Background
On 21 July 2018, a Robinson Helicopter Company R44 Raven II helicopter, registered ZK-HTB, was prepared for a private flight from Wanaka Aerodrome to a lodge in Upper Estuary Burn Valley on the northern shore of Lake Wanaka. The pilot was the sole occupant. The helicopter had been flown earlier that day with passengers and cargo, and a cargo pod on the right side was removed before the accident flight.
Narrative
The pilot began pre-flight inspection around 1230, as recorded by closed-circuit television. After loading the helicopter and refuelling, the pilot started the engine and loaded additional items. At 1257, the pilot radioed departure from the owner’s base and stated an intention to fly to Dublin Bay, a reporting point northwest of Wanaka.
A flight tracking system transmitted position reports every three minutes. The first report at 1300:25 showed the helicopter east of Albert Town at an altitude of 1,772 feet and groundspeed of 86 knots. The final report at 1303:25 placed the helicopter 500 meters south of Stevensons Island at 1,454 feet and 96 knots groundspeed.
At about 1308, a person monitoring the helicopter’s progress noted the lack of a recent position report. After attempts to contact the pilot failed, the helicopter was declared missing. A search was initiated, and an oil slick was observed about 500 meters northwest of Stevensons Island. Wreckage and cargo were found washed ashore on Stevensons Island.
The helicopter and the deceased pilot were recovered from the lake floor on the night of 23-24 July. The wreckage was located at a depth of 47 meters, approximately 500 meters north-northwest of Stevensons Island. About 95% of the helicopter by weight was recovered. The tail section was attached only by electrical cabling when lifted.
Investigation Findings
The Transport Accident Investigation Commission determined that the helicopter likely encountered unexpected turbulence of sufficient magnitude to result in an in-flight breakup. The helicopter’s speed at the last reported position may have increased the risk of an adverse outcome in the mountainous environment.
The investigation also noted that inquiries into loss-of-control or mast bumping accidents involving Robinson Helicopter Company helicopters are hampered by a lack of data and understanding of main rotor performance in adverse conditions.
The Robinson R44 Pilot Operating Handbook used the non-standard term “significant” to describe turbulence without explanation. In June 2020, the manufacturer amended Safety Notice 32 to define the term, aligning with a safety video.