Due to the tremendous increases in demand for performance based navigation and in-flight broadband Internet access in aeronautical industry, Aeronautical Ad-hoc Networks (AANETs) have been deemed as a viable solution to provide aircraft-to-aircraft communications without the aid of ground stations and satellite system. In this paper, we attempt to address the fundamental concerns arising in the AANETs, namely, under what conditions the AANETs are connected such that end-to-end communications between aircrafts are feasible. We consider the aircrafts on a single flight path and introduce a 2-dimensional AANET model based on the specifications of International Civil Aviation Organization (ICAO). Based on this model, the Necessary Transmission Range (NTR) and Sufficient Transmission Range (STR) are derived as a function of the aircraft density, flight path length and airspace separation, which represent the critical conditions under which the AANET is disconnected or connected respectively. The analytical results are validated through simulation studies.