Diagnostics of a civil aircraft engine, which operating under speed variation conditions, is a representative problem encountered in aeronautic industry. One current challenge is to estimate the Instantaneous Angular Speed (IAS) directly from the aircraft engine vibration signal in order to address the case when no encoder is made available due to cost or technological reasons. However, for the current available tacho-less order tracking algorithms, many vital parameters in the need to be manually selected in advance, which raises the question of user-friendliness, even result in false diagnosis. To address this issue, a novel method based on nonlinear mode decomposition (NMD) is proposed and the IAS can be estimated adaptively without pre-knowledge. The vibration signal collected from a civil aircraft engine is used for validation, the experimental results exhibit that the proposed method is more accurate and flexible when compared with the conventional methods.