Covering form of preliminary or final report

On 11 August 2019, the AAII was notified of an air accident of the ASW-19 B glider at LKHB. On the given day, the glider pilot was conducting a conversion flight aimed at practising piloting skills on a new glider type in the area around the airport. The pilot was taking off in aerotow behind the towing UL aircraft SAMBA XXL from RWY 11 at LKHB. The pilot did not respond to instructor’s instruction to immediately release the towline after “not really” a successful take-off run. The glider remained connected to the towing UL aircraft and the aerotow was relatively steeply ascending and gradually decelerating. The pilot of the towing UL aircraft failed to gain speed and at the moment when he saw the glider in the left roll in the rear-view mirror, he released the towline. The glider started immediately falling down in a left spin without any attempt to recover the spin. The glider crashed on the ground and was completely destroyed. The pilot died at the accident site in the glider wreckage.

The towing UL aircraft pilot made a descent and after gaining speed continued flying on a circuit and then landed on RWY 11 at LKHB. On the same day, the AAII inspectors arrived at the location of the air accident and collected data principal for investigating the accident causes.

The critical situation occurred at the moment when the glider pilot did not respond to the instructor’s instruction to immediately release the towline after a dangerous take-off run. Although he had understood the information as he subsequently confirmed it, he failed to release the towline whether because of having both hands on the controls or because of not being able to find the towline release handle in the glider cabin in time. According to Rule L 2, the towing UL aircraft pilot could have also released the towline, but as he saw the glider in the rear-view mirror returning from the right side back to the axis behind the towing UL aircraft, he did not release the towline and continued focusing on a safe take-off.

Immediately after the glider release, the glider floated up to a higher altitude. The pilot responded to the arisen situation by forwarding the stick and the glider reached the desired altitude but, at the same time, its speed accelerated and its position in relation to the towing UL aircraft changed. The manoeuvre executed in such a manner caused sagging of the towline and a subsequent loss of the glider from the towing UL aircraft pilot’s field of vision. The towing UL aircraft, after the short-term towline strain relief, began to ascend automatically and its pilot having lost the visual contact with the glider and on the basis of the towline strain relief sensation evaluated the situation as if the glider was released. Without enquiring the glider pilot what his situation was like, the tow aircraft pilot performed a right avoidance action in an attempt to vacate the space for a safe landing of the glider on RWY 11.

In this phase of flight, the glider was situated to the left from the axis behind the towing UL aircraft and the position of the glider was at this particular moment so unfavourable that the glider pilot was supposed to release the towline immediately. Aborting of the flight in this phase would enable the glider pilot to make a safe straight landing on RWY 11. The glider pilot, however, did not release the towline and the aerotow continued in a relatively steep ascent at a low speed. The glider was flying markedly under the propeller slipstream and the pull of the towline was negatively affecting the flight of both, the glider as well as towing UL aircraft. The vertical component of the towline pull was pulling the rear part of the towing UL aircraft downwards. For this reason, the towing UL aircraft pilot did not manage to reduce the angle of attack and subsequently increase the aerotow speed to the required value. Hence, once he spotted the glider in a sharp left roll, he immediately carried out the towline release from the towing UL aircraft. The glider entered into a nose-dive from the altitude of approx. 50 m above the airfield and it crashed into the ground in a left spin. After gaining speed, the towing UL aircraft continued flying on a circuit and then safely landed on RWY 11 at LKHB.

The cause of the air accident was a concurrence of several factors: the glider pilot with almost no experience on a greater number of types, and zero experience on the given type, did not handle the take-off run phase of the flight properly and was not able to respond adequately to the occurred critical situation leading to the aerotow not gaining necessary flight speed during the ascent. After a forced release of the towing UL aircraft, the fall of the glider into the left spin with a subsequent impact to the ground occurred.

Given the cause of the accident, the AAII issues no safety recommendation.

 

Attached final report in PDF file is in original Czech language.