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