History of the Flight
The pilot and observer arrived at Cumbernauld Airport on the morning of 21 December 2005 for a standard pipeline inspection flight. They started the aircraft and lifted off at 0922 hrs, departing along Runway 26 and turning right to cross the airport boundary heading northeast. No further radio transmissions were received from the aircraft. Onboard GPS equipment recorded their route, which closely followed a gas pipeline heading approximately northeast. Their airspeed varied between 100 and 120 kt, and radar showed their height between 500 and 1,000 ft above ground level.
Just prior to the accident, the aircraft was seen to enter a gentle descent. As it passed through approximately 100 ft agl, a light-colored part of the rear section fell from the helicopter. The helicopter then began a right-hand turn, rolled onto its left side, and pitched nose-down into the ground. Debris exited the helicopter before impact. Both occupants sustained fatal injuries. The wreckage trail extended for several hundred metres, with the vertical fin, tail rotor assembly, and gearbox among the earliest items found along the flight path.
Engineering Findings
The aircraft wreckage was recovered to the AAIB facility at Farnborough for detailed examination. This revealed that the vertical fin had detached from the aircraft in flight. The lower part of the fin entered the tail rotor arc, removing the "stinger" at the base of the fin and damaging the rotor blades. The tail rotor and its gearbox were torn from their mountings shortly after the fin loss.
The fin was attached to the tail boom by four bolts that locate into holes in two fin supports at the front and rear of the tail rotor gearbox platform. On G-WLLY, this platform was of fabricated sheet metal construction. The fin supports were machined forgings; the rear support was riveted in position, forming the rearmost frame of the tail boom. The front support was bolted to the structure. A later design exists with an integral, one-piece forging for the platform and fin supports.
Metallurgical examination of the fin supports revealed fatigue in the fractures through and around all four bolt holes. The most extensive fatigue was found in the upper rear support. Corrosion was evident on many fracture faces, indicating the fatigue cracks had been developing over a period of time, although the duration could not be quantified.
The Aircraft Maintenance Manual requires a visual inspection of the fin supports every 100 hours. Dye penetrant is not normally used, nor is the fin removed. A typical inspection involves removing the tail rotor gearbox fairing to access the inboard faces of the fin supports. The investigation has not revealed an extensive history of cracking in Bell 206 fin supports. The UK Civil Aviation Authority recorded a crack in a rear support during a visual inspection in March 1977, and the manufacturer reported one occurrence of fin support failure in the Gulf of Mexico in April 1991.
It is unclear whether the fatigue damage on G-WLLY is unique to this helicopter or could affect others of the same type. To gather information and protect the fleet, the AAIB proposed a one-off inspection with the fin removed to obtain adequate access to the supports and bolt holes. Reassembly should pay particular attention to the torque requirements specified in the Aircraft Maintenance Manual.
Safety Recommendations
Safety Recommendation 2006-039: The UK Civil Aviation Authority was recommended to require a one-off inspection, within a reasonable timescale, of the vertical fin supports of all Bell and Agusta-Bell 206 series helicopters on the UK register. The inspection should be conducted with the fin removed.
Safety Recommendation 2006-040: Transport Canada, the European Aviation Safety Agency, and the US Federal Aviation Administration were each recommended to consider requiring a similar one-off inspection of the vertical fin supports of all Bell and Agusta-Bell 206 series helicopters within their jurisdictions.
Further investigation will continue to establish the cause of the fatigue crack initiation and why it was not detected prior to the catastrophic fin detachment. A full report will be published in due course.