In the last decade, the aeronautical authorities ex- pressed their need to develop a new system for aeronautical radio- communications. The L-band Digital Aeronautical Communica- tion System (L-DACS) is the part of the future system that will be operating in a part of the aeronautical L-band (960−1164 MHz), already occupied by a large number of radio-frequency legacy systems. This paper aims at studying the L-DACS impact on a legacy system, the Distance Measuring Equipment (DME), under two critical situations. Such topics are fundamental in aeronau- tics, as any communication or radionavigation dysfunction may endanger flight and passengers security. Some obtained results will be used for the L-DACS standardization and its specifications finalization. For the first scenario (air-to-air scenario), we propose a deterministic algorithm to compute the interference level in the frequency domain. Since this seems to be insufficient for the second scenario (co-site scenario), we develop a time-frequency approach to analyze the interference using an aeronautical RFC test-bed that we implemented at Supelec.