What happened
On 19 February 2024, an ST Engineering Aerospace DrN-35LS unmanned aircraft was conducting a beyond visual line-of-sight (BVLOS) demonstration flight. The mission involved traveling from Tanjong Beach, Sentosa, to the Western Anchorage in Singapore. The aircraft was flying autonomously at an altitude of approximately 164 feet above the sea.
During the flight, the aircraft's secondary GNSS receiver transmitted incorrect latitude data to the flight computer. This discrepancy triggered a 'POSITION X-CHECK FAIL' error message and an audible alert. Following its programmed logic, the aircraft entered a hover mode for five seconds. During this brief window, the pilot attempted to regain control using a button on the handheld controller; however, this specific command was ineffective because the necessary manual override required a specific action on the Ground Control Station (GCS) screen.
Because the pilot did not execute the 'Hover-Manual' command on the GCS, the aircraft transitioned into a 'Land – GPS Lost' mode. The drone began a controlled descent at its current location, approximately 2km from the shore, and eventually sank into the sea. There were no injuries to any persons.
The investigation
Investigators examined the flight data and the software capabilities of the DrN-35LS. They found that the secondary GNSS receiver likely experienced external interference, causing it to momentarily report a latitude of +130.0793918°, which is geographically impossible. The flight computer software was not programmed to filter out such invalid data, and the massive discrepancy between the primary and secondary receivers caused a computational loss of precision.
Regarding the human element, the investigation noted that the pilot was startled by the error and failed to use the correct interface to intervene. While the operator provided a Quick Reference Card (QRC) on a separate laptop, the five-second window provided by the system was insufficient for the pilot to consult external documentation.
Findings
- The primary cause of the error was external interference affecting the secondary GNSS receiver.
- The aircraft's flight software lacked a mechanism to identify and ignore invalid latitude or longitude data.
- The sudden magnitude of the positional error caused the flight computer to trigger the 'POSITION X-CHECK FAIL' alert.
- The pilot's attempt to intervene via the handheld controller was the incorrect procedure for this specific error type.
- The current training and assessment regime lacked sufficient opportunities for pilots to be objectively evaluated on emergency procedures in the presence of an instructor.
Safety action
Following the incident, the manufacturer updated the flight computer software to better handle large positional discrepancies and to filter out invalid GNSS data. The operator has also implemented refresher training for all pilots, specifically focusing on emergency procedures and the handling of GNSS errors.