What happened
On July 8, 2010, a Z-37A aircraft, registration OK-JKA, was performing agricultural spraying operations near Plešnice, Czech Republic. The pilot was treating a sugar beet field and had previously returned to the airfield to repair a malfunctioning application device. After refilling the chemical tanks, the pilot resumed the mission.
The flight path involved crossing two intersecting 22 kV overhead power lines. During previous passes, the pilot had successfully cleared these obstacles. However, during the critical pass, the pilot began descending to the required spraying altitude approximately one minute after passing the first set of wires. The pilot subsequently realized that the second set of power lines was still ahead and immediately initiated a climb.
Despite the attempt to climb, the aircraft did not reach a sufficient altitude in time. The right main landing gear tire likely made contact with the upper conductor, causing a short circuit and visible flashes. The impact caused the conductor to detach from the insulators on two utility poles. The aircraft sustained no damage, and the pilot was uninjured.
The investigation
The ÚZPLN investigation focused on the flight sequence and the pilot's awareness of the obstacles. The investigation established that the pilot was fully qualified, medically fit, and experienced, with extensive flight time specifically on the Z-37A. The aircraft was within its maximum takeoff weight, and all technical certifications were valid.
Investigators examined the terrain and the specific requirements for agricultural flight, which mandate a minimum clearance of 10 meters above the highest conductor when crossing power lines. The investigation noted that the pilot had very little time to react due to the proximity of the two intersecting lines and the necessity of descending for the spraying task.
Findings
- The pilot had successfully crossed both sets of power lines in previous passes.
- During the incident, the pilot focused on the first set of wires and forgot the presence of the second set of wires.
- The pilot's realization of the second obstacle occurred too late to allow for a safe climb.
- The aircraft's actual rate of climb, influenced by the aircraft's weight and environmental conditions, was insufficient to clear the wires safely.
- The contact between the main landing gear tire and the conductor was the primary cause of the wire detachment.