What happened
On the evening of 1 June 2023, a Arace Sirin UAS, registered ZT-YKI, was conducting night-time surveillance operations at the Glen Marais Warehouse in Gauteng Province. The operation was being conducted under Beyond Visual Line of Sight (BVLOS) regulations.
During the flight, the pilot-in-command activated the return-to-launch (RTL) function. As the aircraft approached the landing site and descended to approximately 9 meters above the ground, the controller displayed a "potential thrust loss" warning. Following this alert, the aircraft entered a rapid descent, striking the landing surface with significant force. The impact caused the aircraft to bounce before settling upright. While the aircraft sustained minor damage, the impact was severe enough to dislodge the supplementary transponder unit from the airframe. No injuries to personnel on the ground were reported.
The investigation
SACAA AIID examined the flight logs, aircraft specifications, and environmental conditions. The investigation confirmed that the aircraft was operating near its maximum take-off weight, with a recorded weight of 2.906kg against a limit of 2.98kg. The investigation also noted that the aircraft was fitted with an additional transponder unit, which increased the overall weight.
Technical feedback from the Original Equipment Manufacturer (OEM) was also considered. The OEM highlighted that the high-altitude, high-density altitude conditions found in the South African Highveld differ significantly from European operating environments. Furthermore, the manufacturer noted that battery voltage is critical for motor effectiveness and suggested limiting flight times to ensure power levels remain above 50%.
Findings
- The primary cause of the hard landing was likely the aircraft entering a vortex ring state during the high rate of descent following the RTL activation.
- A contributing factor was that the Arace Sirin was operating near its maximum allowable take-off weight, which increased the thrust-to-weight ratio required to maintain flight.
- The presence of the supplementary transponder unit contributed to the increased mass of the aircraft.