What happened
On June 18, 2022, a Z-37A aircraft, registration OK-JKA, was performing agricultural spraying operations over a cornfield near Polánka and Jamolice. The pilot was flying at a height of approximately 3 to 5 meters above the crop tops, moving in an east-west direction.
During the ninth pass of the day, the aircraft encountered two overhead power lines: a 22 kV line and a 110 kV high-voltage line. After successfully crossing the lower 22 kV line, the pilot failed to climb sufficiently before encountering the second line. The aircraft's right landing gear struck a combined 18 mm ground wire belonging to the 110 kV circuit.
While the pilot felt a slight jolt during the flight, they did not immediately recognize the significance of the impact and continued the mission. It was only when the pilot observed emergency services (police and firefighters) arriving at the site of the power lines that they realized a strike had likely occurred. The pilot subsequently landed at the Dobelice field to inspect the aircraft, finding no damage to the landing gear, and then reported the incident to the police.
The investigation
The ÚZPLN investigation established that the pilot was experienced and properly qualified for agricultural flight operations. The aircraft was airworthy and maintained according to regulations.
The investigation focused on the flight path and the pilot's perception of the overhead obstacles. It was determined that the pilot had been crossing both lines during previous passes. However, due to the changing geometry of the 22 kV line, the distance between the two lines was increasing. On the critical pass, the pilot mistakenly believed that both lines had already been cleared after crossing the 22 kV line.
Findings
- The strike of the landing gear with the 110 kV ground wire caused the wire to break and fall to the ground.
- The pilot failed to maintain sufficient altitude when crossing the high-voltage line.
- The pilot incorrectly assumed both power lines had been cleared after passing the first one.
- The ground wire, which contained optical fibers for data transmission, was difficult to see during the flight.