What happened
On September 13, 2023, at 12:55 p.m. EDT, a Eurocopter EC135T2 helicopter, registration N273AM, was conducting a positioning flight under Part 91 regulations near Fargo, Georgia. While cruising at approximately 1,500 feet mean sea level, the pilot reported hearing a loud bang followed by a very strong vibration in the tail rotor pedals. The pilot immediately identified a landing site and performed a precautionary landing without further incident. The aircraft sustained substantial damage, but the pilot and a passenger were not injured.
The investigation
An examination of the Fenestron tail rotor system revealed that one of the ten blades had separated at its root. While the remaining nine blades remained intact in their mounts, the cover for the tail rotor gearbox and pitch-change controls had separated and was found beneath the gearbox. The Fenestron shroud also sustained damage.
Investigators examined the fractured blade (identified as No. 10) and found that the fracture surface exhibited features consistent with fatigue. The blade's construction involved a die-forged aluminum alloy with a sealed chromic acid anodization layer. Microscopic analysis of the fracture origin revealed intergranular features and a higher concentration of oxygen, chromium, aluminum, sulfur, and chlorine.
Review of maintenance records showed that the operator had not documented the completion of 30-day corrosive environment inspections. Furthermore, the operator's records indicated that 7-day corrosive environment inspections were marked as "not applicable" because the aircraft was not considered to be in a corrosive environment. However, FAA guidelines recommend frequent cleaning and inspection for aircraft operating in severe corrosion zones, such as southern Georgia.
Findings
- The fracture originated from localized intergranular stress concentrations.
- Fatigue cracking progressed through approximately 77% of the blade cross-section before the blade failed due to tensile or bending overstress.
- The presence of chlorine and sulfur suggests the blade was exposed to a corrosive environment.
- The protective anodic oxide film was likely compromised by abrasive wear from operation or during abrasive maintenance procedures.
- The resulting corrosion pit at the fracture origin was estimated to be between 100 and 120 microns deep, a condition that would be difficult to detect through standard visual inspections.