System safety assessment and condition monitoring for an electric power train in a tail rotor

The pursuit of emission-free mobility is driving the development of new propulsion concepts for rotorcraft. Electric propulsion systems are being introduced as replacements for state-of-the-art combustion engines. The usability and viability of these systems in terms of emissions, energy consumption, flight characteristics, safety and reliability, as well as costs, must be demonstrated thoroughly. A Special Condition (SC E-19) has been issued by the European Union Aviation Safety Agency (EASA) to certify electric propulsion systems. Together with the CS-27, this framework defines the conditions for the system. In this paper the special characteristics of an electric tail rotor drive are emphasized, and a safety analysis is conducted to assess the preliminary system architecture of the power train. This analysis identifies common failure modes and resulting component requirements, taking into consideration the zonal arrangement and specific risks associated with each component. The resulting requirements are used to revise the system architecture and develop a basic redundancy architecture for the electric power train. For this, preliminary failure rates are determined using Reliability Block Diagrams (RBD). Several redundancy architecture options are considered and the most promising one is selected. Critical failure modes and their probabilities of occurrence are determined using Failure Modes Effects and Criticality Analysis (FMECA), and further calculated via Fault Tree Analysis (FTA) for the resulting system architecture. Additionally, the requirements for condition monitoring within the system, including the physical signals and the associated sensors are identified.