What happened
On September 6, 2006, an Eurocopter AS 355 N was performing aerial work near Tranqueville, France. The aircraft was in a hover out of ground effect above a 400,000-volt power line, transporting a sling load containing two technicians tasked with inspecting the cables.
During the operation, the co-pilot noted an amber REGUL warning light on the dashboard, indicating a non-critical regulation failure in the left engine. While the pilot focused on verifying the fuel lever positions, a red warning light illuminated on the left engine's fuel control lever. The co-pilot observed this light but did not alert the pilot.
Shortly after, a continuous audible alarm signaled an overspeed condition. The pilot misidentified the tone as an indication of low rotor speed and responded by reducing collective pitch. This action contributed to the left engine reaching maximum RPM and subsequently shutting down.
In an attempt to clear the high-voltage lines, the pilot maneuvered the helicopter forward. During this movement, the sling load struck a power cable before breaking free and colliding with nearby trees. The impact caused two injuries to the technicians inside the load. Believing the load had already reached the ground because the tension was gone, the pilot instructed the co-pilot to activate the cable release mechanism. The load then fell through the vegetation. The helicopter landed safely nearby.
The investigation
Investigators examined the left engine regulator and discovered an electrical insulation failure in the wiring. This failure was capable of triggering both the initial amber warning and the subsequent red alarm. Analysis of the flight data recorder confirmed that the red REGUL light had indeed illuminated.
Technical tests also revealed that the red alarm light on the fuel control lever was specifically designed to assist crews in identifying which engine was affected by a failure. However, the pilot did not notice the light on the lever due to his outward focus, and the co-pilot failed to communicate the light's activation to the pilot.
Findings
- The primary cause of the engine shutdown was the misinterpretation of the rotor overspeed alarm, which led the pilot to reduce collective pitch.
- A lack of coordinated crew response and communication contributed to the failure to recognize the severity of the engine malfunction.
- The co-pilot failed to report the red warning light on the fuel control lever, partly due to over-reliance on the commander's experience.
- The crew lacked recent training in specific emergency procedures, as the last individual training session had occurred in January 2006.
- At the time of the accident, the company had not yet implemented formal crew resource management or standardized procedures for recognizing and managing engine failures during simulator training.
Safety action
Following the accident, the operator implemented new internal training procedures, including classroom instruction, simulator sessions, and flight training, to improve emergency response and crew coordination.