History of Flight
On February 5, 2016, at approximately 1430 Pacific daylight time, a McDonnell Douglas Helicopter (MDHI) 600N, registration N745BW, experienced a cracked main rotor blade during a positioning flight from Alturas, California, to Ashland, Oregon. The pilot, a commercial pilot operating under 14 Code of Federal Regulations Part 91 for Brim Aviation, was not injured. The helicopter sustained substantial damage to one main rotor blade. Visual instrument meteorological conditions prevailed, and no flight plan had been filed.
The pilot reported that he was flying one of two helicopters on a ferry flight to Ashland, with three intermediate stops planned. All flight operations were normal, though he noted the main rotor blades seemed minimally out of track. After departing Alturas, he felt a slight hop during an ascending 180° right turn but attributed it to possible gusting winds and door-off operations. In straight and level flight, blade track appeared similar to previous legs. During descent into Ashland, the hop became more apparent when the blades were unloaded. The pilot requested the trailing pilot to observe the rotor system; the trailing pilot reported the blades looked out of track. After landing, the pilot informed maintenance that track and balance checks were needed for both helicopters before the next flight. Maintenance personnel later discovered a crack in one main rotor blade, extending from the trailing edge forward to the spar at a point midspan near the beginning of the trim tab.
Examination Findings
The damaged blade was sent to the NTSB Materials Laboratory for examination. The blade had 2,013.4 hours time in service (TIS), 11,065 torque events, and a retirement index number (RIN) of 587,640, compared to a published service life of 3,200 hours or 1 million RIN.
Visual inspection 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. Scanning electron microscope examination showed striations and other fracture features within the fatigue crack region. The origin was near the trailing edge of the upper skin, with striations pointing to the vicinity of the upper corner. The fatigue crack propagated forward in the upper skin past the "C" channel. At the "V" strip, the crack reinitiated at the upper aft corner and propagated forward in the upper leg of the "V" and down through the lower leg. In the lower skin, additional fatigue crack propagation initiated adjacent to the lower aft corner of the "V" strip, propagating forward and aft. Two additional fatigue crack paths were found in the "C" channel.
The other five main rotor blades were examined by the manufacturer, and no anomalies were detected.