Structuring the safety case for unmanned aircraft system operations in non-segregated airspace

Routine access to non-segregated airspace is a key enabler for the civilian Unmanned Aircraft System (UAS) industry. Approvals for UAS operations in this airspace are contingent on the provision of a safety case, which details how the risk of a Mid-Air Collision (MAC) accident will be managed to an acceptable level. There is no accepted framework for structuring operational safety cases for UAS and this gives rise to a number of challenges to the application of the regulation by "safety target" approach. Further, a wide range of controls has been proposed for mitigating the risk, however the effectiveness of the controls is not known. A reconciliation and extension of existing causal models describing the MAC accident sequence is provided in this paper. A barrier bow tie model is developed as a means for structuring the safety case for generic UAS operations in non-segregated airspace. The model is applied to the classification of over 50 commonly used risk controls and the relationship between the control and the manner in which the reduction in MAC risk is achieved is determined. A case-study application is also presented validating the utility of the tool in the development and communication of safety cases for UAS operations in civilian airspace.