The computation of the electromagnetic (EM) fields of a return stroke is important to compare, evaluate and enhance the engineering models. Due the increasing costs of software packages rise compare to the funds for the academic research reduce it is economically interesting to take a closer look at open source software. Within the scope of engineering and scientific numerical computation SCILAB ([Scilab-Enterprises, 2014]) is such a package. SCILAB is re- leased as cross-platform open source application under the CeCILL license ([CEA-CNRS-INRIA, 2013]). For that reason four engineering models of the current distribution along the lightning return stroke channel and the resulting electrical and magneti- cally fields are programmed in SCILAB and compared with published results of other computations. Additionally the measured currents of lightning discharges are recorded as stroke to an elevated object for obvious reasons. Therefore also the strike to a tall object is implemented in SCILAB, the fields are calculated and compared with published data.