# Preliminary Impact Assessment of the Ad Hoc Separation Minima: A New Separation Mode
## 1 Introduction
An analysis of the impact of these concepts on the environment and cost-efficiency was conducted for Free Route airspace in [20; 21], and for AMAN/DMAN in [22; 23; 24]. In the last SESAR call [25], a proposed research line related to airspace capacity is the evolution of separation minima. Separation minima values were determined decades ago. SESAR encourages research into advanced separation modes (e.g., dynamic separation) because new horizons are opening in the ATM system. This is due to recent and expected future technological developments (European ATM Masterplan, Level 4 and Level 5 of Automation [26]) and their impact on the area of separation minima and separation modes. Ad Hoc or Variable separation minima (ADSM) concept is one of these new topics, among others, such as Time-Based Separation (TBS), Airborne Separation Assurance Systems (ASAS), or European Recategorization Pairwise (RECAT-EU PWS). This last solution [27] was recently developed and is based on the definition of different Minimum Wake Vortex Separation (MWS) values, depending on the aircraft models (considering over a hundred) and according to their individual characteristics. Ad Hoc separation refers to the application of different separation minima values between aircraft pairs in the same volume of airspace depending on a set of conditions, such as aircraft model, aircraft mass, FL, and encounter geometry, among others. These pairwise separation minima (Ad Hoc separation) also imply a reduction in separation values in those situations in which this is safe. On the one hand, this new separation mode is expected, to some extent, to increase airspace capacity. On the other hand, the effort to reduce separation minima values aims, among other objectives, to minimize the necessity for frequent tactical interventions by ATCOs during in-flight operations. Consequently, the reduction in ATCO interventions holds the potential to decrease aircraft deviations from their intended trajectories, resulting in savings in fuel, carbon dioxide emissions (\(CO_{2}\)), and mitigating delays. In this research, an analysis is presented of the impact of this novel concept in various en-route scenarios. The objective of this analysis is to assess whether applying this concept to a scenario is beneficial. It is considered non-beneficial if the application of this concept increases system complexity without yielding any benefits in terms of capacity, environment, and airspace user cost-efficiency. This analysis serves as a Go/No-Go test for the next stages of development. This study is structured as follows: Section 2 provides a brief description of the Ad Hoc separation operational concept (CONOPS), while Section 3 outlines the methodology that is employed to assess the impact of implementing this concept on the capacity, environmental, and cost-efficiency KPAs. The results of the analysis of different scenarios are presented in Section 4. Section 5 includes a discussion of the results. Finally, Section 6 provides conclusions drawn from the findings and outlines potential avenues for future research.