ERA23LA369 · Eurocopter Deutschland Gmbh EC135T2

The pilot of the helicopter reported that she had been in cruise flight for about 1 hour about 1,500 ft mean sea level. All systems were normal when she heard a “bang” and felt a “very strong” vibration in the tail rotor pedals. The pilot immediately identified a site for a precautionary landing, which she completed without further incident. Initial examination of the Fenestron, a 10-bladed tail rotor that acts as a ducted fan for anti-torque control, revealed that one blade was separated at its root. The blades were numbered 1-10, with the fractured blade identified as No. 10. The separated portion of the No. 10 blade was not recovered. The examination of the fracture surface of the No. 10 blade revealed features consistent with fatigue. The general construction of each blade comprised a die-forged aluminum alloy blade and integrated root protected with a sealed chromic acid anodization layer. A chromium oxide layer was deposited on the inner and outer bearing surfaces of the blade root followed by a low-friction varnish. The fracture origin area consisted of localized intergranular features (stress concentrations) from which fatigue cracking progressed through approximately 77% of the blade cross-section before ultimately separating in tensile, or bending, overstress. The initiating event could not be identified. The most likely initiation of intergranular cracking was a compromised anodic oxide film that allowed exposure to a corrosive environment (i.e. marine and/or industrial), as evidenced by the presence of chlorine and sulfur on the blade surfaces. The corrosive exposure resulted in the formation of a network of intergranular cracks at the blade surface. The anodic oxide film was most likely compromised due to abrasive wear (erosion) during operation and/or removal during abrasive maintenance processes. The appearance of the fracture origin area on the pressure side surface prior to blade separation was likely a corrosion pit on the order of 100–120 microns (0.004 – 0.005 inch) with similar depth estimated from oxidation patterns on the fracture surface at the origin area. Due to the intergranular nature of cracking at the fracture origin area, the appearance would not resemble a dent or a score, and it would likely be difficult to detect visually or with the aid of other typical non-destructive inspection techniques.

Verbatim NTSB analysis and probable cause from the NTSB dataset