On the compatibility of ACAS with the concept of segmented independent parallel approach

Although air traffic's growth rate has slowed, airport capacity and environmental issues, especially noise mitigation, are identified as major challenges that face aviation. From the perspective of flight guidance, the development of segmented or curved approach procedures based on required navigation performance (RNP) allow for rerouting around congested areas and is thus a promising approach in coping with these issues. Considering airports with parallel runway systems, an increased airport capacity can be reached when runways are operated independently. This, however, requires straight precision approaches to be conducted according to current guidelines issued by the International Civil Aviation Organization (ICAO). To help pave the way for independent operations of segmented approaches at parallel runway systems, an elementary safety concept was developed in previous work that utilizes the ground-based system of precision runway monitoring. Within this concept, a generic airport scenario with two parallel runways was considered, at which a straight approach is proposed for one of the parallel runways and a segmented or curved approach is assumed for a second runway. Based on this, it was shown that a modified concept of precision runway monitoring is capable of achieving target levels of safety comparable to or even more stringent than those defined in the literature for independent parallel precision approaches. Although the Airborne Collision Avoidance System (ACAS) is by definition not part of this kind of safety assessment, an important question to answer is whether the current ACAS implementation can be characterized as compatible to the proposed approach geometry. Because classical approaches towards parallel runways are already critical with respect to generating nuisance ACAS alerts, this issue might even be exacerbated due to the segmented routing. To approach this issue, a method will be presented in this work that allows for calculating an approach geometry-dependent critical runway spacing that will not result in nuisance ACAS alerts. The obtained value can be compared to a given runway layout to determine whether ACAS may be used or should be deactivated for a given procedure.