On June 18, 2021, around 0845 eastern daylight time, a McDonnell Douglas Helicopter 369E, N371EE, was substantially damaged when it was involved in an accident near Swan Lake, New York. The pilot and lineman sustained serious injuries. The helicopter was being operated by Haverfield Corporation as a Title 14 Code of Federal Regulations Part 133 rotorcraft external load operation. The helicopter was performing maintenance work to electric power transmission equipment. The pilot visually confirmed there was adequate clearance to work as he approached the tower and positioned the helicopter near one of the tower structures. The lineman, seated on a platform attached externally on the skids, removed the safety voltage detector to make room for his shunt and started the maintenance. The pilot described that during this process it felt a “bang came from behind them” and the helicopter began to shake violently while various cockpit alarms sounded. He stated that he had “limited control authority” as he pulled away from the tower and looked for a safe area to land. He descended the helicopter through the tree branches while attempting to keep it level. The lineman reported that as he began the maintenance, he heard a “bang” and then repeated “bang” noises and felt the helicopter shaking as it pulled away from the tower. He attempted to turn around to see if they had struck something but was unable to do so given the force that was pushing him onto the platform. His equipment began to fly off the platform and he tucked himself into a ball on the platform when he saw that the helicopter was descending through the trees. The helicopter impacted trees and came to rest upright in a heavily wooded area. The tailboom remained attached to the fuselage with left bending and crushing deformation forward of the empennage. A photo of the powerline tower being worked on revealed damage to a lifeline support bracket for the tower catwalk (see figure). Figure - Tower where lineman was working with close-up view of damage. A postaccident microscopic examination of one main rotor blade tip showed witness marks consistent with metal impact. The tail rotor assembly exhibited little to no damage. The examination revealed no evidence of any preimpact failure or malfunction of the helicopter’s controls or engine that would have precluded normal operation. A postaccident examination of the tower revealed that, unlike most of the towers being maintained, it was equipped with a catwalk which had a lifeline support bracket protruding from the top of it for the tower catwalk. Review of the helicopter technical description as well as a diagram of the tower at which the upset occurred revealed that there was a negative vertical and lateral clearance from the main rotor blades to the lifeline support bracket for the tower catwalk.