Published on 30 July 2025
SR 589: Revision of conditions for triggering the automatic flight termination system for drones
In the course of the safety investigation concerning the accident involving the drone SUI-9903 on 9 May 2019 near the city of Zurich (ZH), the Swiss Transportation Safety Investigation Board STSB identified a safety deficit and issued Safety Recommendation SR No. 587 in its final report No. 2390.
Ensure that the manufacturer revises the conditions for triggering the automatic flight termination system for drones
The Federal Office of Civil Aviation (FOCA) should ensure that the manufacturer revises the conditions for triggering the automatic flight termination system so that suitable measures (contingency procedures) are taken to achieve a controlled flight abortion before the parachute is triggered and the drone descends to the ground in an uncontrolled manner.
Amendment instructions, controlled flight abortion and sensor technology
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 de-stroyed.
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 resili-ence 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.In the case of the accident involving the largely identical drone SUI-9909 on 25 January 2019 (cf. summary report), the FTS was also immediately triggered due to a loss of the GPS signal. As the investigation showed, the drone's flight attitude was still stable at this time and thus a landing under engine power, either manually controlled on sight or autonomously, would not have been fundamentally impossible.
When operating under extreme climatic conditions, corresponding flight-critical parameters such as ambient temperature and humidity are not included in practice. This would allow a flight mission to be aborted early or not carried out at all under certain conditions.
Implementation and FOCA opinion
In its letter dated 15 November 2023, the Swiss Transportation Safety Investigation Board STSB informs the Federal Office of Civil Aviation FOCA that it has assessed SE No. 589 as "implemented".
Das Bundesamt für Zivilluftfahrt unterstützt die Sicherheitsempfehlung (SE) Nr. 589 grundsätzlich.
Die Rollen und Verantwortlichkeiten der zuständigen Behörden werden in der Durchführungsverordnung (EU) 2019/947, bzw. den Applicable Means of Compliance (AMC) 1, Art. 11, Abs. 1.5 (f), festgelegt:
According to Regulation (EU) 2018/1139 (the EASA ‘Basic Regulation’), EASA is the authoritycompetent in the European Union to verify compliance of the UAS design and its components withthe applicable rules, while the authority that is designated by the Member State is competent toverify compliance with the operational requirements and compliance of the personnel’s competencywith those rules
Diese Sicherheitsempfehlung bezieht sich auf Drohnenhersteller und Designüberlegungen, dementsprechend ist für die Überwachung der Regelkonformität bezüglich Konstruktion (design) die Agentur der Europäischen Union für Flugsicherheit (EASA) die zuständige Behörde.
Darüber hinaus wird die SE 589 bereits vollumfänglich durch den risikobasierten Ansatz gemäss SORA (Art. 11 der Durchführungsverordnung (EU) 2019/947) berücksichtigt.
SORA verlangt die Bestimmung von betrieblichen Verfahren für den Fall eines Auftretens technischer Probleme bei unbemannten Luftfahrzeugen (unmanned aircraft systems, UAS) sowie den Umgang mitgestörten Systemen, um den sicheren Betrieb von UAS zu gewährleisten. Das BAZL richtet sich hierbeinach den Anforderungen an den Betrieb (operational safety objectives, OSO), wie sie heute in derVerordnung (EU) 2019/947 respektive in den dazugehörigen AMC festgehalten werden.Verfahren zur Bewältigung von Ausnahmesituationen (contingency procedures) und solche zur Bewältigung von Notfallsituationen (emergency procedures) werden bereits heuteim Rahmen des SORA-Prozesses sowohl durch den Betreiber als auch vom BAZL bewertet respektive validiert.
Das BAZL erachtet die Sicherheitsempfehlung Nr. 589 hiermit als vollständig umgesetzt und abgeschlossen.
SUST Summarischer Bericht zum Unfall mit SUI-9909
Sicherheitsempfehlung Nr. 587
Sicherheitsempfehlung Nr. 588
Sicherheitsempfehlung Nr. 589
Sicherheitsempfehlung Nr. 590
Sicherheitshinweis Nr. 54 - Hersteller von Drohnenkomponenten und zugehöriger Software
Federal Office of Civil Aviation
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