What happened
On September 16, 2019, at approximately 12:03 JST, a demonstration flight near Komatsu Airport resulted in the unintentional release of a tow rope. The operation involved a Diamond Aircraft HK36TTC Super Dimona, registered JA01KY, acting as the tow plane, and an Alexander Schleicher ASK21, registered JA2471, acting as the towed glider. Both aircraft were operated by the Japan Student Aviation League.
The flight sequence began with a takeoff from Fukui Airport. After completing initial demonstration patterns at an altitude of approximately 500 to 600 feet, the aircraft entered a left-hand circling maneuver for a third round. During this turn, the pilot of the tow plane felt a sudden impact and realized the tow rope had fractured. The glider pilot, upon feeling the separation, attempted to detach the remaining rope but followed the instructions of the second crew member to avoid ground damage. The glider landed safely on a taxiway, while the tow plane returned to Fukuit Airport. A 61-meter section of the 7mm diameter tow rope was subsequently recovered from a grassy area near the airport's runway 24 threshold.
The investigation
The JTSB investigation focused on the physical condition of the tow rope and the mechanical integrity of the connection points. Investigators examined the fractured ends of the rope, specifically the "stop egg" (an egg-shaped fitting on the tow plane side) and the "end piece" (the attachment on the glider side).
Technical analysis revealed that the rope used was an aftermarket polyester rope rather than the manufacturer's original specification. The investigation also scrutinized the knotting methods used at both ends of the rope. Furthermore, the JTSB reviewed the maintenance records and the adequacy of the flight manual instructions regarding tow rope inspections and installation procedures.