Eurocopter EC130 N11QK accident near Kalapana, Hawaii

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ANC20LA028 for the full record.