What aircraft was involved and where did it happen?

The accident on 1999-03-27 involved a AGUSTA A109A, registration G-USTA, at Hurstpierpoint, Sussex, GB.

What was the probable cause of the accident?

The tail rotor blade failed due to a fatigue crack caused by stress concentrations at the end of the bonded reinforcement, leading to blade separation and subsequent loss of tail rotor control.

Tail Rotor Failure Leads to Heavy Landing of Agusta A109A

A private Agusta A109A helicopter experienced a sudden tail rotor separation during final approach, resulting in a heavy landing near Hurstpierpoint.

What happened

On 27 March 1999, an Agusta A109A, registration G-USTA, was performing a private flight near Hurstpierpoint. During the final approach to landing, while at approximately 30 feet altitude and flying at low speed, a loud bang was heard, followed by a rapid rightward rotation of the aircraft. The pilot, perceiving a loss of tail rotor authority, lowered the collective to mitigate the rotation rate. The helicopter subsequently performed approximately two rotations before a heavy landing occurred. While the aircraft remained upright, the impact caused the landing gear to be driven upwards and deformed the underside of the fuselage. The pilot and one passenger were able to exit the cabin, and neither occupant suffered significant injuries.

The investigation

Investigators found that the tail rotor assembly and 90° gearbox had been torn from the aircraft. The separation occurred while the helicopter was still airborne, with the gearbox and parts of the tail rotor blade (TRB) found along the approach path. /nDetailed metallurgical examination of the failed TRB revealed a chordwise fatigue fracture on the outboard-facing skin of the blade. The fracture was located near the end of a bonded reinforcing doubler. The investigation established that the gearbox had detached due to the extreme out-of-balance forces generated by the blade separation.

Records indicated that a dye penetrant inspection had been performed on the blades 47 flight hours prior to the accident. However, investigators found no evidence of dye residue on the fracture surfaces. It was suggested that during the previous inspection, a slurry of dissolved lacquer might have seeped into the crack, effectively sealing it and preventing the dye from penetrating the defect.

Findings

The primary cause of the accident was the separation of the tail rotor blade due to a fatigue crack.
The fatigue crack originated from stress concentrations at the point where the bonded doubler terminated.
The fatigue crack was likely present and potentially detectable during the last recorded dye penetrant inspection.
The effectiveness of the previous inspection may have been compromised by the paint removal process, which potentially occluded the crack with dissolved lacquer residue.