SA 44: Amendment instructions in regard to drone components
In the course of the safety investigation concerning the accident involving the drone Matternet M2 V9, SUI-9903, on 9 May 2019 near Zurich (ZH), the Swiss Transportation Safety Investigation Board (STSB) identified a safety deficit and issued safety advice no. 44 in its final report 2390.
Target group(s)
Manufacturer of drone components and associated software
Safety Advice
Manufacturers of drones and drone components should ensure that Service Bulletins (SB) relating to drone components or their associated software are clearly listed, identified and dated, and that the timing of any revisions can be identified by the operator. Prompt communication to the operators concerned should also be ensured.
Safety deficit
About one minute after take-off at the University of Zurich (UZH) Irchel, the M2 V9 drone automatically triggered the flight termination system (FTS) and initiated an emergency descent with a parachute. After ejecting the parachute, the connecting rope broke and the drone hit the forest floor without deceleration and was destroyed.
As the investigation showed, the applied firmware of the flight controller based on the software version ArduCopter 3.5.0-rc5 was only capable of using two of the three available inertial measurement units (IMU) for the flight control of the drone. As a result, the flight controller's software lacked the ability, known as resilience, not to fail completely in the event of malfunctions or the failure of individual components, but to maintain control of the drone.
Only from software version 3.6.12 onwards was it possible to achieve this resilience with the corresponding configuration of the safety-critical parameter ("EK2_IMU_MASK = 7"), as published in a discussion forum of the flight controller manufacturer as Service Bulletin SB 0000002.At a later date, the manufacturer stipulated that "All Copter users to be on at least ArduCopter 4.x and SB2 parameters, regardless of cube age", (cf. https://discuss.cubepilot.org/c/MSB/16), without specifying a release date.