What happened
On 21 December 2005, a Bell 2D6B Jet Ranger II, registration G-WLLY, was conducting a routine pipeline inspection flight near Coupar Angus, Tayside. The flight was part of a mission to transport attachment supports between Cumbernauld Airport and Aberdeen. Approximately 45 minutes after takeoff, the aircraft began a descent at low altitude. Witnesses observed debris falling from the rear of the helicopter, and noted that the aircraft was missing its tail rotor and vertical stabiliser.
Following the loss of these components, the helicopter entered a right-hand turn and began to roll onto its left side. The aircraft then pitched nose-down and struck a field at a steep angle. The impact resulted in two fatalities, involving both the pilot and the observer. The helicopter was destroyed in the accident.
The investigation
The AAIB investigation focused on the sequence of structural failures. Examination of the wreckage trail revealed that the vertical fin, tail rotor assembly, and gearbox had separated from the tail boom during flight. The investigation established that the vertical fin detached first, subsequently striking the tail rotor. This impact caused the tail rotor and its associated gearbox to also become detached from the tail boom. The loss of approximately 20 kg of mass from the rear of the aircraft caused the centre of gravity to shift forward beyond controllable limits.
Metallurgical analysis of the fin supports conducted by QinetiQ revealed fatigue cracks at all four attachment points. The cracks originated from the outboard edges of the bolt and rivet holes. The investigation also noted that the torque levels found on the attachment fasteners were significantly below the manufacturer's specified values.
Findings
- The primary cause of the accident was the in-flight detachment of the vertical fin due to fatigue cracking in the fin supports.
- The fatigue cracks were driven by a lack of sufficient assembly torque in the fin attachment fasteners, which reduced the clamping force and structural rigidity.
- The loss of the vertical fin caused the tail rotor to strike the fin, leading to the secondary detachment of the tail rotor and gearbox.
- The subsequent loss of mass and directional stability caused the centre of gravity to move outside controllable limits, leading to the fatal impact.