What happened
On June 23, 2006, an Eurocopter AS 355 N helicopter, registered F-GVHF, was performing a medical evacuation flight from Thiviers to Limoges. The crew, consisting of the pilot, a physician, and a patient, was cruising at approximately 1,200 feet when the pilot experienced violent vibrations. Despite engine parameters remaining normal, the pilot initiated an autorotation and performed a side-slip landing on a sloping meadow near Bussière-Galant.
The investigation
Investigators examined the tail rotor blades and identified that one blade had suffered a fatigue failure. A section of the blade, measuring approximately 40 cm in length and 10 cm in width, had broken away from the trailing edge. The failure originated at a rivet hole on the tab side. This specific hole was ovalized because it had been created by two intersecting holes.
Further inspection revealed that the structural composite fabrics on both sides of the trailing edge had been weakened by sanding during previous repair work. The investigation also noted that the crack was difficult to detect because it was located on the internal face of the blade, an area where sealant cracks are commonly found, making it hard for both flight and maintenance crews to identify.
Findings
- The primary cause of the failure was inadequate repairs performed on the tail rotor blade.
- The structural integrity of the blade was compromised by the sanding of composite layers during multiple repair cycles, the most recent occurring in 2003.
- The resulting imbalance caused by the blade loss generated severe vibrations, which subsequently tore away parts of the rear transmission box mountings and damaged the vertical stabilizer.
- The failure propagation was likely rapid, and the delamination of the composite fabrics was not visually detectable.