ERA23LA369

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.

On September 13, 2023, at 1255 eastern daylight time, a Eurocopter EC135T2, N273AM, was substantially damaged when it was involved in an accident near Fargo, Georgia. The airline transport pilot and a passenger were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. In a written statement, the pilot stated the helicopter had been in cruise flight approximately 1 hour about 1,500 ft mean sea level (msl) when the accident occurred. She stated that there were no faults or warnings displayed and all systems were in the “normal” range 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 cover for the tail rotor gearbox and pitch-change controls was separated and found directly beneath the gearbox. The surrounding Fenestron shroud was damaged. All blade grips remained in their mounts, and the remaining 9 blades appeared intact. The Fenestron blades were retained and forwarded to the NTSB Materials Laboratory in Washington, DC, for examination. 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 primary fracture origin was examined more closely with a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS) detector. A higher concentration of oxygen was observed within the intergranular region at the fracture origin. Chromium, along with aluminum and oxygen, was identified on the adjacent surface, consistent with the presence of a sealed anodic oxide film. Sulfur and chlorine were also observed on the adjacent surface. The stress riser or initiating event/damage could not be identified. The intact blades exhibited varying degrees of contact marks near the blade tips and varying degrees of coating wear about the bearing surfaces. The leading edges of the blades were generally pitted; however, the minimum residual blade chord lengths were within aircraft maintenance manual requirements. According to Aircraft Maintenance ManualEC135 Tail Rotor – Inspection (AMM 64-22-00, 6-4) the tail rotor blade should be inspected for signs of cracks, mechanical damage, corrosion, erosion, damage/wear to paint/bearing surfaces, and correct installation of the balance mass in the blade root. Allowable damage limits vary from a 0.01 mm (0.0003 inch) deep score or a 0.05 – 0.1 mm (0.001 – 0.0039 inch) deep dent depending on the location on blade surface. Repairable damage limits on the blade surface vary from a 0.05 – 0.15 mm (0.001 – 0.005 inch) deep score or a 0.2 mm (0.007 inch) deep dent depending on the location on blade surface. Cracks or mechanical damage outside of the repairable damage limits require replacement of the blade. Mechanical damage or corrosion within the repairable damage limits allow for repair as detailed below. According to Aircraft Maintenance Manual Repair – Tail Rotor Blade (AMM 64-22-00, 8-1), a typical tail rotor blade repair consists of removing the tail rotor blades, locally sanding the mechanical damage, polishing the repair area with 400 grit abrasive cloth, inspecting the repair using a ruler and feeler gauge, followed by applying a chemical conversion coating to the repair area. Chemical conversion coatings are typically 0.25 – 1 micron (0.00001 – 0.00004 inch) thick films of mixed metal oxides and hydroxides that are formed spontaneously (without application of current or voltage) on aluminum surfaces via chemical oxidation of the substrate and electrochemical and chemical reactions of species present in the aqueous coating solution. Chemical conversion coatings are generally less robust than anodic oxide films formed during anodization processes. Review of the operator’s maintenance documents revealed no evidence of corrosive environment inspection tasks completed on the accident helicopter. Maintenance tasks referenced a “7 day corrosive environment inspection,” which was noted as “Not applicable – aircraft not in corrosive environment.” Maintenance tasks also referenced a “30 day corrosive environment inspection,” which was not documented as being completed during the maintenance records provided, which dated back to December 6, 2022. FAA Advisory Circular 43-4B, Corrosion Control for Aircraft, dated September 11, 2018, recommends thorough cleaning, inspection, lubrication, and preservation every 15 calendar days for operations conducted in severe corrosion zones, which includes southern Georgia and Florida.