What happened
On 17 June 2018, a DJI Phantom 4 Pro was conducting a commercial surveying mission in a built-up area of Worcester. During the third flight of the day, the aircraft was operating with a battery charge of approximately 51%. After several minutes of stable flight, the controller suddenly displayed a 'landing' notification without any prior warning or pilot input. This was immediately followed by messages indicating that obstacle sensing would be disabled and that the aircraft was nearing its home point, triggering an automatic landing sequence.
Following these notifications, the drone entered a stationary hover. The pilot attempted to regain manual control by switching to Attitude (A) mode and initiating a Return to Home (RTH) command, but the aircraft remained unresponsive. In an effort to resolve the issue, the pilot rebooted the flight application and even switched to a different portable electronic device, yet the aircraft continued to hover without responding to any commands. After approximately six and a height minutes of hovering, the battery level had dropped to 11% when the aircraft began a descent. During this uncontrolled descent, the drone struck the side of a building and fell 30 feet onto a flat roof, resulting in extensive damage to the aircraft.
The investigation
An initial inspection by a UK repair agency could not identify a specific cause, describing the event as "unknown behaviour." The aircraft was subsequently sent to the manufacturer for detailed analysis. The manufacturer's investigation concluded that the descent was a result of a critical low battery landing. However, the manufacturer could not determine why the aircraft failed to respond to pilot inputs or why the landing sequence initiated so far in advance of the battery reaching a critical state.
Findings
- The aircraft's flight mode changed from GPS to landing mode without any input from the pilot.
- The pilot received no low battery warnings prior to the landing sequence, despite the manufacturer's user manual stating such warnings should be present.
- There was a significant delay between the initial 'landing' notification and the actual commencement of the descent.
- The aircraft became completely unresponsive to pilot commands, including manual mode changes and RTH requests.