What happened
On 11 August 2008, a Kawasaki-Hughes 369D helicopter, registration ZK-HWE, departed from Haast to assist with Department of Conservation track maintenance. The flight carried the pilot, three track workers, and approximately 150 kilograms of equipment.
The mission involved a landing at Maori Saddle Hut, a location characterized by tall trees that limited available approach paths. During the final approach, the aircraft encountered tailwinds ranging from 11 to 21 knots. As the pilot transitioned into an out-of-ground-effect hover, the helicopter experienced an uncommanded yaw to the right. Despite efforts to regain control, the aircraft struck a tree and crashed. While there was no post-crash fire, the pilot and one passenger sustained serious injuries.
The investigation
Investigators examined the mechanical state of the aircraft and the pilot's preparation. The engine and flight controls were found to be functioning normally, and there was no evidence of a technical malfunction. While the aircraft was heavily loaded, it was not exceeding its maximum weight, and performance charts indicated it had sufficient capability to maintain a hover out of ground effect at that altitude.
However, the investigation highlighted several lapses in flight preparation. The pilot was found to have been complacent regarding cargo oversight, failing to personally verify the accuracy of the load weight. Additionally, the pilot did not ensure that passengers were properly briefed on safety procedures or that shoulder harnesses were accessible and being worn.
Findings
The investigation concluded that the crash was caused by a loss of tail rotor effectiveness (LTE). This phenomenon occurred because the approach was conducted under specific environmental conditions identified in the flight manual as high-risk for LTE.
Several contributing factors converged to trigger the uncommanded yaw:
- The presence of weathercock stability during the approach.
- The loss of translational lift caused by the downwind approach.
- The high power demand required as the helicopter transitioned into a hover.