Secondary spectrum access emerges as a meansto increase spectrum utilization significantly in a near future.However, the technical availability and the economic value of thesecondary access have not been fully investigated. Particularly,the impact of the secondary access on the real-life primary systemsremains mostly unaddressed. Among the primary systemsof interest are aeronautical communication systems operating inthe radio frequencies under 6 GHz. In this paper, we presentan overview of various aeronautical communication systems asthe potential primary users. We describe the purposes, characteristics,and operational facts of not only existing but alsofuture aeronautical systems. We also discuss the opportunitiesand challenges of secondary access to aeronautical spectrumand suggest a secondary use case suitable for the aeronautical spectrum.