Risk-Based Method for Establishing Ad Hoc Separation Minima Between Aircraft in En-Route Airspace

Separation minima are the minimum required distances between aircraft in vertical or horizontal dimensions, established to ensure safety levels in controlled airspace. Nevertheless, current separation standards are fixed and have remained unchanged for decades, limiting airspace capacity. As a result, separation management is an area where improvement is sought. This research proposes an innovative concept of operation: Ad Hoc (or variable) separation, which involves redefining both the separation values and the way they are applied (separation mode). This concept refers to the application of different separation minima values between aircraft pairs in the same airspace volume based on factors such as aircraft model and mass, encounter geometry, and flight level, among others. A novel collision risk model is developed to compute these Ad Hoc separation minima values from Monte Carlo and Fast Time Simulations. The methodology was applied to a Spanish en-route sector (LECMZMU) to determine safe horizontal Ad Hoc separations. The best results show reductions from 5 NM (current horizontal separation minima value) to 4 NM in over 90% of the evaluated situations, and from 5 NM to 3 NM in more than 80% of encounters for low intersection angles (around 15°). In contrast, for intersection angles around 45°, reductions from 5 NM to 4 NM are possible in 68% of encounters, and from 5 NM to 3 NM in only 11%. These findings confirm the feasibility of Ad Hoc separation and its potential to enhance airspace capacity while maintaining safety levels, supporting its integration into future operational concept.