What happened
On March 5, 1977, a Hughes 269 C helicopter, registration HB-XDW, was conducting a commercial flight for the Geneva section of the AéCS. Shortly after takeoff from Geneva, while the aircraft was climbing through approximately 2,000 feet, the pilot heard a sharp noise and experienced a sudden engine surge. This was immediately followed by a total loss of power. The pilot promptly initiated an autorotation and identified a suitable landing site in a nearby field. The forced landing resulted in no fatalities and no injuries to the two passengers or the pilot, though the aircraft sustained minor damage to the rear landing gear.
The investigation
Investigators examined the mechanical state of the aircraft and the condition of the engine components. The aircraft had recently undergone a 1,000-hour inspection and an engine replacement just five hours prior to the accident. Laboratory analysis of the lower pulley coupling shaft (part no 269A5559-3) by the Federal Laboratory for Materials Testing (LFEM) revealed evidence of a fatigue fracture. The examination identified a longitudinal crack that originated from a manufacturing defect. While the shaft was not undersized for its operational loads, the presence of this manufacturing flaw led to the eventual failure.
Findings
- The primary cause of the power loss was the fatigue failure of the coupling shaft between the engine and the clutch.
- The fracture originated from a longitudinal surface crack caused by a manufacturing error.
- This specific defect was not detected during previous inspections, including the recent 1,000-hour maintenance check.
- It was noted that similar longitudinal cracks could potentially be caused by torsional oscillations occurring while the engine is running with the rotor disengaged.
Safety action
Following the accident, the Federal Office of Civil Aviation implemented stricter airworthiness directives. In addition to the manufacturer's recommended dye penetrant inspections, new regulations now mandate a specialized inspection every 100 flight hours and the mandatory replacement of the shaft with a new, inspected unit after 1,000 hours of service.