Safety Risk Assessment of Unmanned Aircraft System Operations for Urban Air Mobility

Technology developments has enabled Unmanned Aircraft System (UAS) to be adopted for various applications, including Urban Air Mobility (UAM) – an air transportation system for passengers and cargo in and around urban environments. The operations of UAS in urban environment inevitably raises concerns about the safety impact of UAS.
The operational characteristic of UAS is largely different from the conventional commercial aviation. which brings novel safety issues for which the safety learning process has just started. To address these novel safety issues of UAS operations, it is essential to systematically study them within a formal setting of safety risk assessment.
Safety risk assessment involves a process that comprises risk indicators, risk analysis and risk evaluation. In recent years, regulators and researcher have dedicated significant efforts to developing risk assessment for UAS operations. These approaches are largely adopted from safety risk assessment of commercial aviation. However, it is essential to recognize that UAS operations have large differences with commercial aviation. Therefore there remains shortcomings and improvements to be made to the risk assessment of UAS operations.
This thesis addresses the further development of risk assessment methods for UAS operations for Urban Air Mobility (UAM). The main risk posed by UAM is third party risk (TPR) posed to people on the ground. Therefore, the focus of this thesis is on improving risk assessment methods for ground TPR.
The first study focuses on the TPR indicators for UAS operations. Based on these TPR indicators of commercial aviation, novel TPR indicators and nine separate third party fatality terms are identified. Subsequently, current UAS regulations are evaluated regarding their coverage of these nine third party fatality terms. By doing so, the research provides a more comprehensive understanding of the overall third party risk posed by UAS operations.
The second study aims to develop a safety risk assessment method for the novel ground TPR indicators proposed in the first study. To achieve this, a Monte Carlo simulation based risk assessment approach is proposed and applied to a hypothetical UAS urban parcel delivery case. The results show that the proposed annual ground TPR model and indicators provide an accumulated understanding of the risk posed to people on the ground. The non-negligible level of uncertainty in the models adopted highlights the need for further development of more accurate sub models for UAS ground TPR assessment.
The third study aims to improve the accuracy of the common ground TPR model, where a key limitation lays in the assumption that the product of impact PoF and size of impact area are independent of each other. To address this, an improved characterization is developed and evaluated using dynamical simulation of MBS model of a UAS impacting a human body. The comparison of the novel approach to existing approaches shows significant advantages of the novel developed approach.
The fourth study applies the novel approach developed in the third study to an urban parcel delivery UAS, weighting 15kg, equipped with airbag and parachute. A key motivation is that existing models do not address the risk mitigating effects of equipping a UAS with a combination of airbag and parachute. For the UAS equipped with an airbag Multi Body System (MBS) and Finite Element (FE) models are developed. Subsequently, these models are used to assess ground TPR for different cases with and without airbag and parachute. This analysis show that the method developed in the third study is able to quantify the risk reducing effects of the combination parachute and airbag.
The four interrelated series of studies have developed novel insights and methods in Third Party Risk assessment of UAS operations. These novel insights and methods can provide enhanced safety feedback to a UAS design process, and can stimulate further development of UAS regulation.