Abstract This article describes a safety assessment of a new Air Traffic Management (ATM) operational concept—RNP Parallel Approach Transitions (RPAT)—at a hypothetical major international-hub airport. The methodology employed is that developed initially for the innovative and highly complex Single European Sky ATM Research programme but further refined by the authors and applied to a wide range of safety-related projects. Though the described application of the methodology is based on experience gained from a specific ATM project, this article does not take the form of a case study. Rather, the operational environment (OE) has been generalised and the article explains how the safety of RPAT operations would be assessed for this generalised application. Part 1 describes how the required safety properties of the RPAT concept are derived at the ATM-service level—this is the highest level of abstraction, and addresses what the ATM service needs to achieve for the safe movement of aircraft in the airspace (and on the airport surface), without any reference to the underlying end-to-end ATM system. Part 2 describes how those safety properties are then allocated to the ATM system architectural elements and how they are shown to have been met in the implemented system.