What happened
On April 30, 2002, a New Zealand Helicopters Hughes 369D helicopter, registration ZK-HRV, was conducting a routine scenic flight near Mount Tarawera. The flight, which included three passengers, was traveling from Mount Tarawera toward the company base near Rotorua. During the cruise-climb phase of the flight, the pilot noticed the engine chip light illuminate on the instrument panel.
As the pilot searched for a suitable landing site, the engine oil pressure and temperature gauges dropped significantly. Shortly after, the engine RPM warning sounded, followed by a sudden loss of power and the activation of the engine out light. At an altitude of approximately 700 feet, the pilot initiated an autorotational landing on a farm track. During the touchdown, the helicopter' 1900-foot-elevation terrain caused the tail stinger to strike the ground, resulting in the aircraft rolling onto its right side. There were no injuries among the four occupants.
The investigation
Investigators examined the wreckage and the engine components to determine the cause of the power loss. The inspection revealed that the engine oil tank was nearly empty. Examination of the instrument panel area uncovered a fractured nylon TEE fitting located on the rear of the engine torque gauge. This fracture allowed engine oil to escape from the line and leak down behind the instruments and beneath the cabin floor.
Further analysis of the engine showed that the turbine section bearings had failed due to oil starvation. The investigation also looked into the pilot's monitoring of the aircraft's instruments. It was noted that the pilot had been primarily monitoring the turbine outlet temperature (TOT) gauge rather than the torque gauge, which meant the initial indications of the oil leak—specifically abnormal torque readings—went unnoticed.
Findings
- The engine failure was caused by oil starvation following a fracture in a nylon TEE fitting in the oil line.
- The fracture occurred at a sharp corner of the fitting, which may have been a point of stress concentration.
- The pilot was unable to detect the early stages of the failure because they were not actively monitoring the torque gauge.
- The forced landing on uneven terrain resulted in the helicopter rolling over.
Safety action
Following the investigation, the manufacturer, MD Helicopters Incorporated (MDHI), planned to update flight manuals to emphasize the importance of monitoring engine oil pressure and the necessary pilot response to such indications. Additionally, the Commission recommended that the Director of Civil Aviation publish advisory information to ensure pilots and operators better understand the torque gauge oil line system and the significance of abnormal indications.