A Robinson R44 helicopter was operating at a high density altitude, and likely above its maximum gross weight, when it impacted rocky terrain shortly after take-off in Western Australia's Pilbara region, an ATSB investigation has found.
On 3 November 2022, the R44 was departing a cultural heritage site in the Collier Ranges, with a pilot and three passengers, who were members of a survey team, on board.
During the take-off, travelling about 27 kt and just above treetop height, the pilot experienced a severe drop in the helicopter's performance, and the low rotor RPM warning sounded.
"The pilot conducted the low rotor RPM recovery actions, but was unable to arrest the descent," ATSB Director Transport Safety Dr Stuart Godley said.
The helicopter impacted rocky terrain about 150-200 m from its take-off location, and rolled onto its left side, resulting in substantial damage, serious injuries to one passenger, and minor injuries to the pilot and other two passengers.
The ATSB's final report notes the take-off occurred at an elevation of 2,170 ft above mean sea level, with a calculated density altitude of 4,210 ft.
Compounding this, the pilot used estimated weights for the passengers, based on figures provided by one of the passengers. This resulted in the calculated helicopter gross weight below the maximum take-off weight, although the helicopter was likely being about 30 kg over the maximum take-off weight.
"More power than the engine could provide was needed to safely conduct the take-off, given the overweight helicopter condition, confined take-off area and high density altitude," Dr Godley said.
"This highlights the importance of using accurate figures when calculating weight and balance, and expected performance, especially when operating at full capacity and near the maximum gross weight."
The investigation report notes the drop in performance observed by the pilot coincided with the helicopter transitioning out-of-ground effect. (Before this, when the helicopter was in‑ground‑effect when closer to the ground, where the air being drawn down through the rotor collects under the helicopter and provides a 'cushion' of air, requiring slightly less power than otherwise required).
"Regularly reviewing and being prepared to amend flight plans, such as by reducing passenger numbers to increase performance margins, is central to safe operations," Dr Godley added.
While not found to have contributed to the accident, the ATSB investigation also found the flight was operated under CASA's Part 138 regulations, which are for aerial work operations.
The flight should have been operated under Part 133, for air transport operations, as it was a passenger carrying flight. Further, the operator was only approved for aerial work operations.
