Ideally, an airport security system should be transparent, yet reliable; it should provide both a high degree of accessibility to the travelling public and an equally high level of security. However, airport security in the United States is often compromised due to technical and/or legal limitations. This paper examines the technical and strategic measures that have been implemented to address the key issues in providing optimum levels of security without compromising the public's accessibility to airports. It focuses on the technical and organisational initiatives implemented in the eleven years since the Pan Am Flight 103 disaster on 21 December 1988. A literature review provides evidence that a variety of screening techniques must be used to provide reliable security, yet most airports do not apply such techniques. Current security systems at most airports are single-layered, and so they act at best as a deterrent. The paper concludes that the classic law of requisite variety (an intelligent enemy requires an intelligent fortification) applies to airport security systems. The paper therefore presents a dynamic model of aviation security systems, to help ensure the continuous safety and security of the travelling public.