What happened
On July 29, 2018, at approximately 12:05 local time, a DR 400/180 motorized aircraft, registration D-EFTW, was performing a glider towing operation near Bovec Airport (LJBO) under Visual Flight Rules (VFR). During the flight, the towing rope failed, causing the aircraft to lose its connection to the glider. This failure resulted in the aircraft crashing near the airfield.
The investigation
The investigation focused on the mechanical integrity of the towing equipment and the operational procedures used during the glider towing mission. Investigators performed a mechanical and chemical analysis of the used towing rope and examined the damage to the aircraft's propeller. The investigation also reviewed the aircraft's fuel system, specifically the fuel selector valve and fuel quantity indicators, as well as the maintenance records for the towing components. Additionally, the investigators examined the training and oversight procedures of the flight school involved in the operation.
Findings
- The primary cause of the accident was the failure of the towing rope.
- The towing rope and associated components, such as the towing sleeve, were not being managed with proper traceability regarding their age or purchase date.
- There was no documented log of the number of towing operations performed with the rope.
- The operators failed to follow established manufacturer procedures for the maintenance and storage of the rope, such as protecting it from sunlight and ensuring proper drying.
- The investigation found that the flight training organization lacked sufficient internal oversight regarding the use of certified towing sleeves, appropriate ropes, and specialized hardware as required by technical standards.