What happened
On 18 March 2022, a DJI Phantom 4 Pro with registration ZT-UKY was being used for a surveillance mission at Camps Drift, near Pietermaritzburg, KwaZulu-Natal. The operation was being conducted under visual line of sight (VLOS) in accordance with Part 101 regulations.
After successfully completing an initial automated survey flight, the pilot replaced the batteries to begin a second automated mission. Once the second survey was finished, the pilot initiated a return-to-home command. While the aircraft was approximately 250 metres above ground level, the pilot transitioned from automated to manual flight mode. However, when the drone was roughly 500 metres from the base and at 30% battery capacity, it lost connection with the controller. The aircraft subsequently struck a tree, resulting in substantial damage to the landing gear and gimbal.
The investigation
The investigation examined the pilot's credentials, the aircraft's maintenance history, and the condition of the power source. The pilot held a valid Remote Pilot Licence (RPL) with appropriate ratings and a current medical certificate. The aircraft had undergone a recent inspection earlier in February 2022.
Post-accident analysis by the operator focused on the battery used during the flight. Testing revealed that the fourth battery cell exhibited a higher voltage deviation compared to the other cells. The battery in question had been manufactured in September 2017 and purchased in July 2019.
Findings
- The primary cause of the accident was a battery cell failure during flight.
- This electrical failure led to a loss of command link between the pilot and the aircraft.
- The loss of connection resulted in the aircraft colliding with a tree during its return-to-home sequence.
- The operator noted that the failure may have been due to the prolonged use of the battery and a potential lack of strictness in retiring batteries that showed digital signs of wear.
- Weather conditions at the time of the incident were fine.