Flight Details
On 19 February 2024, a ST Engineering Aerospace DrN-35LS unmanned aircraft (UA) operated by Skyports Deliveries Pte Ltd was performing a demonstration flight. The UA took off at 11:37:58 hrs from Tanjong Beach, Sentosa island, Singapore, and flew autonomously along a predetermined route at about 164 feet (50 m) above the sea towards the Western Anchorage. The flight was conducted in beyond visual line-of-sight (BVLOS) mode, controlled by a pilot via a Ground Control Station (GCS) at a remote site.
GNSS Error and Sequence of Events
At 11:45:50 hrs, a 'POSITION X-CHECK FAIL' error message appeared on the GCS screen, accompanied by an aural alert. This error occurred because the UA's secondary Global Navigation Satellite System (GNSS) receiver had transmitted an invalid latitude value (+130.0793918°) to the flight computer, while the primary receiver provided correct data. The UA's flight computer was not programmed to filter out invalid latitude data. Upon the error, the UA automatically halted horizontal movement and hovered for five seconds. If the pilot did not assume manual control by clicking the 'Hover-Manual' command on the GCS screen within that window, the UA would transition to a 'Land – GPS Lost' mode and descend to land at its current location.
Pilot Response
The pilot attempted to take over control by pressing the 'Hover' button on the UA controller, but this method requires valid GNSS data from both receivers, which was not available. The correct action—clicking the 'Hover-Manual' command button on the GCS screen—was not performed. After five seconds of hovering, the UA entered 'Land – GPS Lost' mode and began a controlled descent. During the descent, the pilot could still have manually regained control by clicking 'Hover-Manual' but did not. Approximately 60 seconds after the error message, the pilot realized the correct action, but by then the UA had already ditched into the sea and was sinking.
Outcome
The UA was not recovered and sank. No injuries to persons were reported. The aircraft carried a payload, and its maximum take-off weight is 36.3 kg. The incident occurred during daylight with visibility 14 km, temperature 33°C, and wind 050° at 10 knots, all within operating limits.
Investigation
The Transport Safety Investigation Bureau of Singapore classified the occurrence as an incident. The investigation reviewed flight data recorded at the GCS. The UA manufacturer confirmed the software did not filter invalid GNSS data. The pilot held a valid Unmanned Aircraft Pilot Licence (Class A & B Rotorcraft) issued by the Civil Aviation Authority of Singapore, with 98 hours 51 minutes total flying time and 15 hours 53 minutes on type. The pilot's last Final Handling Test was on 24 March 2023.