History of the Flight
The aircraft, a Sikorsky S76A (Modified) registered G-BHBF, was on a public transport flight from Norwich to the offshore Clipper gas field. While cruising at 2,000 feet in instrument meteorological conditions (IMC), just above the cloud base, it was struck by lightning. Approximately 30 seconds later, the crew noticed a smell of burning. They rapidly descended below cloud and returned visually to Norwich. The landing was uneventful. No injuries were reported among the two crew and three passengers.
Aircraft Examination
Initial examination revealed lightning damage to all four main rotor blades, two main rotor tip caps, both tail-rotor paddle assemblies, and the upper surface of the horizontal stabiliser. Arcing or burning was observed on main rotor jumper leads and a bifilar arm. A tangential field meter detected excessive residual magnetism in numerous components, including all four main rotor spindles, main-rotor drive shaft, swashplate guide, exterior surfaces of both engines, No 2 engine drive shaft, main gearbox tail rotor drive shaft flange, tail-rotor drive shaft bearing supports, intermediate gearbox flanges, red and yellow tail rotor pitch rods, one tail rotor paddle, horizontal stabiliser support, and tailcone attachment bolts.
Further inspection followed manufacturer advice to identify subtle damage. All tail-rotor shaft bearings were replaced due to high residual magnetism indicating possible arcing. The horizontal stabiliser was scrapped as beyond economic repair. Many major components affected were not returned to service; new or overhauled parts were used instead, as repair procedures or effective inspection techniques for lightning events were often unavailable.
Aerodynamic Component Structural Composition
The main rotor blades have a titanium structure with woven fibreglass skin, nomex honeycomb, and graphite trailing-edge strips. A wire-mesh system provides lightning protection. Tail-rotor blades use fibreglass skins with aluminium alloy mesh protection, with fibreglass/nomex and aluminium honeycomb cores. The horizontal stabiliser has a kevlar skin over kevlar honeycomb core with a spar of kevlar wrap over aluminium honeycomb and graphite epoxy straps.
Meteorological Data
Meteorological data indicated an unstable north-northwesterly airstream with a band of showers over Norfolk. Cumulonimbus cloud and a thunderstorm with rain were observed at Coltishall at the time of the strike (1450 UTC). The crew reported an outside air temperature of 0.5°C. Neither the Met Office's Arrival Time Delay system nor the Lightning Location System recorded data for that time and position, both operating correctly. The nearest automated lightning data was a very weak indication over 20 km away.
Summary
It was concluded that the aircraft was struck by a relatively low magnitude, negative, airborne lightning strike, probably triggered by the aircraft. Existing lightning warning systems could not warn of such events. The damage was not a severe hazard for the short flight back to Norwich, but would have impaired longer-term airworthiness, making further flight unwise without inspection and component replacement.