Kalapana, HI, USA
N11QK
Eurocopter EC130
The pilot was conducting a precautionary landing following the onset of a significant, high frequency vibration. As the helicopter slowed, he raised the collective and applied right tail rotor pedal, but the nose of the helicopter veered to the left. The pilot eventually applied full right tail rotor pedal, but the nose of the helicopter continued to the left. About 200 feet above ground level, with the right tail rotor pedal fully depressed, the helicopter began to spin to the left. The pilot attempted to stop the spin to no avail and eventually closed the throttle and performed a hovering autorotation. He stated that the helicopter descended, touched down hard, and subsequently rolled onto its right side, sustaining substantial damage. A postaccident examination revealed that one of the 10 tail rotor fenestron blades had been liberated at the blade root, at the level of the blade’s drain port; the blade root remained attached and installed to the fenestron hub. Visual examination of the nine remaining blades revealed three that had cracks aligned with the drain hole with features consistent with fatigue. The investigation determined that the ruptured blade failure was a fatigue fracture that originated near the blade’s drain hole; however, the fracture surface’s origins were damaged, and the type of failure near the point of origin could not be determined. The water drain hole geometry and rib thickness were determined by the manufacturer to be the main factors in the development of the crack. Additional factors include blade loading stresses that were not anticipated during certification, and helicopter operations that involve sideslip maneuvers.
On March 5, 2020, about 1130 Hawaii-Aleutian standard time, an Airbus EC130 B4 helicopter, N11QK, was substantially damaged when it was involved in an accident near Kalapana, Hawaii. Of the six occupants on board, the commercial pilot and three passengers were uninjured, and two passengers sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 air tour flight. The pilot reported that the accident helicopter was the second of two commercial air tour helicopters departing Hilo International Airport (PHTO). After departure, they flew in a southerly direction and remained slightly offshore for a short time before turning west along the shoreline. The two helicopters proceeded to a geographic area known as the "Old Ocean Entry." As the helicopter passed over the shoreline, the pilot noticed a significant, high frequency airframe vibration. He said that as soon as the vibration started, the tail rotor chip annunciator light briefly illuminated, and as the vibration continued, the tail rotor chip light "flickered." The vibration and noise stopped after a few seconds and the chip light extinguished. The pilot selected a large open area as a precautionary landing site and slowed the helicopter on the approach. As the helicopter slowed, he raised the collective, and applied right tail rotor pedal, but the nose of the helicopter veered to the left. The pilot noted that he eventually applied full right tail rotor pedal, but the nose of the helicopter continued to the left. About 200 feet above ground level, with the right tail rotor pedal fully depressed, the helicopter began to spin to the left. To stop the spin, he attempted to gain forward airspeed but eventually closed the engine throttle and preformed a hovering autorotation. He stated that the helicopter descended, touched down hard, and subsequently rolled onto its right side, sustaining substantial damage to the fuselage, tail boom, and the main rotor drive system. A postaccident examination revealed that one of the 10 tail rotor fenestron blades had been liberated at the blade root, at the level of the blade’s drain port; the blade root remained attached and installed to the fenestron hub. The liberated blade section was located within the fenestron outer fairing/shell. The fracture surface was flat starting at the aft end of the blade, up until about two-thirds of the distance to the leading edge, while the forward section exhibited shear lips. The tail rotor chip detector cannon plug was found to have an intermittent connection. Further examination of the ruptured blade’s fracture surface found that the failure initiated from two separate origin areas, one on either side of the drain hole at or near its intersection with the aft rib face and grew progressively along the blade chord through about 60% of the blade before fracturing in overstress. The origin areas had been damaged with no identifiable features. Further from the origin, variable-spaced fatigue striations were observed, consistent with high cycle fatigue crack growth. The blade exhibited some geometric anomalies, its rib thickness was measured at 1.27-1.70 mm and the drain hole offset measured 0.23 mm. The acceptable engineering limit specified on the technical drawing was 2.3±0.3mm and ±0.2 mm respectively. Examination of the remaining nine blades revealed three additional blades that had cracks aligned with the blade drain hole on the suction side of the blade. The cracks were opened and revealed fracture features consistent with fatigue. Airbus Helicopters engineers performed a root cause analysis that identified numerous factors that contributed to the crack development; water drain hole geometry and rib thickness were determined to be the main factors that preceded crack development. Additional factors were attributed to underestimated blade load predictions at the time of aircraft certification, and customer flight data has demonstrated more flight conditions that involve sideslip maneuvers than previously anticipated. Airbus Helicopters has issued Alert Service Bulletin No. EC130 05A033 to identify and replace fenestron blades that meet criteria for which they are susceptible to developing a crack.
A loss of control due to the fatigue fracture of a tail rotor fenestron blade due to the geometry of the water drain hole positioning on the rib.
Source: NTSB Aviation Accident Database
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