What happened
On July 7, 2019, a towing operation involving two aircraft at Sky-port KITAMI, a temporary airfield in Kitami City, Hokkaido, resulted in a serious incident. An Avions Pierre Robin DR400/180R, registered JA4027, was towing an Alexander Schleicher ASK21 glider, registered JA2288, to an altitude of approximately 3,000 feet.
During the ascent, the aircraft encountered rough airflow due to strong ascending currents. The glider's motion became unstable, causing the tow rope to become significantly slack. As the glider moved closer to the towing aircraft at increased speed, the slack line was suddenly and rapidly pulled taut. This sudden tension caused the tow rope to fracture.
Upon feeling the impact, the pilot of the towing aircraft initiated a descending left turn and radioed a warning regarding the rope break. Simultaneously, the pilot of the glider, who had not yet realized the rope had snapped but feared being pulled into the towing aircraft due to its sudden turn, activated the release lever. This action caused the remaining portion of the rope to drop to the ground, where it was later recovered approximately 1 km south of the release point.
The investigation
The JTSB investigation focused on the mechanical failure of the tow rope and the subsequent actions of the flight crew. Investigators examined the tow rope, which was a 7 mm diameter polyester synthetic fiber rope. They found that the fracture occurred at a knot located within the sleeve of the retractor winch on JA4027.
Technical analysis of the winch mechanism revealed that the sleeve was designed to move backward in a tube and be stopped by a stopper to prevent the rope from being pulled too far from the drum. The investigation also reviewed the flight history and the maintenance of the equipment, noting that the rope was well within its replacement interval.