What happened
On 14 November 2016, a Robinson Helicopter Company R66, registered ZK-HAG, was performing agricultural spraying duties in the Hokonui Hills, Southland. During the final spray run of the day, the pilot noticed a warning indicating low rotor revolutions-per-minute (RPM). Because the aircraft was at a low altitude, the pilot could not immediately recover the RPM and instead attempted a run-on landing to avoid a more serious stall.
The helicopter struck the ground heavily, causing the main rotor blades to impact the terrain. The impact caused the aircraft to pitch forward and the tail boom to separate. While the helicopter was significantly damaged, the pilot escaped the wreckage with only minor injuries.
The investigation
The Transport Accident Investigation Commission (TAIC) examined the wreckage, the engine, and the fuel system. Investigators analyzed data from the engine monitoring unit and performed laboratory testing on fuel samples. The investigation also looked into the operator's refueling practices at remote sites and inspected the aircraft's fuel-filtering capabilities.
Findings
Technical analysis revealed that there were no mechanical failures within the engine's power plant itself. Instead, the investigation determined that the low rotor RPM was likely caused by a momentary drop in engine power. This power loss was attributed to contaminated fuel containing fine clay particles.
The investigation also identified that the operator's methods for refueling at remote, dusty locations provided several opportunities for such contaminants to enter the fuel supply. The presence of debris in the fuel chain was a critical contributing factor to the engine's performance degradation.