This article describes a series of validation tests of an active flap system (AFS) on a multi-megawatt wind turbine. A single blade of a 4MW turbine with 130m rotor diameter (SWT-4.0-130) is retrofitted in the outer 15-20m with the AFS. The AFS is controlled remotely with a pneumatic pressure supply system located in the hub of the turbine. The measurements were performed between October 2017 and June 2019 using two different AFS configurations on the blade. A description of the system setup is given, as well as comparisons of measurements and aeroelastic simulations. The measurements quantify the static load control authority of the AFS in atmospheric conditions, providing a preliminary estimate of load impact potential for the concept. This article presents, furthermore, a new method for the characterization of the load impact of such a system and its dynamic response under atmospheric conditions based on a blade-to-blade load comparison.