The assessment of the service durability of aerospace components and assemblies has become an important segment of design. In order to meet strict safety requirements, a number of complex and long experiments are carried out. The use of finite element method (FEM) and extended finite element method (XFEM) for the estimation of fatigue life and fatigue crack growth predictions has been proved as a good alternative to the expensive experimental methods.In this paper, both experimental and numerical analyses of 2024-T3 aluminum spar of a light aircraft under variable amplitude loading are presented. FEM has been used for estimation of the spar life to crack initiation, whereas XFEM has been used for fatigue crack growth predictions and fatigue life estimation of damaged spar. The values of stress intensity factors were extracted from the XFEM solution in MorfeoCrack for Abaqus software. Numerical results (i.e. number of cycles to failure) have corresponded well with the experimental values obtained for the spar made of 2024-T3 aluminum alloy.The investigations have shown that it's possible, using XFEM, to obtain not only the good estimation of fatigue life of the assembly such as the spar of the light aircraft, but also a good prediction of a number of load cycles which will propagate a crack to a certain length. On the basis of these results, it is possible to determine the proper inspections intervals which could prevent the catastrophic failure of the aircraft structure under variable amplitude loading.