What happened
On January 25, 2019, a Matternet M2 V9 unmanned aircraft, registration SUI-9909, was conducting a commercial delivery mission for the Swiss Post. The mission involved transporting blood samples from the Klinik im Park to the Zentral Labor Zürich. The flight was being monitored by a Flight Director located in a mission control unit in Dübendorf.
Shortly after takeoff, while the drone was cruising over the western shore of Lake Zurich, it experienced a sudden loss of GPS signal. Following its programmed emergency logic, the drone's Flight Termination System (FTS) activated, deploying a parachute to initiate a controlled descent. The aircraft descended parabolically and struck the surface of the lake at a vertical speed of approximately 4 m/s. The drone subsequently sank to a depth of about 20 meters. While the medical samples remained intact and no injuries occurred, the aircraft was destroyed due to water ingress causing a short circuit in the power electronics.
The investigation
SUST examined the wreckage recovered from the lake bed on January 28, 2019. The investigation focused on the electrical systems and the sequence of events leading to the GPS signal loss. Investigators analyzed the flight data logs, which showed that the flight battery voltage remained stable during the descent, but the loss of positioning data triggered the FTS.
Technical examination of the hardware revealed that the 5V power supply for the GPS receiver and the precision landing sensor had failed. This failure was traced to a modified ODU connector used by Matternet, where the pins were positioned very closely together. The investigation also noted that while the parachute deployment was successful, the impact of the deceleration caused a piece of the aircraft's fairing to break off due to an eccentric attachment of the parachute line.
Findings
- The primary cause of the emergency landing was a short circuit within a connector serving the GPS receiver and landing sensors.
- This short circuit was likely caused by residual moisture within the housing, as the pins on the modified ODU connector were in extremely close proximity.
- The loss of GPS signal triggered the Flight Termination System's automated emergency landing procedure.
- The aircraft sustained total loss due to the ingress of lake water into the battery management system and power electronics upon impact.
- The investigation found that the FTS design relies on an electrical fuse, which lacks the high-level redundancy found in manned aviation systems.