Based on the telemetry data, the technology to select the real modes for the aircraft operational mode derived by ambient excitation is investigated through ARMANExT method in this paper. Firstly, the theory of the ARMANExT method is introduced. Secondly, the mode indication is proposed, and the filter strategy is studied based on stabilization diagram, frequency and timefrequency analysis. Eventually, a case study is carried out. It is found that the identified modes reasonably match with the result of the frequency and timefrequency analysis.