Investigation Summary
The accident occurred as the Super Puma was navigating through a line of cumulus clouds. Investigators determined that the helicopter likely triggered a lightning strike by flying through a highly charged area of the atmosphere. The bolt struck the tail rotor, which was constructed from carbon fiber reinforced plastic (CFRP) with a brass lightning protection strip. The investigation found that the lightning current did not follow the intended path along the protection strip; instead, it entered the composite structure, causing the internal resins to vaporize instantly. The resulting internal pressure caused the rotor blade to explode and delaminate. The crew initially felt a massive jolt and heard a bang, followed by severe vibrations. As they attempted to maneuver for an emergency landing, the tail rotor assembly failed completely, necessitating an immediate autorotation. The pilot managed to level the aircraft just before impact with the water, and the emergency flotation bags deployed successfully, keeping the airframe buoyant long enough for a full evacuation.
Final Conclusions
The Air Accidents Investigation Branch (AAIB) concluded that the primary cause was a lightning strike that resulted in the catastrophic failure of the tail rotor. The design of the tail rotor blades, while meeting contemporary certification standards, was found to be inadequate for protecting the composite material from the high-energy discharge of a triggered lightning strike. The report noted that the brass bonding strips were insufficient to prevent the lightning from penetrating the carbon fiber skin. The successful outcome was attributed to the crew's professional handling of the emergency and the effectiveness of the helicopter's flotation equipment and the passengers' survival suits.
Video Analysis
Damaged Tail Rotor Causes Bristow Helicopters Flight 56C To Crash Land | Mayday — A look at how lightning damaged the tail rotor of a Super Puma over the North Sea, forcing an emergency ditching and rescue operation.
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Airframe & Maintenance
Crew Experience
Captain
Cedric Roberts, aged 44.
First Officer
Lionel Sole, aged 39.
Systems & Failure Modes
Tail Rotor
The tail rotor provides antitorque and directional control; its failure in this incident led to an uncontrollable spin that required an immediate ditching.
Emergency Flotation System
Inflatable bags designed to keep the helicopter buoyant after a water landing, which functioned correctly and allowed the crew and passengers to exit safely.
Lightning Protection System
A series of brass strips intended to conduct lightning strikes away from the composite structure; the system failed to prevent damage to the rotor's internal resin.
Interesting Facts
- 01All 18 people on board survived the ditching and subsequent rescue.
- 02The lightning strike was likely 'triggered' by the presence of the helicopter in a charged cloud.
- 03The tail rotor blades were made of carbon fiber composite with brass lightning protection strips.
- 04The rescue was conducted in sea conditions featuring waves approximately 2 meters high.
- 05The helicopter was named 'Cullen' and had been in service since the 1980s.
Safety Actions & Advisories
Rotor Blade Design Review
The investigation led to a re-evaluation of lightning protection systems on composite rotor blades, resulting in design changes to better conduct high-voltage strikes.
Weather Radar Procedures
Recommendations were made to improve pilot training regarding the avoidance of localized, highly charged cumulus clouds that may not appear severe on standard weather radar.