Accident
On September 13, 2023, at 1255 eastern daylight time, a Eurocopter EC135T2, registration N273AM, was substantially damaged during a positioning flight near Fargo, Georgia. The helicopter, operated under Title 14 Code of Federal Regulations Part 91, was occupied by an airline transport pilot and a passenger, both of whom were not injured.
According to the pilot's written statement, the helicopter had been in cruise flight at approximately 1,500 ft mean sea level (msl) for about one hour. She reported no faults or warnings and all systems in the normal range when she heard a bang and felt a very strong vibration in the tail rotor pedals. She immediately identified a site for a precautionary landing and completed it without further incident.
Examination Findings
Initial examination revealed that one blade of the Fenestron—a 10-bladed tail rotor that acts as a ducted fan for anti-torque control—had 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 nine blades appeared intact.
NTSB Materials Laboratory examined the retained blades (numbered 1-10). The separated portion of blade No. 10 was not recovered. Examination of the fracture surface revealed features consistent with fatigue. The fracture origin area exhibited localized intergranular features (stress concentrations) from which fatigue cracking progressed through approximately 77% of the blade cross section before final separation in tensile (bending) overstress.
Further analysis with scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) indicated a higher concentration of oxygen within the intergranular region, and chromium, aluminum, and oxygen on the adjacent surface—consistent with a sealed anodic oxide film. Sulfur and chlorine were also detected. The stress riser or initiating event/damage could not be identified.
The intact blades showed varying contact marks near the tips and coating wear on bearing surfaces. Leading edges were generally pitted, but minimum residual chord lengths remained within aircraft maintenance manual (AMM) requirements.
Maintenance Context
According to the AMM for the EC135 tail rotor (AMM 64-22-00), blades must be inspected for cracks, mechanical damage, corrosion, erosion, paint/bearing surface wear, and balance mass installation. Allowable damage limits vary by location. Cracks or damage beyond repairable limits require blade replacement. Repairable damage limits allow for local sanding, polishing, and application of a chemical conversion coating, which is less robust than the anodic oxide film from manufacturing.
Review of the operator’s maintenance documents found no evidence of corrosive environment inspection tasks completed on the accident helicopter. A “7 day corrosive environment inspection” was noted as “not applicable—aircraft not in corrosive environment.” A “30 day corrosive environment inspection” was not documented as completed in the records provided (dating from December 6, 2022). FAA Advisory Circular 43-4B recommends cleaning and inspection every 15 calendar days for operations in severe corrosion zones, including southern Georgia and Florida.