Mcdonnell Douglas Helicopter 600N N745BW accident near Ashland, Oregon

Probable cause

Fatigue cracking of a main rotor blade for reasons that could not be determined based on the available information.

NTSB analysis

The commercial pilot reported that, during a cross-country helicopter flight, the main rotor blades seemed minimally out of track. After departing on the last leg of the flight, he noticed a slight "hop" as he made an ascending 180° right turn, but later stated that he had experienced worse with gusting winds and door-off operations. In straight and level flight, the blade track appeared to be no different than on the previous legs. During the descent toward the destination, he noticed that the hop became more apparent when the blades were unloaded. Maintenance personnel subsequently found a crack in one main rotor blade from the trailing edge forward to the spar at a point midspan near the beginning of the trim tab.

Examination revealed a visible crack on the top skin, with an opposing crack on the bottom skin. The crack surfaces were flat and light grey with features indicative of fatigue cracking from the trailing edges of the upper and lower skins forward to an internal "C" channel. No gross mechanical damage was visible, and abrasions visible after stripping paint from the blade appeared to be too small to initiate the fatigue crack. The blade had been in service for about 2/3 of its service hour life and more than half of its cycle life at the time of discovery. The reason for crack initiation could not be determined.

Text from the NTSB. Open the NTSB docket for WPR16LA081 for the full record.