The paper presents results of research on thin-walled load-bearing structure model representing a fragment of plane wing torsion box. The basis for obtaining data on deformation character and stability loss process was a model experiment conducted using a dedicated experimental stand. On the basis of experiment results, adequate numerical simulations were conducted using software based on finite elements method. Results of non-linear numerical analyses allowed to determine the character of stress distribution and formulate conclusions regarding behaviour of the post-critical deformations of the examined wing part.