What happened
On 18 January 2010, an Agusta A1obE, registration G-TYCN, was conducting a private flight in Dorset. After departing a private field near Blandford Forum to refuel at Henstridge, the pilot returned to the original site. The aircraft's takeoff weight for the return leg was calculated to be 1 kg below the maximum allowable limit of 2,850 kg.
During the approach to the private field, the pilot began a right-hand turn to intercept an approach angle similar to the one used an hour earlier. At approximately 100 ft agl, the pilot observed a marked increase in the rate of descent. Efforts to arrest this descent by increasing collective pitch and raising the aircraft's nose provided only marginal relief. As the aircraft neared the ground, the pilot applied maximum torque and leveled the aircraft attitude.
The helicopter made a heavy touchdown and briefly bounced into the air. Upon bouncing, a rapid yaw to the right developed, which could not be corrected via the anti-torque pedals. The pilot believed the tail rotor had struck the ground. After a second touchdown, the aircraft rotated approximately 250° before coming to rest. There were no fatalities and no injuries to the two occupants.
The investigation
The investigation examined the flight conditions and the aircraft's performance during the final stages of the approach. The investigators noted that while the pilot had experienced intermittent 10 kt gusts from the south during the earlier flight, these were not present during the landing. The investigation focused on the aerodynamic state of the main rotor during the high rate of descent.
Findings
- The aircraft entered a vortex ring state during the final stages of the approach.
- The conditions for this state were facilitated by the high mass of the aircraft and a slight tailwind.
- The high rate of descent prevented the pilot from having sufficient power available to arrest the descent using standard recovery techniques.
- The aircraft sustained damage to the landing gear, tail rotor, tail rotor gearbox, and the left horizontal stabiliser.