## 1 Introduction  
An engine typically encounters extreme environments which will result in the inlet distortion of the engine [1]. The inlet distortion, which caused the high-cycle fatigue of turbomachinery components, is the main reason for the disfunction of the engine's components. When a mild inlet distortion occurs, the vibration of the compressor becomes abnormal, reducing the efficiency of flight propulsion. Whereas when a severe inlet distortion occurs, there will be unrecoverable damage to the engine [2,3]. This can even surge the compressor, leading to a shutdown of the engine, affecting the safety of the engine. Therefore, it is important to research the inlet distortion of the engine.  
From the mid-20 th century, a great number of studies have been conducted on the inlet distortion of the engine throughout the world. [PERSON] [4] discovered that during the approaching of the designed speed of the compressor, the loss of surge pressure ratio, due to inlet flow mal distribution, was mainly dependent on the circumferential variation of inlet total pressure. [PERSON] [5] measured the blade response to an inlet total pressure distortion in an integral blade disk. The variations of measured stress were found to be significantly affected by unsteady aerodynamic coupling. [PERSON] [6] demonstrated the feasibility of using the stereo particle image velocimetry techniques for determining the flow field at the exit of convoluted intakes with a spatial resolution higher than the typical pressure measurements.  
[PERSON] and [PERSON] [7] developed a computational approach to assess the effect of the inlet distortion on the stability of a large bypass ratio turbofan engine. However, all the above researches were carried out based on some fundamental analysis. The accuracy of the experimental models and the results need further verification.  
The traditional nondestructive monitoring methods of engine mainly include the vibration monitoring technique, the electrostatic monitoring technique, and the oil monitoring technique [8; 9; 10; 11]. Although these monitoring methods can meet the requirements of health monitoring to a certain extent, some serious limitations still exist. For instance, the vibration monitoring technique is limited to the structure of the engine. The optimal sensor placement in different systems is a great challenge. Furthermore, the signal picked up by the vibration sensor reflects the overall engine's vibration but not specific components' vibration. Therefore, it is difficult to locate the fault [12]. Furthermore, the electrostatic monitoring technique is only sensitive in monitoring gas path faults, while the oil monitoring technique has a delay for the identification of the early faults, which can easily miss the appropriate time of diagnosing [13].  
The acoustic emission (AE) monitoring technique is a widely used nondestructive monitoring method that can identify and locate various types of faults [14; 15], reducing the daily maintenance workload of engines, improving the engine's efficiency. It shows an excellent performance in comparison with other traditional methods. In the field of aviation, some studies undertook the AE monitoring technique on the fatigue components of an engine, such as the main body, the horizontal tail, the landing gear, and the turbine blades [16; 17; 18; 19; 20]. However, the application of the AE technique on the inlet distortion of the engine remains blank. This paper applies the AE technique to explore the inlet distortion of the engine, and the study proves that the AE technique is an effective method in monitoring the status of the engine. It can precisely identify the inlet distortion of the engine through an intelligent method based on a unit parameter entropy and the BP neural network.