What happened
On 19 April 2023, an Arace Sirin unmanned aircraft system, registered as ZT-YAS, was conducting a night-time surveillance mission over railway tracks near Nottingham Road, KwaZulu-Natal. Operating under beyond visual line of sight (BVLOS) regulations, the pilot initiated the flight at 1945Z following successful pre-flight inspections. After climbing to an altitude of 330 feet, the aircraft entered a loiter mode.
Approximately 28 minutes into the mission, while the aircraft was hovering near the launch site, it experienced a sudden loss of thrust from two of its motors. This mechanical instability caused the drone to roll sharply and descend rapidly, resulting in a hard impact with the ground roughly 20 metres from the launch pad. The impact caused substantial damage to the aircraft, specifically affecting two propellers and three of the landing legs. No injuries were reported during the incident.
The investigation
SACAA AIID examined flight logs and consulted with the Original Equipment Manufacturer (OEM) to determine the cause of the instability. The investigation reviewed the pilot's credentials, including a valid Remote Pilot Licence with BVLOS endorsement, and the aircraft's maintenance history, noting that a periodic inspection had been completed shortly before the accident.
Analysis of the flight data revealed that the loss of control was consistent with a vortex ring state or a significant imbalance in motor output. The investigation also considered environmental factors, noting that the high density altitude in South Africa differs significantly from the European conditions under which the OEM designs its systems. Additionally, the OEM highlighted that maintaining battery voltage is critical for motor effectiveness and suggested that flight times should be limited to prevent power levels from dropping below 50%.
Findings
- The primary cause of the crash was a rapid depletion of battery power below 50%, which led to a loss of thrust in motors 2 and 4.
- This loss of thrust created a motor imbalance, triggering a sharp roll and subsequent loss of altitude.
- The aircraft's flight logs confirmed the loss of thrust in the specific motors during the descent.