch in the power supply that triggered during the intense physical shock of the strike, cutting power to the recorder. Meteorological analysis using SFERICS and EA Technology data suggested the strike was an isolated event, as no preceding lightning activity was detected in the immediate vicinity of the aircraft.
Findings
- The aircraft was struck by an isolated lightning strike of extreme severity.
- The electrical discharge was significantly more powerful than the aircraft's original certification standards, reaching levels consistent with modern, more demanding civil requirements.
- The strike caused the vaporization of the brass earth-path strips on two blades, leading to secondary damage through arcing and delamination.
- The aircraft's stability during the return flight was maintained because the mass loss from the blade tip damage was symmetrically distributed on opposite sides of the rotor head, preventing extreme imbalance.
Safety action
- The AAIB issued Safety Recommendation 99-24, requesting that the CAA reassess the use of G-switches in CVFDR power supplies, as their premature operation can prevent investigators from obtaining critical data following an incident.