What happened
On the night of 24 February 2023, an Arace Sirin remotely piloted aircraft (RPA), registered ZT-XXN, was conducting a beyond visual line of sight (BVLOS) surveillance mission near Pullens Hope, Mpumalanga. The flight was being operated under Part 101 regulations. Following standard pre-flight procedures, the pilot coordinated with another RPA operator in the vicinity before launching the aircraft to an altitude of 300 feet above ground level.
As the aircraft followed a railway line for approximately 3 kilometres, it encountered a loss of signal. The pilot attempted to refresh the controller unit and even drove toward the last known position in an effort to re-establish the link, but the connection could not be recovered. While a search was attempted that night, the aircraft was not located until the following morning, when it was discovered inside a cooling tower at the Hendrina Power Station. The impact resulted in substantial damage to the aircraft, though there were no injuries reported.
The investigation
SACAA AIID examined the flight parameters and the operational environment of the mission. The investigation confirmed that the pilot held a valid Remote Pilot Licence with the necessary BVLOS rating and that the aircraft had undergone its mandatory periodic inspection recently. The weather conditions at the time of the incident were favorable, with good visibility and light winds.
Investigators focused on the flight path and the automated responses of the RPA. The aircraft was programmed to initiate a return-to-launch (RTL) sequence automatically if the command link was severed. The investigation scrutinized the trajectory of this automated maneuver in relation to the physical obstacles present at the site.
Findings
Technical analysis revealed that the loss of signal occurred because the pilot flew the RPA behind the power station's cooling towers, which obstructed the link. Once the signal was lost, the RPA engaged its programmed return-to-launch mode, which is designed to fly in a straight line back to the takeoff point. Because the flight path followed a curved railway line, the straight-line RTL trajectory intersected with the structure of the cooling tower. The aircraft's programmed RTL altitude of 300 feet caused it to collide directly with the tower.