CEN17FA127 · McDonnell Douglas Heli Co 369FF

As part of a power line construction project, the helicopter was stringing sock line between power line towers. The pilot was in the process of hooking a needle that was attached to a 50-ft long line to the tower structure in order to pull a sock line that was attached to the needle through the center of the tower. The helicopter was equipped with a side pull hook assembly that attached a cargo hook to the left side of the helicopter. The 50-ft long line was attached to the cargo hook, and a grappling hook was attached to the other end of the long line. The grappling hook was connected to the metal needle, which was to be temporarily attached to a horizontal cross-member of the tower. A cell phone video of the accident sequence revealed that the pilot made two unsuccessful attempts to hook the needle to a horizontal cross-member. On the third attempt, the helicopter flew backward until the needle became entangled with the tower's vertical lattice, which tethered the helicopter to the tower via the long line and resulted in the pilot losing control of the helicopter. As the helicopter continued to apply force on the long line, the needle's aft loop impacted the tower and subsequently separated from the needle. When the needle fractured it ended the helicopter's tether to the tower. The helicopter continued backward to a near vertical pitch attitude then rotated about its vertical axis. As the helicopter rotated and descended, the long line became entangled with the main rotor blades, and the main rotor blades impacted the top of the cabin and the tailboom. The tailboom separated about mid span and impacted the ground next to the rest of the helicopter.

Examination of the fracture surfaces of the needle showed evidence of ductile overstress separation. There was no evidence of a preexisting fracture or crack in the needle. The fracture of the needle was most likely the result of the needle impacting the tower.

The side pull system was certified for a maximum side pull load of 1,900 lbs., which was to be safeguarded by a breakaway swivel and shear pin. The video of the accident sequence revealed that the long line remained attached to the helicopter even after the needle became entangled with and impacted the tower. The breakaway swivel did not appear to separate before the long line became entangled in the helicopter's rotor blades. The breakaway swivel, its shear pin, the two carabiners that hooked to either side of the barrel swivel, and the upper portion of the long line were not found during the investigation. It is likely that the swivel was forcibly disconnected from the side hook when the long line became tightly wrapped around the rotor hub, and then the missing components were ejected from the rotor hub when the long line broke. An examination of these components was not possible; therefore, the investigation could not verify the type of swivel and shear pin that were installed during the accident.

A study of the accident video revealed that shortly before the needle became entangled with the tower, the helicopter initiated a rearward movement. As the helicopter flew backward the needle rotated/rolled about its longitudinal axis from a vertical orientation to a more horizontal orientation, which moved the leading edge of the needle laterally toward the tower until it became entangled with the tower. The helicopter continued to move backward and pulled the needle's aft hook into contact with the tower, which tethered the helicopter to the tower via the long line. The long line force applied on the helicopter before the accident sequence started was calculated to be about 875 lbs, which was less than half the rated force of the side pull hook assembly. Therefore, it is unlikely that the long line force applied on the helicopter before the needle contacted the tower contributed to the accident.

The force required to fracture the needle's aft loop was calculated to be 73,790 lbs., which is nearly 40 times greater than the load required to shear the breakaway swivel shear pin. Since the breakaway swivel could not be located the investigation was unable to determine why it did not separate as designed during the event when the helicopter was tethered to the tower. In the absence of the breakaway swivel and shear pin, there is no evidence that system did not function as designed and certified relevant to the certification basis of the STC. Based on the evidence, the system should have separated due to tensile loads transmitted to the shear pin at the time the helicopter became tethered then moved away from the tower.

Verbatim NTSB analysis and probable cause from the NTSB dataset