We describe our experience with the ongoing development of a safety case for an unmanned aircraft system (UAS), emphasizing autopilot software safety assurance. Our approach combines formal and non-formal reasoning, yielding a semi-automatically assembled safety case, in which part of the argument for autopilot software safety is automatically generated from formal methods. This paper provides a discussion of our experiences pertaining to (a) the methodology for creating and structuring safety arguments containing heterogeneous reasoning and information (b) the comprehensibility of, and the confidence in, the arguments created, and (c) the implications of development and safety assurance processes. The considerations for assuring aviation software safety, when using an approach such as the one in this paper, are also discussed in the context of the relevant standards and existing (process-based) certification guidelines.