Accident Summary
On March 5, 2020, about 1130 Hawaii-Aleutian standard time, an Airbus EC130 B4 helicopter, registration N11QK, was substantially damaged when it was involved in an accident near Kalapana, Hawaii. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 air tour flight. Of the six occupants on board, the commercial pilot and three passengers were uninjured, and two passengers sustained minor injuries.
Sequence of Events
The accident helicopter was the second of two commercial air tour helicopters departing Hilo International Airport (PHTO). After departure, they flew southerly, remained slightly offshore for a short time, then turned west along the shoreline and 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. Immediately after 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 approach. As the helicopter slowed, he raised the collective and applied right tail rotor pedal, but the nose veered to the left. The pilot eventually applied full right tail rotor pedal, but the nose 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 performed a hovering autorotation. 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.
Postaccident Examination
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 limits specified on the technical drawing were 2.3±0.3 mm 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.
Root Cause Analysis
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.