What happened
On 21 December 2005, a Bell 206B helicopter, registration G-WLLY, departed Cumbernauld Airport to perform a routine pipeline inspection. The flight, operated by a crew of two, followed a north-easterly path alongside a gas pipeline at altitudes between 500 and 1,0-00 feet.
During the mission, radar observations indicated the aircraft entered a gentle descent. As it reached approximately 100 feet above the ground, a light-colored component from the rear of the helicopter separated. This triggered a right-hand turn and a roll onto the left side, followed by a nose-down impact. The crash resulted in two fatalities, involving both the pilot and the observer. The wreckage trail was extensive, with the vertical fin, tail rotor assembly, and gearbox being among the first components to strike the ground.
The investigation
Engineers at the AAIB facility examined the wreckage and determined that the vertical fin had detached during flight. The lower portion of the fin entered the path of the tail rotor, causing the rotor blades to strike the fin and subsequently tearing the tail rotor and gearbox from their mountings.
Metallurgical analysis of the fin supports, which are attached to the tail boom via four bolts, revealed fatigue cracks in the fractures. These cracks were present in all four bolt holes, with the most significant damage found in the upper rear support. Evidence of corrosion on the fracture faces suggested that these cracks had been developing over an extended period.
Findings
- The primary cause of the accident was the in-flight detachment of the vertical fin.
- Fatigue and corrosion had weakened the fin supports, leading to structural failure.
- Standard visual inspections, as prescribed by the Aircraft Maintenance Manual, are insufficient to detect such issues because the required access to the inboard faces of the supports is blocked by the fin itself.
- The existing inspection protocol does not involve removing the fin or using dye penetrant processes that would reveal internal cracks in the bolt holes.