Influences of communication structural complexity on operational safety in regional airspace design Neale L. Fulton a,⇑ , Mark Westcott a , Stephen Emery b a Mathematical & Information Sciences, Commonwealth Scientific & Industrial Research Organisation (CSIRO), GPO Box 664, Canberra, ACT 2601, Australia b School of Civil and Environmental Engineering, University of the Witwatersrand, Johannesburg, South Africa article info Article history: Received 3 May 2010 Received in revised form 7 August 2010 Accepted 4 December 2010 Available online 18 May 2011 Keywords: Airspace design Aircraft proximity Design space Communications Mid-air collision abstract As air traffic management systems have evolved and traffic flows have increased, communications pro- tocols have become more complex. Extra frequencies have been introduced, resulting in different fre- quencies being assigned to adjacent airspace volumes around airports. Consequently, pilots in close spatial proximity might not be operating on a common frequency. This could fatally inhibit timely exchange of information critical to successful avoidance of a mid-air collision. This paper considers the physical feasibility of communication between aircraft when they operate near radio frequency bound- aries. It uses a simple but revealing model of aircraft operation within a multiple radio frequency struc- ture. The model allows comprehensive descriptions of operational and failure modes once parameters such as aircraft velocities, radio frequency structures and communication transmission lengths are spec- ified. The paper uses a novel form of nested plot for high-dimensional data to show how the failure modes are influenced by the parameters. An important conclusion is that circumstances in which problems can arise are not easily predicted during flight. Thus operational experience is not necessarily a good basis for evaluating the safety of communication system design. The model is not intended to be complete or exhaustive; its role is to demonstrate design principles and processes. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Air transport management systems, of which Air Traffic Man- agement (ATM) systems are a part, have undergone significant changes over the past two decades. There has been considerable modernisation of ground-based ATM equipment and of the com- munication infrastructure (e.g., introduction of data-link). This im- parts new engineering demands on these systems, which are now being stressed in ways that can either cause failure modes once la- tent to manifest or new failure modes to emerge. Concurrently there has been an increasing use of all airspace by both new and established groups: the number of recreational sports aircraft have increased significantly; the Unmanned Aerial Vehicle (UAV) indus- try is reported to continue to be the most dynamic growth sector of the world aerospace industry, projected to total just over $62 bil- lion in the next ten years (Teal, 2010); the personal jet and regional jet markets are growing strongly (Mozdzanowska et al., 2003); commercial and regular public transport services are increasing aircraft numbers as demand for air travel rises (Babikian et al., 2002). One possible consequence of failure in an air transport manage- ment system is a mid-air collision. This is perhaps the most dra- matic accident that can occur and one that it is particularly important to avoid. Air transport systems rely on various lines of defence in the form of both procedures and communication links to mitigate against the occurrence of a mid-air collision. One of the roles of these links is to enable aircraft to keep well-separated. Should aircraft come into proximity that violate the required phys- ical separation standards then such a failure will be called a prox- imity event. In these situations the dependability (a reliability and safety concept) of the communication links that exist between air- craft emerges as a critical design issue. Such dependability directly influences the probability of a collision given the occurrence of a proximity event. The term communication link is used here, in the sense of Shannon and Weaver (1998), for any means of transferring information between aircraft. It has characteristics of directness, mode and latency. Direct links can form between aircraft. Indirect links rely on provision of a ground-based air traffic service. Broadly speaking, air-to-ground links are of strategic importance while air-to-air links are of tactical importance. The multiple types of communica- tion links that may exist between aircraft can be considered as a parallel, hierarchical, and fault tolerant system comprised of any combination of data-link (including the specialist links of TCAS), voice and see-and-avoid communications. Each means of 0925-7535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssci.2010.12.002 ⇑ Corresponding author. Tel.: +61 2 6216 7058; fax: +61 2 6216 7111. E-mail addresses: neale.fulton@csiro.au (N.L. Fulton), mark.westcott@csiro.au (M. Westcott), emery@iafrica.com (S. Emery). Safety Science 49 (2011) 1099–1109 Contents lists available at ScienceDirect Safety Science journal homepage: www.elsevier.com/locate/ssci