Mcdonnell Douglas Heli Co 369FF N975BW accident near Wikieup, Arizona

Probable cause

A vibration of the helicopter for reasons that could not be determined based on the available information, which resulted in a forced landing and the main rotor contacting the tailboom.

NTSB analysis

The pilot was performing external load operations in the helicopter when he felt a vibration during a left descending turn. The pilot increased the collective to arrest the rate of descent, but the vibration became more prominent. He subsequently performed a forced landing, during which the helicopter impacted the ground and a main rotor blade contacted and severed the tailboom. The pilot did not report any malfunctions with the engine nor any abnormal increase or decrease in main rotor rpm.

Postaccident examination revealed a fractured drive link and a fractured strap pack assembly. Examination of the fractured surfaces revealed signatures consistent with tensile overstress, most likely from the blade contacting the left inlet fairing after the helicopter impacted the ground. There was no other evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation, and the reason for the vibration could not be determined based on the available information.

Text from the NTSB. Open the NTSB docket for WPR18LA152 for the full record.