This paper aims at providing input material into the “NM Top 5 Safety Priorities for the topic of “Operation without a transponder or with a dysfunctional one. The paper proposes an approach for the safety assessment of the impact on ATC from the loss of the aircraft transponder function, i.e. an Aircraft Surveillance Function (ASF) continuity failure. It addresses the case of an aircraft subject to Area Control Service and Approach Control Service. The paper proposes a set of generic operational elements for both the Aircraft and the Ground Domains, as well as recommendations for future work by appropriate stakeholder bodies.