What happened
On 11 September 2001, a Bell 206B II helicopter, registration ZK-HWI, departed from Mount Pisa Station near Cromwell for a scheduled agricultural spraying mission. The flight was being conducted by an instructor pilot and a trainee pilot. The aircraft was performing its tenth spraying flight of the morning and was carrying a standard chemical load.
Shortly after takeoff, while the aircraft was climbing at approximately 50 feet, the instructor pilot observed the power turbine speed (N2) indicator steadily decreasing. Believing the engine was losing power, the instructor pilot immediately took control and instinctively lowered the collective lever to manage what he perceived as an engine or transmission failure. This maneuver caused the helicopter to descend rapidly, impacting the ground with significant forward speed. The impact caused the helicopter to bounce into the air, rotate, and eventually come to rest in an irrigation ditch. While the helicopter sustained substantial damage, both pilots escaped without injury.
The investigation
Investigators examined the aircraft's mechanical state and maintenance history to determine why the power indication dropped. The investigation revealed that the engine itself was functioning normally; there was no actual loss of engine power or transmission drive. Instead, the failure originated in the N2 tachometer generator drive.
Technical analysis showed that a maintenance action performed six months prior had involved replacing the N2 tachometer generator. At that time, the engineer used an aviation sealant (RTV) to temporarily stabilize a worn drive shaft receptacle, intending to replace the component during a subsequent scheduled service. However, the requirement for this follow-up repair was not properly documented in the aircraft's technical log or the operator's maintenance records.