This paper reports on a preliminary investigation into applying work on graphic animation of behavioral models to an air traffic control case study ? the National Air Traffic Services (NATS) Short Term Conflict Alert (STCA) system that advises controllers of potential conflicts between aircraft in controlled airspace. Graphic animation permits a model to be visualized in the context of a problem domain. The paper describes how, in order to construct a satisfactory visualization, the finite state behavioral model of the STCA system is extended with Boolean functions over continuous variables to form a hybrid model. The rationale for constructing this hybrid model and its potential use in requirements and design are discussed.