Rescue swimmers are prone to static discharge shocks during SAR winch operations.There have been times that I have been called to probe the hoist assembly for proper electrical bonding to the helicopter structure due to static discharge incidents.
Static electricity builds up upon hoist assemblies for some reasons.The primary reason is to the very nature of the hoist itself.As the galvanised iron cable runs around the hoist drum in a variable or constant speed ( depending on the hoist type), they rub with each other and friction is resulted.Friction generates charges and since there is no conductive electrical path these charges remain stationary.
Another reason is the movable or rotary blades.Since they rotate at high circumferential and angular speeds,blades collide with air molecules and particles and again friction is the result ( aerodynamic heating- a phenomenon that impair the helicopter structure as well).
A third reason is the whole helicopter structure and generally every structure, assembly that extends to the airflow and beyond the helicopter structure perimeter such as landing gear, skids, tail skid, horizontal stabiliser, search lights,radomes, even the hoist assembly itself.All these are impaired by the aerodynamic heating effect and are the cause of accumulated static charge.
A final reason is the corona effect caused by high electrical power transmission lines along the helicopter structure, that produce tiny electrical discharge causing in their turn the surrunding air to ionize and stationary electrical charges are locally accumulated.
Due to the static charge an accumulated voltage locally of up to 25000Volts can be measured, that severley affects the helicopter communication and navigation equipment and of course poses a threat to aircrew safety ( mainly the rescue divers).
Proper electrical bonding of the hoist assembly is crucial, such as usage of well conditioned bonding braids and conductive (to the helicopter inner structure )paths, for aircrew safety and helicopter wellness.In addition, usage of functional static dischargers and bonding braids suppress accumulated static charge.
Finally, suitable procedures are applied during hoisting operations, by grounding the hook first before the rescue diver touches the sea level or the rescuer touches the ground.