What happened
On 17 November 1999, a Sikorsky S76A (Modified), registration G-BHBF, was performing a public transport flight from Norwich to the Clipper offshore gas field. While cruising at 2,000 feet within instrument meteorological conditions, the aircraft was struck by lightning. Shortly after the strike, the crew detected the smell of burning. In response, the pilot initiated a rapid descent to clear the cloud base and returned to Norwich for a visual approach. The landing was completed without further incident, and there were no injuries to the two crew members or three passengers on board.
The investigation
An investigation by the AAIB examined the aircraft for damage and assessed the meteorological conditions. The inspection revealed widespread light to medium lightning damage across several key areas, including strike marks on all four main rotor blades, the tail-rotor paddle assemblies, and the upper surface of the horizontal stabiliser. Investigators also identified arcing and burning on the main rotor jumper leads and the bifilar arm.
Using a tangential field meter, investigators found excessive residual magnetism in numerous critical components, such as the main rotor spindles, the main-rotor drive shaft, engine exteriors, and various gearbox flanges and supports. Due to the risk of arcing damage between bearings, all tail-rotor shaft bearings were replaced. The horizontal stabiliser was deemed beyond economic repair and scrapped. Because many major components lacked established repair procedures or effective inspection techniques for lightning-induced damage, most were replaced with new or overhauled parts.
Findings
- The aircraft was struck by a low-magnitude, negative, airborne lightning strike, which was likely triggered by the aircraft itself.
- Existing lightning warning systems were unable to detect or provide warning for this specific type of airborne strike.
- While the damage was not severe enough to pose an immediate hazard during the short flight back to Norwich, it was sufficient to compromise the long-term airworthiness of the helicopter.
- Meteorological conditions at the time included an unstable airstream and thunderstorms in the Norfolk area.