What happened
On the morning of 23 March 2001, a Hughes 369D helicopter, registration ZK-HMN, was performing a commercial deer hunting mission near Milford Sound. While the flight began without incident under clear weather conditions, the pilot noticed unusual vibrations and an engine re-ignition warning approximately ten minutes into the flight.
As the pilot attempted to manage the aircraft's descent near a mountain knoll, the engine failed to respond to power demands. The engine experienced a flameout, and the pilot was forced to perform an emergency landing. The aircraft struck the tops of beech trees on a mountain slope at approximately 3000 feet, rolling onto its side. Both the pilot and the crew member escaped the crash without injury.
The investigation
Investigators examined the engine's performance and the maintenance history of the fuel control unit (FCU). The inquiry established that the engine had been exhibiting symptoms of overspeeding for over two years prior to the accident. Maintenance personnel had previously replaced the power turbine governor multiple times, believing it to be the source of the instability.
Technical analysis revealed that the fuel control unit had been previously overhauled by a facility in Australia. During this overhaul, the unit was incorrectly repaired and subsequently released for service. Furthermore, the investigation looked into the management of service bulletins, noting that a manufacturer's bulletin regarding internal spring replacements had not been implemented, although this was not determined to be the direct cause of the flameout.
Findings
- The engine flameout was caused by a defective fuel control unit that induced engine hunting during periods of reduced power.
- The fuel control unit had been incorrectly repaired by an approved Australian overhaul facility.
- Maintenance personnel misdiagnosed the engine overspeeding symptoms as being related to the power turbine governor rather than the fuel control unit.
- A lack of comprehensive troubleshooting following repeated component changes prevented the identification of the faulty FCU.
- There were deficiencies in the monitoring and control of commercial engine bulletins by the maintenance provider.
Safety action
Following the accident, the Australian Transport Safety Bureau initiated a systemic investigation into the performance of the Australian component overhaul facility due to identified quality assurance concerns. Additionally, the Commission issued recommendations to the maintenance provider to improve the monitoring of service bulletins and to implement more rigorous troubleshooting procedures when repeated component replacements fail to resolve known issues.