© Published under licence by IOP Publishing Ltd. In the present study the aerodynamic boundary layer at a rotor blade is investigated while the turbine is working under real operating conditions in the atmosphere. Owing to the complexity of the experimental set-up, up to now most research on transition is conducted in wind tunnels and field measurements are rare. Hence important effects such as the unsteady behavior of the inflow is not taken into account. For the current measurements the blade is equipped with a hot film at the most interesting part of the upper side midspan of the blade in order to detect non-laminar structures in the boundary layer. Furthermore, 34 pressure tubes are installed along the chord length in order to gain information about the flow field. A preliminary analysis of the hot-film measurements combined with a CFD calculation and a stability analysis based on the eN method leads to two results. Firstly it is possible to determine the state of the boundary layer (laminar or turbulent) and secondly we propose to discuss our findings in case of medium rotational speed within so called Tollmien-Schlichting scenario.