History of the Flight
The helicopter was returning to its base after a training exercise at Sandtoft Airfield. The destination was a private landing site at Hollis Farm. The flight was conducted with the instructor acting as both the commander and the handling pilot. The aircraft was brought to a hover and began to manoeuvre towards the landing site. Having turned through 180°, the pilot proceeded to hover-taxi the helicopter when, according to the pilot's report, it began to "vibrate, shake and judder" and turn of its own accord. The pilot reacted quickly by landing the helicopter immediately and shutting down. Both occupants were uninjured and vacated the aircraft without difficulty. On inspection, the pilot observed that the tail rotor gearbox was missing and the empennage, although in one piece, was almost completely detached.
Wreckage Examination
The empennage, including the upper, lower and horizontal stabilisers, was almost detached from the helicopter. Damage to the lower stabiliser was consistent with it having been struck by one of the tail rotor blades whilst they were rotating. The tips of both blades had detached. The rear portion of the tail rotor guard had also separated and was found in several pieces; damage to its tubular construction was consistent with it having been struck from beneath in the area where it attaches to the lower stabiliser. The tail rotor had become detached from the associated casting in the rear end of the tail boom. A metallurgical examination showed that none of the four attachment bolts had fractured. Three of the attachment lugs had fractured by overload bending and the fourth by a low‑cycle, high‑peak, cyclic stress, simple bending fatigue mechanism. It is probable that this fourth lug was the first to separate. It was concluded that all the damage resulted from the tail rotor blades being struck.
Discussion
The helicopter had come to rest at the bottom of the field on an easterly heading; however, the tail rotor debris had been collected before AAIB examination of the site and no ground marks were evident to indicate where the tail had struck. During sloping ground operations, the tail rotor is potentially vulnerable as it is some distance behind the pilot.