Abstract: An apparatus and method of automatically threading successive closed framed openings by helicopter towing a line equipped with a separable coupling. The invention utilizes a short cable loop pre-threaded through each closed frame such as the throat of a stringing block which cable loop is provided at its ends with coupling halves releasably supported at the opposite ends of a trough overlying the framed opening to be threaded. A helicopter is manoeuvred to drop the towing line coupling into a trough where its halves are automatically separated and thereafter recoupled to a respective end of the cable loop. Subsequently, the forward coupling can be similarly separated for insertion of a second cable loop in the towing line at the next threading apparatus in the helicopter's path of travel.