What happened
On March 5, 2020, an Airbus EC130 B4 helicopter, registration N11QK, was operating a Part 135 air tour flight near Kalapana, Hawaii. The flight was the second of two scheduled tours departing Hilo International Airport (PHTO). While flying along the shoreline, the pilot experienced a significant, high-frequency airframe vibration accompanied by a flickering tail rotor chip annunciator light.
In response to the vibration, the pilot selected a large open area for a precautionary landing. As the aircraft slowed, the pilot increased collective and applied right tail rotor pedal; however, the nose of the helicopter veered to the left. Despite applying full right pedal, the aircraft began to spin to the left approximately 200 feet above the ground. The pilot attempted to gain forward airspeed to stop the spin but eventually closed the engine throttle and performed a hovering autorotation. The helicopter touched down hard and rolled onto its right side, resulting in substantial damage to the fuselage, tail boom, and main rotor drive system. Of the six occupants on board, the pilot and three passengers were uninjured, while two passengers sustained minor injuries.
The investigation
Post-accident examination of the fenestron tail rotor revealed that one of the ten blades had been liberated at the blade root near the drain port, though the root remained attached to the hub. The liberated section was found inside the outer fairing. Analysis of the fracture surface showed the failure initiated from two separate areas near the drain hole and progressed through approximately 60% of the blade via high-cycle fatigue crack growth.
Discrepancies were noted in the blade's manufacturing dimensions: the rib thickness was measured between 1.27 mm and 1.70 mm, and the drain hole offset was 0.23 mm. These measurements fell outside the engineering limits of 2.3±0.3 mm and ±0.2 mm, respectively. Additionally, three of the remaining nine blades were found to have cracks aligned with the drain hole on the suction side, showing features consistent with fatigue. The investigation also found that the tail rotor chip detector cannon plug had an intermittent connection.
An analysis by Airbus Helicopters engineers identified that the water drain hole geometry and rib thickness were the primary factors in crack development. Other contributing factors included underestimated blade load predictions during original certification and flight operations involving more sideslip maneuvers than previously anticipated.