Summary
On July 7, 2019, at around 13:45 JST, a serious incident occurred involving two aircraft operated by Non-Profit Organization Aero Sports KITAMI. Aircraft A, an Avions Pierre Robin DR400/180R (registered JA4027), was towing Aircraft B, an Alexander Schleicher ASK21 glider (registered JA2288), near Sky-port KITAMI in Kitami City, Hokkaido Prefecture. During the tow, the tow rope fractured at an altitude of approximately 3,000 ft. The glider pilot subsequently released the remaining rope, which dropped to the ground. There were no injuries to persons on board or on the ground, and no damage to either aircraft or property.
Aircraft Information
Aircraft A (JA4027) was a single-engine airplane manufactured on January 31, 1985, with serial number 1684. Its certificate of airworthiness was valid until June 15, 2020. Aircraft B (JA2288) was a glider manufactured on January 30, 1981, with serial number 21046 and a maximum weight of 600 kg. Its certificate of airworthiness was valid until April 13, 2020. At the time of the incident, the glider weighed 465.1 kg.
Incident Details
The tow was intended to reach a pressure altitude of 3,600 ft (3,000 ft AGL) near an area of relatively strong ascending current. During the climb, the airflow was rough, and the glider's motion was unstable in vertical and lateral directions. When the glider was at about 3,000 ft, it was considerably uplifted by an ascending current and approached Aircraft A at increased speed, causing the tow rope to become widely slacked downward. The glider pilot did not take corrective action such as deceleration. The slack line was then rapidly pulled, accompanied by a significant impact. The airplane pilot, upon feeling the impact, checked the rearview mirror and saw that the tow rope was no longer visible, indicating a break. He began a descending left turn per procedures and reported the rope break by radio, noting that the rope might remain in the glider.
The glider pilot, who was not watching the rope at the moment of impact, felt a significant impact but did not immediately recognize a rope break. Seeing the airplane make a descending left turn, he feared being pulled into the airplane and instantaneously operated the tow rope release lever. Almost simultaneously, he heard the airplane's report of the rope break. The released tow rope, about 49 m long and weighing about 1.5 kg, was later found in a field about 1 km south of the release point.
The tow rope, made of TETRON (polyester synthetic fiber) with a diameter of 7 mm and total length of about 52 m, met the strength specified in the flight manual. It was fractured at a knot in the sleeve of the tow rope retractor winch of Aircraft A. The organization replaced tow ropes every 1,000 launches; this was the 428th launch since the last replacement. The sleeve mechanism was designed to block movement and prevent direct force propagation to the winch drum.
Investigation and Analysis
The investigation determined that it is highly probable that the tow rope was rapidly stretched from a widely slack condition and fractured at the knot in the sleeve by a tensile force exceeding its strength after the sleeve was blocked by the stopper. The glider pilot, not recognizing the rope break but feeling the impact and seeing the airplane's descending turn, judged it dangerous to follow and operated the release lever, inadvertently causing the remaining rope to drop.
Probable Cause
In this serious incident, it is highly probable that, when the tow rope was fractured while Aircraft A was towing Aircraft B, the captain of Aircraft B, who did not recognize the rope break and judged that it was dangerous to follow Aircraft A by seeing it making descending turn to the left, operated the tow rope release lever, that caused the tow rope remaining in Aircraft B to drop.