## 1 Introduction  
The final overall quality of the support frame of an aircraft engine nacelle is generally determined by the machining accuracy of each wall panel and the assembly quality of multiple assembly stations. Currently, there are various research methods to assess multi-level assembly quality, the most representative of which are the influence coefficient method and the finite element method. [PERSON] et al. [1] use measured data to predict the assembly deviation of automotive body-in-white parts with the help of linear regression equations in the numerical analysis, suggesting that the deviation of flexible sheet-metal thin-walled parts can no longer follow the traditional method of accumulating rigid deviations. [PERSON] and [PERSON] [2] discard the traditional methods of deviation analysis, such as the square root method and the Monte Carlo method, and propose using the finite element method to simulate the deviation variation of two-dimensional or three-dimensional sheet metal parts. [PERSON] and [PERSON] [3] consider the deviation characteristics of flexible parts and predict the box assembly deviation of flexible parts for the three most commonly used connection forms (lap, butt and corner) in part assembly, based on the small deformation assumption and finite element ideas, combining engineering structural models and statistical analysis methods. However, the studies described in the previous section are mostly limited to two-dimensional parts or consider only the rigid deviations of the parts. In the actual multi-station assembly process, flexible deviations of 3D parts are still one of the important sources of deviations. In [4], a mixed-variable rigid-flexible modelling approach is proposed for sheet metal parts that are prone to flexible deformation during assembly. Based on a two-dimensional deviation flow model, the deviation transfer mechanism in each assembly step is investigated and the mathematical relationship between the total assembly deviation and the various sources of deviation is established accordingly. In [5], a process-oriented mathematical representation of the positioning datum system is proposed, containing detailed design parameters of the positioning datum system in the multi-station assembly (MSA), which creatively reveals the non-linear cumulative relationship between fixture deviations and errors in the positioning holes and slots of the part. In [6; 7], both rigid and flexible deviations are considered and the impact of the deviations generated by each assembly process is analyzed and through the interaction between stations on the assembly quality by means of a transfer function mechanism. Nowadays, in order to meet the increasingly stringent requirements for assembly quality in the aircraft and automotive industries, the issue of assembly contact during multi-level assembly has received more attention, and many scholars have achieved fruitful results in the field of predicting and controlling the contact state of assembly contact surfaces and the bearing forces in the assembly contact area. [PERSON] and [PERSON] [8] improve on the influence coefficient method by means of a finite element analysis, consider the influence of contact forces on the assembly deviation of parts during the assembly process and propose a contact algorithm that can be implemented in the influence coefficient method. In [9; 10], the technical characteristics of the aero-engine rotor assembly process are analyzed, with a focus on the detection method of coaxiality in assembly quality, and a solution for solving the parallel chain problem with deterministic deviation gravity is proposed. On the other hand, in [11; 12], generalized elastic forces for anisotropic and orthotropic materials are derived from the point of view of material properties, taking into account material inhomogeneities and geometrical non-linearities, and force balance equations are used to investigate the deviation of thin-walled structural parts during assembly. [PERSON] et al.