What happened
On 10 May 2023, an Arace Sirin unmanned aircraft system, registered as ZT-XVM, was conducting a night-time security surveillance mission in Vrede, Free State. Operating under beyond visual line of sight (BVLOS) regulations, the pilot launched the aircraft from a landing pad with 94% battery power.
During the mission, the aircraft experienced several uncommanded changes in flight mode. Approximately twelve minutes into the flight, the system reverted to return-to-launch (RTL) mode due to a radio failsafe, which the pilot manually corrected. Twenty minutes into the operation, the aircraft entered a critical battery failsafe mode, prompting an immediate landing. The pilot chose to cancel this landing mode and instead switched the aircraft to loiter mode.
As the aircraft was returning to the launch site, the pilot received a warning regarding potential loss of motor thrust. The aircraft subsequently lost connection with the remote pilot station and crashed approximately 900 metres from the launch pad. The impact resulted in substantial damage to the aircraft's gimbal and landing gear, though no injuries were reported on the ground.
The investigation
An investigation by the SACAA AIID examined the flight logs and the physical condition of the aircraft's power source. Analysis of the battery revealed a significant failure in one of the cells, with one cell recorded at 0% capacity while others remained at higher levels. This imbalance indicated a failure in the battery's internal integrity. The investigation also reviewed the pilot's credentials, noting a valid Remote Pilot Licence and medical certificate, and confirmed the operator held the necessary certificates for the operation.
Findings
- The primary cause of the accident was a battery cell failure, which created unstable power output.
- This power instability led to multiple flight mode fluctuations and eventually caused the aircraft to disconnect from the ground station.
- The pilot contributed to the sequence of events by cancelling the critical battery failsafe mode instead of allowing the aircraft to land safely.
- The aircraft experienced a loss of thrust on motor 2 during the final descent.
- While the operator had implemented a rule to test batteries older than one year, they were unable to precisely measure the specific number of flight cycles completed by the battery.