Overview
On Monday, 31 October 2016, a Robinson R44 helicopter registered ZK-HTH was conducting aerial spraying operations in Glenbervie Forest, north of Whāngārei. The helicopter was being used to perform a pre-spraying survey with a forestry contractor to verify block boundaries and identify hazards.
Accident Sequence
The helicopter was flying away from the survey blocks and had just crossed a ridge when it crashed into dense bush and caught fire. The aircraft was destroyed, and both occupants sustained fatal injuries.
Investigation
The Transport Accident Investigation Commission (TAIC) opened an inquiry on 1 November 2016. Investigators visited the site that day, assisted by the Royal New Zealand Air Force and later by a representative from Robinson Helicopter Company. The wreckage was removed on 3 November 2016 and taken to TAIC's workshop in Wellington for detailed examination. A metallurgical examination of main rotor control system components was commissioned on 8 November 2016.
Site and Wreckage Examination
The wreckage was confined to a compact area around the initial impact point. Parts of the landing skids and spray booms detached as the helicopter fell through trees. The damage indicated a high rate of descent and low forward speed at impact. An intense fire destroyed most of the fuselage forward of the tail boom, including the main rotor transmission housing. The fire did not spread beyond the main wreckage. The tail assembly detached as a unit from the boom; the horizontal stabilizer showed crushing consistent with striking a tree.
Both main rotor blades were attached to the hub and undamaged by fire. Outboard sections of both blades separated near a tree identified as the first impact point. Both pitch links were attached to the upper swashplate but disconnected from the main rotor blades: one broke at the upper rod-end thread, and the other was missing its upper attachment bolt.
Key Findings
The damage and lack of incontrovertible evidence meant the accident cause could not be determined. A missing bolt attaching a pitch link to a pitch horn on one main rotor was virtually certain to have come out during the impact sequence and did not contribute to the accident. It was not possible to determine why a cyclic extension for the left seat was installed or if it was in use at the time.
Mast Bumping Consideration
TAIC noted its concern regarding mast bumping in Robinson helicopters, which it added to its watchlist in October 2016. However, based on the confined wreckage field and lack of blade strikes on the cabin, skids, or tail boom, mast bumping was deemed very unlikely to have caused the accident.
Conclusion
The investigation could not establish a definitive cause for the crash. No new safety recommendations were made, though TAIC reiterated a previous recommendation for on-board recorders on certain helicopter classes to aid future investigations.