One of the primary hazards associated with the operation of Unmanned Aircraft (UA) is the controlled or uncontrolled impact of the UA with terrain or objects on the terrain (e.g., people or structures). National Aviation Authorities (NAAs) have the responsibility of ensuring that the risks associated with this hazard are managed to an acceptable level. The NAA can mandate a range of technical (e.g., design standards) and operational (e.g., restrictions on flight) regulatory requirements. However, work to develop these regulations for UA is ongoing. Underpinning this rule-making process is a safety case showing how the regulatory requirements put in place ensure that the UA operation is acceptably safe for the given application and environment. There is no accepted framework for structuring or assessing the safety case for UA operations over populous areas, or for assessing the effectiveness of the wide range of potential technical and operational measures that can be potentially employed as part of a safety case. This paper presents a Barrier Bow Tie (BBT) model as a suitable template for the development and assessment of safety cases for UA operations over populous areas. The components of the BBT model and its application to UA operations over populous areas are presented. A case study is used to highlight the desirable features of the BBT model. The BBT model provides a systematic means for classifying and assessing proposed risk controls (the points of regulation) and how these controls contribute (in isolation and in combination) towards a reduction in risk.