What happened
On 2 January 1997, a Hughes 369D helicopter, registration ZK-HQA, was performing a charter flight to transport hunters and a dog from Poronui to the Kaimanawa Ranges. Shortly after departing at 1515 hours, while the aircraft was in a steady cruise-climb at approximately 3400 feet, the engine experienced a sudden and unexpected loss of power. The pilot noted a distinct leftward yaw and the activation of the engine power-out warning system.
With the aircraft positioned over dense beech forest, the pilot identified a small gap in the trees and immediately initiated an autorotational descent. During the descent through the canopy, the helicopter struck the ground heavily. While the aircraft remained upright, the main rotor blades were destroyed by contact with trees. The impact resulted in two serious injuries to passengers, including a head injury and fractured ribs, while the pilot, the remaining passenger, and the dog escaped unhurt. The helicopter sustained substantial damage.
The investigation
The investigation focused on identifying the cause of the engine power reduction. Investigators examined the fuel system, the engine components, and the maintenance history of the aircraft. The engine, an Allison 250-C20B, was inspected both locally and by the manufacturer in the United States.
Key aspects of the examination included:
- Fuel quality: Samples taken prior to the flight were clear and showed no signs of water or contamination.
- Engine mechanics: Post-accident testing showed the compressor, gas producer turbine, and power turbine rotated freely. No significant debris was found on magnetic chip plugs.
- Fuel control system: While no obvious leaks were detected during pressure testing, investigators noted that the power turbine governor actuator was found at the bottom of its operating range, though the cause for this position remained unknown.
- Maintenance: The aircraft had undergone a 100-hour inspection less than two weeks prior to the accident, and the engine had been recently overhauled.
Findings
- The sudden loss of engine power occurred during a routine cruise-climb, necessitating an emergency autorotational landing.
- No conclusive reason was found to account for the sudden loss of engine power.
- Although the engine continued to run after the impact, the investigation could not rule out a transient air leak in the fuel control system, a factor identified in similar historical occurrences of the Hughes 369D type.