The present work describes a method for calculating inviscid, incompressible and steady flows around an aircraft. The proposed method is based upon a combination of the surface source panel method for the body, and the vortex lattice method for the lifting surfaces and their separated flows. The configuration as a whole is treated in a single global three-dimensional system of equations. Solving this three-dimensional system of equations we can get the strengths of the singularities, hence, the pressure distribution on the configuration surfaces and the resultant aerodynamic. properties. Experimental work. is made in M.T.C. wind tunnel on a jet trainer aircraft model .In order to demonstrate the capability of the present method to predict the pressure distribution on the configuration surface and hence explain the aerodynamic phenomena of wing-body-tail combinations, comparisons are made with the experimental results. These results demonstrate the same general trends obtained from calculations.